Сценарий большого взрыва это

Как появилась наша Вселенная? Как она превратилась в кажущееся на первый взгляд бесконечное пространство? И чем она станет спустя многие миллионы и миллиарды лет? Эти вопросы терзали (и продолжают терзать) умы философов и ученых, кажется, еще с начала времен, породив при этом множество интересных и порой даже безумных теорий. Сегодня большинство астрономов и космологов пришли к общему согласию относительно того, что Вселенная, которую мы знаем, появилась в результате гигантского взрыва, породившего не только основную часть материи, но явившегося источником основных физических законов, согласно которым существует тот космос, который нас окружает. Все это называется теорией Большого взрыва.

Основы теории Большого взрыва относительно просты. Если кратко, согласно ей вся существовавшая и существующая сейчас во Вселенной материя появилась в одно и то же время — около 13,8 миллиарда лет назад. В тот момент времени вся материя существовала в виде очень компактного абстрактного шара (или точки) с бесконечной плотностью и температурой. Это состояние носило название сингулярности. Неожиданно сингулярность начала расширяться и породила ту Вселенную, которую мы знаем.

Стоит отметить, что теория Большого Взрывая является лишь одной из многих предложенных гипотез возникновения Вселенной (например, есть еще теория стационарной Вселенной), однако она получила самое широкое признание и популярность. Она не только объясняет источник всей известной материи, законов физики и большую структуру Вселенной, она также описывает причины расширения Вселенной и многие другие аспекты и феномены.

Хронология событий в теории Большого Взрыва

Основываясь на знаниях о нынешнем состоянии Вселенной, ученые предполагают, что все должно было начаться с единственной точки с бесконечной плотностью и конечным временем, которые начали расширяться. После первоначального расширения, как гласит теория, Вселенная прошла фазу охлаждения, которая позволила появиться субатомным частицам и позже простым атомам. Гигантские облака этих древних элементов позже, благодаря гравитации, начали образовывать звезды и галактики.

Все это, по догадкам ученых, началось около 13,8 миллиарда лет назад, и поэтому эта отправная точка считается возрастом Вселенной. Путем исследования различных теоретических принципов, проведения экспериментов с привлечением ускорителей частиц и высокоэнергетических состояний, а также путем проведения астрономических исследований дальних уголков Вселенной ученые вывели и предложили хронологию событий, которые начались с Большого взрыва и привели Вселенную в конечном итоге к тому состоянию космической эволюции, которое имеет место быть сейчас.

Ученые считают, что самые ранние периоды зарождения Вселенной — продлившиеся от 10^-43 до 10^-11 секунды после Большого взрыва, — по прежнему являются предметом споров и обсуждений. Если учесть, что те законы физики, которые нам сейчас известны, не могли существовать в это время, то очень сложно понять, каким же образом регулировались процессы в этой ранней Вселенной. Кроме того, экспериментов с использованием тех возможных видов энергий, которые могли присутствовать в то время, до сих пор не проводилось. Как бы там ни было, многие теории о возникновении Вселенной в конечном итоге согласны с тем, что в какой-то период времени имелась отправная точка, с которой все началось.

Тайны сингулярности

Также известная как планковская эпоха (или планковская эра) принимается за самый ранний из известных периодов эволюции Вселенной. В это время вся материя содержалась в единственной точке бесконечной плотности и температуры. Во время этого периода, как считают ученые, квантовые эффекты гравитационного взаимодействия доминировали над физическим, и ни одна из физических сил не была равна по силе гравитации.

Ученые обнаружили неизвестный источник гравитационных волн

Планковская эра предположительно длилась от 0 до 10^-43 секунды и названа она так потому, что измерить ее продолжительность можно только планковским временем. Ввиду экстремальных температур и бесконечной плотности материи состояние Вселенной в этот период времени было крайне нестабильным. После этого произошли периоды расширения и охлаждения, которые привели к возникновению фундаментальных сил физики.

Приблизительно в период с 10^-43 до 10^-36 секунды во Вселенной происходил процесс столкновения состояний переходных температур. Считается, что именно в этот момент фундаментальные силы, которые управляют нынешней Вселенной, начали отделяться друг от друга. Первым шагом этого отделения явилось появление гравитационных сил, сильных и слабых ядерных взаимодействий и электромагнетизма.

В период примерно с 10^-36 до 10^-32 секунды после Большого взрыва температура Вселенной стала достаточно низкой (1028 К), что привело к разделению электромагнитных сил (сильное взаимодействие) и слабого ядерного взаимодействия (слабого взаимодействия).

Эпоха инфляции

С появлением первых фундаментальных сил во Вселенной началась эпоха инфляции, которая продлилась с 10^-32 секунды по планковскому времени до неизвестной точки во времени. Большинство космологических моделей предполагают, что Вселенная в этот период была равномерно заполнена энергией высокой плотности, а невероятно высокие температура и давление привели к ее быстрому расширению и охлаждению.

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Это началось на 10^-37 секунде, когда за фазой перехода, вызвавшей отделение сил, последовало расширение Вселенной в геометрической прогрессии. В этот же период времени Вселенная находилась в состоянии бариогенезиса, когда температура была настолько высокой, что беспорядочное движение частиц в пространстве происходило с околосветовой скоростью.

В это время образуются и сразу же сталкиваясь разрушаются пары из частиц — античастиц, что, как считается, привело к доминированию материи над антиматерией в современной Вселенной. После прекращения инфляции Вселенная состояла из кварк-глюоновой плазмы и других элементарных частиц. С этого момента Вселенная стала остывать, начала образовываться и соединяться материя.

Охлаждение Вселенной

Со снижением плотности и температуры внутри Вселенной начало происходить и снижение энергии в каждой частице. Это переходное состояние длилось до тех пор, пока фундаментальные силы и элементарные частицы не пришли к своей нынешней форме. Так как энергия частиц опустилась до значений, которые можно сегодня достичь в рамках экспериментов, действительное возможное наличие этого временного периода вызывает у ученых куда меньше споров.

Как думаете, как как космос изменит человечество в будущем?

Например, ученые считают, что на 10^-11 секунде после Большого взрыва энергия частиц значительно уменьшилась. Примерно на 10^-6 секунде кварки и глюоны начали образовывать барионы — протоны и нейтроны. Кварки стали преобладать над антикварками, что в свою очередь привело к преобладанию барионов над антибарионами.

Так как температура была уже недостаточно высокой для создания новых протонно-антипротонных пар (или нейтронно-антинейтронных пар), последовало массовое разрушение этих частиц, что привело к остатку только 1/1010 количества изначальных протонов и нейтронов и полному исчезновению их античастиц. Аналогичный процесс произошел спустя около 1 секунды после Большого взрыва. Только «жертвами» на этот раз стали электроны и позитроны. После массового уничтожения оставшиеся протоны, нейтроны и электроны прекратили свое беспорядочное движение, а энергетическая плотность Вселенной была заполнена фотонами и в меньшей степени нейтрино.

В течение первых минут расширения Вселенной начался период нуклеосинтеза (синтез химических элементов). Благодаря падению температуры до 1 миллиарда кельвинов и снижения плотности энергии примерно до значений, эквивалентных плотности воздуха, нейтроны и протоны начали смешиваться и образовывать первый стабильный изотоп водорода (дейтерий), а также атомы гелия. Тем не менее большинство протонов во Вселенной остались в качестве несвязных ядер атомов водорода.

Спустя около 379 000 лет электроны объединились с этими ядрами водорода и образовали атомы (опять же преимущественно водорода), в то время как радиация отделилась от материи и продолжила практически беспрепятственно расширяться через пространство. Эту радиацию принято называть реликтовым излучением, и она является самым древнейшим источником света во Вселенной.

С расширением реликтовое излучение постепенно теряло свою плотность и энергию и в настоящий момент его температура составляет 2,7260 ± 0,0013 К (-270,424 °C), а энергетическая плотность 0,25 эВ (или 4,005×10^-14 Дж/м³; 400–500 фотонов/см³). Реликтовое излучение простирается во всех направлениях и на расстояние около 13,8 миллиарда световых лет, однако оценка его фактического распространения говорит примерно о 46 миллиардах световых годах от центра Вселенной.

Структурирование Вселенной

В последующие несколько миллиардов лет более плотные регионы почти равномерно распределенной во Вселенной материи начали притягиваться друг к другу. В результате этого они стали еще плотнее, начали образовывать облака газа, звезды, галактики и другие астрономические структуры, за которыми мы можем наблюдать в настоящее время. Этот период носит название иерархической эпохи. В это время та Вселенная, которую мы видим сейчас, начала приобретать свою форму. Материя начала объединяться в структуры различных размеров — звезды, планеты, галактики, галактические скопления, а также галактические сверхскопления, разделенные межгалактическими перемычками, содержащими всего лишь несколько галактик.

Детали этого процесса могут быть описаны согласно представлению о количестве и типе материи, распределенной во Вселенной, которая представлена в виде холодной, теплой, горячей темной материи и барионного вещества. Однако современной стандартной космологической моделью Большого взрыва является модель Лямбда-CDM, согласно которой частицы темной материи двигаются медленнее скорости света. Выбрана она была потому, что решает все противоречия, которые появлялись в других космологических моделях.

Согласно этой модели на холодную темную материю приходится около 23 процентов всей материи/энергии во Вселенной. Доля барионного вещества составляет около 4,6 процента. Лямбда-CDM ссылается на так называемую космологическую постоянную: теорию, предложенную Альбертом Эйнштейном, которая характеризует свойства вакуума и показывает соотношение баланса между массой и энергией как постоянную статичную величину. В этом случае она связана с темной энергией, которая служит в качестве акселератора расширения Вселенной и поддерживает гигантские космологические структуры в значительной степени однородными.

Что будет со Вселенной

Гипотезы относительно того, что эволюция Вселенной обладает отправной точкой, естественным способом подводят ученых к вопросам о возможной конечной точке этого процесса. Если Вселенная начала свою историю из маленькой точки с бесконечной плотностью, которая вдруг начала расширяться, не означает ли это, что расширяться она тоже будет бесконечно? Или же однажды у нее закончится экспансивная сила и начнется обратный процесс сжатия, конечным итогом которого станет все та же бесконечно плотная точка?

Разгадка у нас в руках? Исследователи нашли возможную причину Большого Взрыва

Ответы на эти вопросы были основной целью космологов с самого начала споров о том, какая же космологическая модель Вселенной является верной. С принятием теории Большого взрыва, но по большей части благодаря наблюдению за темной энергией в 1990-х годах, ученые пришли к согласию в отношении двух наиболее вероятных сценариев эволюции Вселенной.

Согласно первому, получившему название «большое сжатие», Вселенная достигнет своего максимального размера и начнет разрушаться. Такой вариант развития событий будет возможен, если только плотность массы Вселенной станет больше, чем сама критическая плотность. Другими словами, если плотность материи достигнет определенного значения или станет выше этого значения (1-3×10^-26 кг материи на м³), Вселенная начнет сжиматься.

Большой взрыв — в таком виде

Альтернативой служит другой сценарий, который гласит, что если плотность во Вселенной будет равна или ниже значения критической плотности, то ее расширение замедлится, однако никогда не остановится полностью. Согласно этой гипотезе, получившей название «тепловая смерть Вселенной», расширение продолжится до тех пор, пока звездообразования не перестанут потреблять межзвездный газ внутри каждой из окружающих галактик. То есть полностью прекратится передача энергии и материи от одного объекта к другому. Все существующие звезды в этом случае выгорят и превратятся в белых карликов, нейтронные звезды и черные дыры.

Постепенно черные дыры будут сталкиваться с другими черными дырами, что привет к образованию все более и более крупных. Средняя температура Вселенной приблизится к абсолютному нулю. Черные дыры в итоге «испарятся», выпустив свое последнее излучение Хокинга. В конце концов термодинамическая энтропия во Вселенной станет максимальной. Наступит тепловая смерть.

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Современные наблюдения, которые учитывают наличие темной энергии и ее влияние на расширение космоса, натолкнули ученых на вывод, согласно которому со временем все больше и больше пространства Вселенной будет проходить за пределами нашего горизонта событий и станет невидимым для нас. Конечный и логичный результат этого ученым пока не известен, однако «тепловая смерть» вполне может оказаться конечной точкой подобных событий.

Есть и другие гипотезы относительно распределения темной энергии, а точнее, ее возможных видов (например фантомной энергии). Согласно им галактические скопления, звезды, планеты, атомы, ядра атомов и материя сама по себе будут разорваны на части в результате ее бесконечного расширения. Такой сценарий эволюции носит название «большого разрыва». Причиной гибели Вселенной согласно этому сценарию является само расширение.

История теории Большого взрыва

Самое раннее упоминание Большого взрыва относится к началу 20-го века и связано с наблюдениями за космосом. В 1912 году американский астроном Весто Слайфер провел серию наблюдений за спиральными галактиками (которые изначально представлялись туманностями) и измерил их доплеровское красное смещение. Почти во всех случаях наблюдения показали, что спиральные галактики отдаляются от нашего Млечного Пути.

В 1922 году выдающийся российский математик и космолог Александр Фридман вывел из уравнений Эйнштейна для общей теории относительности так называемые уравнения Фридмана. Несмотря продвижения Эйнштейном теории в пользу наличия космологической постоянной, работа Фридмана показала, что Вселенная скорее находится в состоянии расширения.

Большой Взрыв, темная материя… Могут ли космологи нас обманывать?

В 1924 году измерения Эдвина Хаббла дистанции до ближайшей спиральной туманности показали, что эти системы на самом деле являются действительно другими галактиками. В то же время Хаббл приступил к разработке ряда показателей для вычета расстояния, используя 2,5-метровый телескоп Хукера в обсерватории Маунт Вилсон. К 1929 году Хаббл обнаружил взаимосвязь между расстоянием и скоростью удаления галактик, что впоследствии стало законом Хаббла.

В 1927 году бельгийский математик, физик и католический священник Жорж Леметр независимо пришел к тем же результатам, какие показывали уравнения Фридмана, и первым сформулировал зависимость между расстоянием и скоростью галактик, предложив первую оценку коэффициента этой зависимости. Леметр считал, что в какой-то период времени в прошлом вся масса Вселенной была сосредоточена в одной точке (атоме).

Эти открытия и предположения вызывали много споров между физиками в 20-х и 30-х годах, большинство из которых считало, что Вселенная находится в стационарном состоянии. Согласно устоявшейся в то время модели, новая материя создается наряду с бесконечным расширением Вселенной, равномерно и равнозначно по плотности распределяясь на всей ее протяженности. Среди ученых, поддерживающих ее, идея Большого взрыва казалась больше теологической, нежели научной. В адрес Леметра звучала критика о предвзятости на основе религиозных предубеждений.

Следует отметить, что в то же время существовали и другие теории. Например, модель Вселенной Милна и циклическая модель. Обе основывались на постулатах общей теории относительности Эйнштейна и впоследствии получили поддержку самого ученого. Согласно этим моделям Вселенная существует в бесконечном потоке повторяющихся циклов расширений и коллапсов.

Только представьте, в космосе зафиксирован самый мощный взрыв со времен Большого взрыва

После Второй мировой войны между сторонниками стационарной модели Вселенной (которая фактически была описана астрономом и физиком Фредом Хойлом) и сторонниками теории Большого взрыва, быстро набиравшей популярность среди научного сообщества, разгорелись жаркие дебаты. По иронии судьбы, именно Хойл вывел фразу «большой взрыв», впоследствии ставшую названием новой теории. Произошло это в марте 1949 года на британском радио BBC.

В конце концов дальнейшие научные исследования и наблюдения все больше и больше говорили в пользу теории Большого взрыва и все чаще ставили под сомнение модель стационарной Вселенной. Обнаружение и подтверждение реликтового излучения в 1965 году окончательно укрепили Большой взрыв в качестве лучшей теории происхождения и эволюции Вселенной. С конца 60-х годов и вплоть до 1990-х астрономы и космологи провели еще больше исследований вопроса Большого взрыва и нашли решения для многих теоретических проблем, стоящих на пути у данной теории.

Все самые свежие новости из мира высоких технологий вы также можете найти в Google News.

Среди этих решений, например, работа Стивена Хокинга и других физиков, которые доказали, что сингулярность являлась неоспоримым начальным состоянием общей относительности и космологической модели Большого взрыва. В 1981 году физик Алан Гут вывел теорию, описывающую период быстрого космического расширения (эпохи инфляции), которая решила множество ранее нерешенных теоретических вопросов и проблем.

В 1990-х наблюдался повышенный интерес к темной энергии, которую рассматривали как ключ к решению многих нерешенных вопросов космологии. Помимо желания найти ответ на вопрос о том, почему Вселенная теряет свою массу наряду с темной матерей (гипотеза была предложена еще в 1932 году Яном Оортом), также было необходимо найти объяснение тому, почему Вселенная по-прежнему ускоряется.

Дальнейший прогресс изучения обязан созданию более продвинутых телескопов, спутников и компьютерных моделей, которые позволили астрономам и космологам заглянуть дальше во Вселенной и лучше понять ее истинный возраст. Развитие космических телескопов и появление таких, как, например, Cosmic Background Explorer (или COBE), космический телескоп Хаббла, Wilkinson Microwave Anisotropy Probe (WMAP) и космическая обсерватория Планка, тоже внесло бесценный вклад в исследование вопроса.

Сегодня космологи могут с довольно высокой точностью проводить измерения различных параметров и характеристик модели теории Большого взрыва, не говоря уже о более точных вычислениях возраста окружающего нас космоса. А ведь все началось с обычного наблюдения за массивными космическими объектами, расположенными во многих световых годах от нас и медленно продолжающих от нас отдаляться. И несмотря на то, что мы понятия не имеем, чем это все закончится, чтобы выяснить это, по космологическим меркам на это потребуется не так уж и много времени.

The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature.^[1] Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form.^[2][3][4] These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure. The overall uniformity of the Universe, known as the flatness problem, is explained through cosmic inflation: a sudden and very rapid expansion of space during the earliest moments. However, physics currently lacks a widely accepted theory of quantum gravity that can successfully model the earliest conditions of the Big Bang.

Crucially, these models are compatible with the Hubble–Lemaître law—the observation that the farther away a galaxy is, the faster it is moving away from Earth. Extrapolating this cosmic expansion backwards in time using the known laws of physics, the models describe an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named «the Big Bang singularity»).^[5] In 1964 the CMB was discovered, which convinced many cosmologists that the competing steady-state model of cosmic evolution was falsified,^[6] since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in the distant past. A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted.^[7] Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787±0.020 billion years ago, which is considered the age of the universe.^[8]

There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. The unequal abundances of matter and antimatter that allowed this to occur is an unexplained effect known as baryon asymmetry. These primordial elements—mostly hydrogen, with some helium and lithium—later coalesced through gravity, forming early stars and galaxies. Astronomers observe the gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by baryonic matter, such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to an unexplained phenomenon known as dark energy.^[9]

Features of the models

The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements, the CMB, large-scale structure, and Hubble’s law.^[10] The models depend on two major assumptions: the universality of physical laws and the cosmological principle. The universality of physical laws is one of the underlying principles of the theory of relativity. The cosmological principle states that on large scales the universe is homogeneous and isotropic—appearing the same in all directions regardless of location.^[11]

These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10⁻⁵.^[12] Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars.^{[13][14][notes 1]}

The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10⁻⁵ via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995.^[15]

Expansion of space

The expansion of the Universe was inferred from early twentieth century astronomical observations and is an essential ingredient of the Big Bang models. Mathematically, general relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be relative to galaxies, stars, or other objects, are specified using a coordinate chart or «grid» that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This metric contains a scale factor, which describes how the size of the universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system, the grid expands along with the universe, and objects that are moving only because of the expansion of the universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such co-moving points expands proportionally with the scale factor of the universe.^[16]

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distances between comoving points. In other words, the Big Bang is not an explosion in space, but rather an expansion of space.^[1] Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local concentrations of matter such as our galaxy do not necessarily expand with the same speed as the whole Universe.^[17]

Horizons

An important feature of the Big Bang spacetime is the presence of particle horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never «catch up» to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe.^[18]

Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.^[18]

Thermalization

Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium. Others were fast enough to reach thermalization. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter. The larger the ratio, the more time particles had to thermalize before they were too far away from each other.^[19]

Timeline

According to the Big Bang models, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling down.

Singularity

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.^[20] This irregular behavior, known as the gravitational singularity, indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward the singularity.^[5]

This primordial singularity is itself sometimes called «the Big Bang»,^[21] but the term can also refer to a more generic early hot, dense phase^{[22][notes 2]} of the universe. In either case, «the Big Bang» as an event is also colloquially referred to as the «birth» of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics) work. Based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the «age of the universe»—is 13.8 billion years.^[23]

Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible density gradient.^[24]

Inflation and baryogenesis

The earliest phases of the Big Bang are subject to much speculation, since astronomical data about them are not available. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures, and was very rapidly expanding and cooling. The period from 0 to 10⁻⁴³ seconds into the expansion, the Planck epoch, was a phase in which the four fundamental forces — the electromagnetic force, the strong nuclear force, the weak nuclear force, and the gravitational force, were unified as one.^[25] In this stage, the characteristic scale length of the universe was the Planck length, 1.6×10⁻³⁵ m, and consequently had a temperature of approximately 10³² degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of quantum gravity.^[26][27] The Planck epoch was succeeded by the grand unification epoch beginning at 10⁻⁴³ seconds, where gravitation separated from the other forces as the universe’s temperature fell.^[25]

At approximately 10⁻³⁷ seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially, unconstrained by the light speed invariance, and temperatures dropped by a factor of 100,000. This concept is motivated by the flatness problem, where the density of matter and energy is very close to the critical density needed to produce a flat universe. That is, the shape of the universe has no overall geometric curvature due to gravitational influence. Microscopic quantum fluctuations that occurred because of Heisenberg’s uncertainty principle were «frozen in» by inflation, becoming amplified into the seeds that would later form the large-scale structure of the universe.^[28] At a time around 10⁻³⁶ seconds, the electroweak epoch begins when the strong nuclear force separates from the other forces, with only the electromagnetic force and weak nuclear force remaining unified.^[29]

Inflation stopped at around the 10⁻³³ to 10⁻³² seconds mark, with the universe’s volume having increased by a factor of at least 10⁷⁸. Reheating occurred until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles.^[30][31] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions.^[1] At some point, an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.^[32]

Cooling

The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10⁻¹² seconds.^[29][33] After about 10⁻¹¹ seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators. At about 10⁻⁶ seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass annihilation immediately followed, leaving just one in 10⁸ of the original matter particles and none of their antiparticles.^[34] A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth’s atmosphere, neutrons combined with protons to form the universe’s deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN).^[35] Most protons remained uncombined as hydrogen nuclei.^[36]

As the universe cooled, the rest energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years, the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.^[36]

Structure formation

Over a long period of time, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today.^[1] The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter (CDM), warm dark matter, hot dark matter, and baryonic matter. The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold. (Warm dark matter is ruled out by early reionization.)^[38] This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%.^[39]

In an «extended model» which includes hot dark matter in the form of neutrinos,^[40] then the «physical baryon density» is estimated at 0.023. (This is different from the ‘baryon density’ expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density is about 0.11, and the corresponding neutrino density is estimated to be less than 0.0062.^[39]

Cosmic acceleration

Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which appears to homogeneously permeate all of space. Observations suggest that 73% of the total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together gravity predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate.^[9]

Dark energy in its simplest formulation is modeled by a cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory.^[9]

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the lambda-CDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10⁻¹⁵ seconds.^[41] Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics.

Concept history

Etymology

English astronomer Fred Hoyle is credited with coining the term «Big Bang» during a talk for a March 1949 BBC Radio broadcast,^[42] saying: «These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past.»^[43][44] However, it did not catch on until the 1970s.^[44]

It is popularly reported that Hoyle, who favored an alternative «steady-state» cosmological model, intended this to be pejorative,^[45][46][47] but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.^[48][49][51] Helge Kragh writes that the evidence for the claim that it was meant as a pejorative is «unconvincing», and mentions a number of indications that it was not a pejorative.^[44]

The term itself is a misnomer as it implies the occurrence of an explosion.^[44][52] However, an explosion implies expansion from a center point out into the surrounding space. Rather than expanding into space, the Big Bang was the expansion/stretching of space itself, everywhere simultaneously (not from a single point), causing the universe to cool down and the density to be lowered.^[53][54] Another issue pointed out by Santhosh Mathew is that bang implies sound, which would require a vibrating particle and medium through which it travels. Since this is the beginning of anything we can imagine, there is no basis for any sound, and thus the Big Bang was likely silent.^[46] An attempt to find a more suitable alternative was not successful.^[44][47]

Development

The Big Bang models developed from observations of the structure of the universe and from theoretical considerations. In 1912, Vesto Slipher measured the first Doppler shift of a «spiral nebula» (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were «island universes» outside our Milky Way.^[57][58] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from the Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time.^[59]

In 1924, American astronomer Edwin Hubble’s measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity—now known as Hubble’s law.^[60][61]

Independently deriving Friedmann’s equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the recession of the nebulae was due to the expansion of the universe.^[62] He inferred the relation that Hubble would later observe, given the cosmological principle.^[9] In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a «primeval atom» where and when the fabric of time and space came into existence.^[63]

In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady-state theory.^[64] This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest.^[65] Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was «repugnant» to him.^[66][67] Lemaître, however, disagreed:

If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.^[68]

During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble’s observations, including the Milne model,^[69] the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman)^[70] and Fritz Zwicky’s tired light hypothesis.^[71]

After World War II, two distinct possibilities emerged. One was Fred Hoyle’s steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time.^[72] The other was Lemaître’s Big Bang theory, advocated and developed by George Gamow, who introduced BBN^[73] and whose associates, Ralph Alpher and Robert Herman, predicted the CMB.^[74] Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître’s theory, referring to it as «this big bang idea» during a BBC Radio broadcast in March 1949.^{[49][44][notes 3]} For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe.^[75]

In 1968 and 1970, Roger Penrose, Stephen Hawking, and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of the Big Bang.^[76][77] Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called «inflation».^[78] Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant^[79] and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe).^[80]

In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters.^[81]

Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE),^[82] the Hubble Space Telescope and WMAP.^[83] Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.^[84][85]

On 30 January 2023, news reported the discovery of a 1964 20-minute interview, considered lost, of cosmologist Georges Lemaître, who pioneered the notion of the Big Bang theory.^[86]

Observational evidence

The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble’s law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis (BBN). More recent evidence includes observations of galaxy formation and evolution, and the distribution of large-scale cosmic structures,^[88] These are sometimes called the «four pillars» of the Big Bang models.^[89]

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations.^[90] Remaining issues include the cuspy halo problem^[91] and the dwarf galaxy problem^[92] of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.^[93] Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.

Hubble’s law and the expansion of space

Observations of distant galaxies and quasars show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble’s law is observed:^[60]

where

Hubble’s law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable under the assumption of the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Friedmann in 1922^[59] and Lemaître in 1927,^[62] well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker.

The theory requires the relation to hold at all times, where is the proper distance, v is the recessional velocity, and , , and vary as the universe expands (hence we write to denote the present-day Hubble «constant»). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity . However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected.^[94]

That space is undergoing metric expansion is shown by direct observational evidence of the cosmological principle and the Copernican principle, which together with Hubble’s law have no other explanation. Astronomical redshifts are extremely isotropic and homogeneous,^[60] supporting the cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position.^[95] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

Cosmic microwave background radiation

In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band.^[75] Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 Nobel Prize in Physics.

The surface of last scattering corresponding to emission of the CMB occurs shortly after recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly scattered from free charged particles. Peaking at around 372±14 kyr,^[38] the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.

In 1989, NASA launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 10⁴, and measured a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.7255 K); then in 1992, further COBE measurements discovered tiny fluctuations (anisotropies) in the CMB temperature across the sky, at a level of about one part in 10⁵.^[82] John C. Mather and George Smoot were awarded the 2006 Nobel Prize in Physics for their leadership in these results.

During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the shape of the universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies.^{[100][101][102]}

In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general.^[83] The Planck space probe was launched in May 2009. Other ground and balloon-based cosmic microwave background experiments are ongoing.

Abundance of primordial elements

Using the Big Bang model, it is possible to calculate the concentration of the isotopes helium-4 (⁴He), helium-3 (³He), deuterium (²H), and lithium-7 (⁷Li) in the universe as ratios to the amount of ordinary hydrogen.^[35] The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by abundance) are about 0.25 for ⁴He:H, about 10⁻³ for ²H:H, about 10⁻⁴ for ³He:H, and about 10⁻⁹ for ⁷Li:H.^[35]

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for ⁴He, and off by a factor of two for ⁷Li (this anomaly is known as the cosmological lithium problem); in the latter two cases, there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to «tune» the Big Bang to produce much more or less than 20–30% helium.^[103] Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products, should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too.^{[104]: 182–185}

Galactic evolution and distribution

Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang models. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then, larger structures have been forming, such as galaxy clusters and superclusters.^[105]

Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently, appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures, agree well with Big Bang simulations of the formation of structure in the universe, and are helping to complete details of the theory.^[105][106]

Primordial gas clouds

In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. Despite being sensitive to carbon, oxygen, and silicon, these three elements were not detected in these two clouds.^[111][112] Since the clouds of gas have no detectable levels of heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.

Other lines of evidence

The age of the universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.^[113] It is also in good agreement with age estimates based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background.^[23] The agreement of independent measurements of this age supports the Lambda-CDM (ΛCDM) model, since the model is used to relate some of the measurements to an age estimate, and all estimates turn out to agree. Still, some observations of objects from the relatively early universe (in particular quasar APM 08279+5255) raise concern as to whether these objects had enough time to form so early in the ΛCDM model.^[114][115]

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift.^[116] This prediction also implies that the amplitude of the Sunyaev–Zel’dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.^[117][118]

Future observations

Future gravitational-wave observatories might be able to detect primordial gravitational waves, relics of the early universe, up to less than a second after the Big Bang.^[119][120]

Problems and related issues in physics

As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang models. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the horizon problem, the magnetic monopole problem, and the flatness problem are most commonly resolved with inflation theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven.^{[121][122][123][124]} What follows are a list of the mysterious aspects of the Big Bang concept still under intense investigation by cosmologists and astrophysicists.

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter.^[32] It is generally assumed that when the universe was young and very hot it was in statistical equilibrium and contained equal numbers of baryons and antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium.^[125] All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.

Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed «dark energy».^[9]

Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.^[9] Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the «missing» energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses,^[126] and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.^[39] According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein’s cosmological constant but also extending to more exotic forms of quintessence or other modified gravity schemes.^[127] A cosmological constant problem, sometimes called the «most embarrassing problem in physics», results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from Planck units.^[128]

Dark matter

During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.^[129]

Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.^[130]

Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem^[92] and the cuspy halo problem.^[91] Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.^[131]

Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact.^[132] The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.^{[104]: 191–202}

A resolution to this apparent inconsistency is offered by inflation theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other’s particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.^{[28]: 180–186}

Heisenberg’s uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe.^[104]: 207 Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.^{[83]: sec 6}

If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.^{[28]: 180–186}

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before electroweak symmetry breaking occurs, so inflation should not be able to prevent large-scale discontinuities in the electroweak vacuum since distant parts of the observable universe were causally separate when the electroweak epoch ended.^[133]

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unified theories (GUTs) predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.^[132]

Flatness problem

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW.^[132] The universe may have positive, negative, or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be flat. Observations indicate the universe is consistent with being flat.^[134][135]

The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.^{[notes 4]} Given that a natural timescale for departure from flatness might be the Planck time, 10⁻⁴³ seconds,^[1] the fact that the universe has reached neither a heat death nor a Big Crunch after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10¹⁴ of its critical value, or it would not exist as it does today.^[136]

Misconceptions

One of the common misconceptions about the Big Bang model is that it fully explains the origin of the universe. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state.^[137] It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe.^[17]

Hubble’s law predicts that galaxies that are beyond Hubble distance recede faster than the speed of light. However, special relativity does not apply beyond motion through space. Hubble’s law describes velocity that results from expansion of space, rather than through space.^[17]

Astronomers often refer to the cosmological redshift as a Doppler shift which can lead to a misconception.^[17] Although similar, the cosmological redshift is not identical to the classically derived Doppler redshift because most elementary derivations of the Doppler redshift do not accommodate the expansion of space. Accurate derivation of the cosmological redshift requires the use of general relativity, and while a treatment using simpler Doppler effect arguments gives nearly identical results for nearby galaxies, interpreting the redshift of more distant galaxies as due to the simplest Doppler redshift treatments can cause confusion.^[17]

Implications

Given current understanding, scientific extrapolations about the future of the universe are only possible for finite durations, albeit for much longer periods than the current age of the universe. Anything beyond that becomes increasingly speculative. Likewise, at present, a proper understanding of the origin of the universe can only be subject to conjecture.^[138]

Pre–Big Bang cosmology

The Big Bang explains the evolution of the universe from a starting density and temperature that is well beyond humanity’s capability to replicate, so extrapolations to the most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the «primeval atom» while Gamow called the material «ylem«. How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, specific laws of nature most likely came to existence in a random way, but as inflation models show, some combinations of these are far more probable.^[139] A flat universe implies a balance between gravitational potential energy and other energy forms, requiring no additional energy to be created.^[134][135]

The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and correcting this will require the development of a correct treatment of quantum gravity.^[20] Certain quantum gravity treatments, such as the Wheeler–DeWitt equation, imply that time itself could be an emergent property.^[140] As such, physics may conclude that time did not exist before the Big Bang.^[141][142]

While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds on the subject of «cosmogony».

Some speculative proposals in this regard, each of which entails untested hypotheses, are:

• The simplest models, in which the Big Bang was caused by quantum fluctuations. That scenario had very little chance of happening, but, according to the totalitarian principle, even the most improbable event will eventually happen. It took place instantly, in our perspective, due to the absence of perceived time before the Big Bang.^{[143][144][145][146]}
• Models in which the whole of spacetime is finite, including the Hartle–Hawking no-boundary condition. For these cases, the Big Bang does represent the limit of time but without a singularity.^[147] In such a case, the universe is self-sufficient.^[148]
• Brane cosmology models, in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the universe cycles from one process to the other.^{[149][150][151][152]}
• Eternal inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe, expanding from its own big bang.^[153][154]

Proposals in the last two categories see the Big Bang as an event in either a much larger and older universe or in a multiverse.

Ultimate fate of the universe

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch.^[18]

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving white dwarfs, neutron stars, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach absolute zero—a Big Freeze.^[155] Moreover, if protons are unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.^[156]

Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.^[157]

Religious and philosophical interpretations

As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy.^[158][159] As a result, it has become one of the liveliest areas in the discourse between science and religion.^[160] Some believe the Big Bang implies a creator,^[161][162] while others argue that Big Bang cosmology makes the notion of a creator superfluous.^[159][163]

See also

• Anthropic principle – philosophical principle about the occurrence of sapient life in the Universe
• Big Bounce – hypothetical cosmological model for the origin of the known universe
• Big Crunch – theoretical scenario for the ultimate fate of the universe
• Cold Big Bang – designation of an absolute zero temperature at the beginning of the Universe
• Cosmic Calendar – method to visualize the chronology of the universe
• Cosmogony – branch of science or a theory concerning the origin of the universe
• Eureka: A Prose Poem – lengthy non-fiction work by American author Edgar Allan Poe, a Big Bang speculation
• Future of an expanding universe – future scenario if the expansion of the universe will continue forever or not
• Heat death of the universe – possible fate of the universe. Also known as the Big Chill and the Big Freeze
• Shape of the universe – the local and global geometry of the universe
• Steady-state model – model of the evolution of the universe, a discredited theory that denied the Big Bang and posited that the universe always existed.

Notes

References

Bibliography

Further reading

• Alpher, Ralph A.; Herman, Robert (August 1988). «Reflections on Early Work on ‘Big Bang’ Cosmology». Physics Today. 41 (8): 24–34. Bibcode:1988PhT….41h..24A. doi:10.1063/1.881126.
• Barrow, John D. (1994). The Origin of the Universe. Science Masters. London: Weidenfeld & Nicolson. ISBN 978-0-297-81497-9. LCCN 94006343. OCLC 490957073.
• Davies, Paul (1992). The Mind of God: The Scientific Basis for a Rational World. New York: Simon & Schuster. ISBN 978-0-671-71069-9. LCCN 91028606. OCLC 59940452.
• Lineweaver, Charles H.; Davis, Tamara M. (March 2005). «Misconceptions about the Big Bang» (PDF). Scientific American. Vol. 292, no. 3. pp. 36–45. Archived (PDF) from the original on 9 October 2019. Retrieved 23 December 2019.
• Mather, John C.; Boslough, John (1996). The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe (1st ed.). New York: Basic Books. ISBN 978-0-465-01575-7. LCCN 96010781. OCLC 34357391.
• Riordan, Michael; Zajc, William A. (May 2006). «The First Few Microseconds» (PDF). Scientific American. Vol. 294, no. 5. pp. 34–41. Bibcode:2006SciAm.294e..34R. doi:10.1038/scientificamerican0506-34a. Archived (PDF) from the original on 30 November 2014.
• Singh, Simon (2005) [First U.S. edition published 2004]. Big Bang: The Origin of the Universe (Harper Perennial; illustrated ed.). New York, NY: Harper Perennial. ISBN 978-0007162215.
• Weinberg, Steven (1993) [Originally published 1977]. The First Three Minutes: A Modern View of the Origin of the Universe (Updated ed.). New York: Basic Books. ISBN 978-0-465-02437-7. LCCN 93232406. OCLC 488469247. 1st edition is available from the Internet Archive. Retrieved 23 December 2019.

External links

• Once Upon a Universe Archived 22 June 2020 at the Wayback Machine – STFC funded project explaining the history of the universe in easy-to-understand language
• «Big Bang Cosmology» – NASA/WMAP Science Team
• «The Big Bang» – NASA Science
• «Big Bang, Big Bewilderment» – Big bang model with animated graphics by Johannes Koelman
• Cosmology at Curlie

The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature.^[1] Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form.^[2][3][4] These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure. The overall uniformity of the Universe, known as the flatness problem, is explained through cosmic inflation: a sudden and very rapid expansion of space during the earliest moments. However, physics currently lacks a widely accepted theory of quantum gravity that can successfully model the earliest conditions of the Big Bang.

Crucially, these models are compatible with the Hubble–Lemaître law—the observation that the farther away a galaxy is, the faster it is moving away from Earth. Extrapolating this cosmic expansion backwards in time using the known laws of physics, the models describe an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named «the Big Bang singularity»).^[5] In 1964 the CMB was discovered, which convinced many cosmologists that the competing steady-state model of cosmic evolution was falsified,^[6] since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in the distant past. A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted.^[7] Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787±0.020 billion years ago, which is considered the age of the universe.^[8]

There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. The unequal abundances of matter and antimatter that allowed this to occur is an unexplained effect known as baryon asymmetry. These primordial elements—mostly hydrogen, with some helium and lithium—later coalesced through gravity, forming early stars and galaxies. Astronomers observe the gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by baryonic matter, such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to an unexplained phenomenon known as dark energy.^[9]

Features of the models

The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements, the CMB, large-scale structure, and Hubble’s law.^[10] The models depend on two major assumptions: the universality of physical laws and the cosmological principle. The universality of physical laws is one of the underlying principles of the theory of relativity. The cosmological principle states that on large scales the universe is homogeneous and isotropic—appearing the same in all directions regardless of location.^[11]

These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10⁻⁵.^[12] Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars.^{[13][14][notes 1]}

The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10⁻⁵ via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995.^[15]

Expansion of space

The expansion of the Universe was inferred from early twentieth century astronomical observations and is an essential ingredient of the Big Bang models. Mathematically, general relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be relative to galaxies, stars, or other objects, are specified using a coordinate chart or «grid» that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This metric contains a scale factor, which describes how the size of the universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system, the grid expands along with the universe, and objects that are moving only because of the expansion of the universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such co-moving points expands proportionally with the scale factor of the universe.^[16]

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distances between comoving points. In other words, the Big Bang is not an explosion in space, but rather an expansion of space.^[1] Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local concentrations of matter such as our galaxy do not necessarily expand with the same speed as the whole Universe.^[17]

Horizons

An important feature of the Big Bang spacetime is the presence of particle horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never «catch up» to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe.^[18]

Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.^[18]

Thermalization

Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium. Others were fast enough to reach thermalization. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter. The larger the ratio, the more time particles had to thermalize before they were too far away from each other.^[19]

Timeline

According to the Big Bang models, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling down.

Singularity

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.^[20] This irregular behavior, known as the gravitational singularity, indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward the singularity.^[5]

This primordial singularity is itself sometimes called «the Big Bang»,^[21] but the term can also refer to a more generic early hot, dense phase^{[22][notes 2]} of the universe. In either case, «the Big Bang» as an event is also colloquially referred to as the «birth» of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics) work. Based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the «age of the universe»—is 13.8 billion years.^[23]

Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible density gradient.^[24]

Inflation and baryogenesis

The earliest phases of the Big Bang are subject to much speculation, since astronomical data about them are not available. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures, and was very rapidly expanding and cooling. The period from 0 to 10⁻⁴³ seconds into the expansion, the Planck epoch, was a phase in which the four fundamental forces — the electromagnetic force, the strong nuclear force, the weak nuclear force, and the gravitational force, were unified as one.^[25] In this stage, the characteristic scale length of the universe was the Planck length, 1.6×10⁻³⁵ m, and consequently had a temperature of approximately 10³² degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of quantum gravity.^[26][27] The Planck epoch was succeeded by the grand unification epoch beginning at 10⁻⁴³ seconds, where gravitation separated from the other forces as the universe’s temperature fell.^[25]

At approximately 10⁻³⁷ seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially, unconstrained by the light speed invariance, and temperatures dropped by a factor of 100,000. This concept is motivated by the flatness problem, where the density of matter and energy is very close to the critical density needed to produce a flat universe. That is, the shape of the universe has no overall geometric curvature due to gravitational influence. Microscopic quantum fluctuations that occurred because of Heisenberg’s uncertainty principle were «frozen in» by inflation, becoming amplified into the seeds that would later form the large-scale structure of the universe.^[28] At a time around 10⁻³⁶ seconds, the electroweak epoch begins when the strong nuclear force separates from the other forces, with only the electromagnetic force and weak nuclear force remaining unified.^[29]

Inflation stopped at around the 10⁻³³ to 10⁻³² seconds mark, with the universe’s volume having increased by a factor of at least 10⁷⁸. Reheating occurred until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles.^[30][31] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions.^[1] At some point, an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.^[32]

Cooling

The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10⁻¹² seconds.^[29][33] After about 10⁻¹¹ seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators. At about 10⁻⁶ seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass annihilation immediately followed, leaving just one in 10⁸ of the original matter particles and none of their antiparticles.^[34] A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth’s atmosphere, neutrons combined with protons to form the universe’s deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN).^[35] Most protons remained uncombined as hydrogen nuclei.^[36]

As the universe cooled, the rest energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years, the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.^[36]

Structure formation

Over a long period of time, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today.^[1] The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter (CDM), warm dark matter, hot dark matter, and baryonic matter. The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold. (Warm dark matter is ruled out by early reionization.)^[38] This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%.^[39]

In an «extended model» which includes hot dark matter in the form of neutrinos,^[40] then the «physical baryon density» is estimated at 0.023. (This is different from the ‘baryon density’ expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density is about 0.11, and the corresponding neutrino density is estimated to be less than 0.0062.^[39]

Cosmic acceleration

Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which appears to homogeneously permeate all of space. Observations suggest that 73% of the total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together gravity predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate.^[9]

Dark energy in its simplest formulation is modeled by a cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory.^[9]

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the lambda-CDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10⁻¹⁵ seconds.^[41] Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics.

Concept history

Etymology

English astronomer Fred Hoyle is credited with coining the term «Big Bang» during a talk for a March 1949 BBC Radio broadcast,^[42] saying: «These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past.»^[43][44] However, it did not catch on until the 1970s.^[44]

It is popularly reported that Hoyle, who favored an alternative «steady-state» cosmological model, intended this to be pejorative,^[45][46][47] but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.^[48][49][51] Helge Kragh writes that the evidence for the claim that it was meant as a pejorative is «unconvincing», and mentions a number of indications that it was not a pejorative.^[44]

The term itself is a misnomer as it implies the occurrence of an explosion.^[44][52] However, an explosion implies expansion from a center point out into the surrounding space. Rather than expanding into space, the Big Bang was the expansion/stretching of space itself, everywhere simultaneously (not from a single point), causing the universe to cool down and the density to be lowered.^[53][54] Another issue pointed out by Santhosh Mathew is that bang implies sound, which would require a vibrating particle and medium through which it travels. Since this is the beginning of anything we can imagine, there is no basis for any sound, and thus the Big Bang was likely silent.^[46] An attempt to find a more suitable alternative was not successful.^[44][47]

Development

The Big Bang models developed from observations of the structure of the universe and from theoretical considerations. In 1912, Vesto Slipher measured the first Doppler shift of a «spiral nebula» (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were «island universes» outside our Milky Way.^[57][58] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from the Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time.^[59]

In 1924, American astronomer Edwin Hubble’s measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity—now known as Hubble’s law.^[60][61]

Independently deriving Friedmann’s equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the recession of the nebulae was due to the expansion of the universe.^[62] He inferred the relation that Hubble would later observe, given the cosmological principle.^[9] In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a «primeval atom» where and when the fabric of time and space came into existence.^[63]

In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady-state theory.^[64] This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest.^[65] Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was «repugnant» to him.^[66][67] Lemaître, however, disagreed:

If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.^[68]

During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble’s observations, including the Milne model,^[69] the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman)^[70] and Fritz Zwicky’s tired light hypothesis.^[71]

After World War II, two distinct possibilities emerged. One was Fred Hoyle’s steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time.^[72] The other was Lemaître’s Big Bang theory, advocated and developed by George Gamow, who introduced BBN^[73] and whose associates, Ralph Alpher and Robert Herman, predicted the CMB.^[74] Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître’s theory, referring to it as «this big bang idea» during a BBC Radio broadcast in March 1949.^{[49][44][notes 3]} For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe.^[75]

In 1968 and 1970, Roger Penrose, Stephen Hawking, and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of the Big Bang.^[76][77] Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called «inflation».^[78] Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant^[79] and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe).^[80]

In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters.^[81]

Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE),^[82] the Hubble Space Telescope and WMAP.^[83] Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.^[84][85]

On 30 January 2023, news reported the discovery of a 1964 20-minute interview, considered lost, of cosmologist Georges Lemaître, who pioneered the notion of the Big Bang theory.^[86]

Observational evidence

The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble’s law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis (BBN). More recent evidence includes observations of galaxy formation and evolution, and the distribution of large-scale cosmic structures,^[88] These are sometimes called the «four pillars» of the Big Bang models.^[89]

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations.^[90] Remaining issues include the cuspy halo problem^[91] and the dwarf galaxy problem^[92] of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.^[93] Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.

Hubble’s law and the expansion of space

Observations of distant galaxies and quasars show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble’s law is observed:^[60]

where

Hubble’s law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable under the assumption of the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Friedmann in 1922^[59] and Lemaître in 1927,^[62] well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker.

The theory requires the relation to hold at all times, where is the proper distance, v is the recessional velocity, and , , and vary as the universe expands (hence we write to denote the present-day Hubble «constant»). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity . However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected.^[94]

That space is undergoing metric expansion is shown by direct observational evidence of the cosmological principle and the Copernican principle, which together with Hubble’s law have no other explanation. Astronomical redshifts are extremely isotropic and homogeneous,^[60] supporting the cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position.^[95] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

Cosmic microwave background radiation

In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band.^[75] Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 Nobel Prize in Physics.

The surface of last scattering corresponding to emission of the CMB occurs shortly after recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly scattered from free charged particles. Peaking at around 372±14 kyr,^[38] the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.

In 1989, NASA launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 10⁴, and measured a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.7255 K); then in 1992, further COBE measurements discovered tiny fluctuations (anisotropies) in the CMB temperature across the sky, at a level of about one part in 10⁵.^[82] John C. Mather and George Smoot were awarded the 2006 Nobel Prize in Physics for their leadership in these results.

During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the shape of the universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies.^{[100][101][102]}

In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general.^[83] The Planck space probe was launched in May 2009. Other ground and balloon-based cosmic microwave background experiments are ongoing.

Abundance of primordial elements

Using the Big Bang model, it is possible to calculate the concentration of the isotopes helium-4 (⁴He), helium-3 (³He), deuterium (²H), and lithium-7 (⁷Li) in the universe as ratios to the amount of ordinary hydrogen.^[35] The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by abundance) are about 0.25 for ⁴He:H, about 10⁻³ for ²H:H, about 10⁻⁴ for ³He:H, and about 10⁻⁹ for ⁷Li:H.^[35]

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for ⁴He, and off by a factor of two for ⁷Li (this anomaly is known as the cosmological lithium problem); in the latter two cases, there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to «tune» the Big Bang to produce much more or less than 20–30% helium.^[103] Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products, should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too.^{[104]: 182–185}

Galactic evolution and distribution

Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang models. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then, larger structures have been forming, such as galaxy clusters and superclusters.^[105]

Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently, appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures, agree well with Big Bang simulations of the formation of structure in the universe, and are helping to complete details of the theory.^[105][106]

Primordial gas clouds

In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. Despite being sensitive to carbon, oxygen, and silicon, these three elements were not detected in these two clouds.^[111][112] Since the clouds of gas have no detectable levels of heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.

Other lines of evidence

The age of the universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.^[113] It is also in good agreement with age estimates based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background.^[23] The agreement of independent measurements of this age supports the Lambda-CDM (ΛCDM) model, since the model is used to relate some of the measurements to an age estimate, and all estimates turn out to agree. Still, some observations of objects from the relatively early universe (in particular quasar APM 08279+5255) raise concern as to whether these objects had enough time to form so early in the ΛCDM model.^[114][115]

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift.^[116] This prediction also implies that the amplitude of the Sunyaev–Zel’dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.^[117][118]

Future observations

Future gravitational-wave observatories might be able to detect primordial gravitational waves, relics of the early universe, up to less than a second after the Big Bang.^[119][120]

Problems and related issues in physics

As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang models. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the horizon problem, the magnetic monopole problem, and the flatness problem are most commonly resolved with inflation theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven.^{[121][122][123][124]} What follows are a list of the mysterious aspects of the Big Bang concept still under intense investigation by cosmologists and astrophysicists.

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter.^[32] It is generally assumed that when the universe was young and very hot it was in statistical equilibrium and contained equal numbers of baryons and antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium.^[125] All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.

Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed «dark energy».^[9]

Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.^[9] Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the «missing» energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses,^[126] and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.^[39] According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein’s cosmological constant but also extending to more exotic forms of quintessence or other modified gravity schemes.^[127] A cosmological constant problem, sometimes called the «most embarrassing problem in physics», results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from Planck units.^[128]

Dark matter

During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.^[129]

Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.^[130]

Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem^[92] and the cuspy halo problem.^[91] Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.^[131]

Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact.^[132] The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.^{[104]: 191–202}

A resolution to this apparent inconsistency is offered by inflation theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other’s particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.^{[28]: 180–186}

Heisenberg’s uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe.^[104]: 207 Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.^{[83]: sec 6}

If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.^{[28]: 180–186}

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before electroweak symmetry breaking occurs, so inflation should not be able to prevent large-scale discontinuities in the electroweak vacuum since distant parts of the observable universe were causally separate when the electroweak epoch ended.^[133]

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unified theories (GUTs) predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.^[132]

Flatness problem

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW.^[132] The universe may have positive, negative, or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be flat. Observations indicate the universe is consistent with being flat.^[134][135]

The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.^{[notes 4]} Given that a natural timescale for departure from flatness might be the Planck time, 10⁻⁴³ seconds,^[1] the fact that the universe has reached neither a heat death nor a Big Crunch after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10¹⁴ of its critical value, or it would not exist as it does today.^[136]

Misconceptions

One of the common misconceptions about the Big Bang model is that it fully explains the origin of the universe. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state.^[137] It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe.^[17]

Hubble’s law predicts that galaxies that are beyond Hubble distance recede faster than the speed of light. However, special relativity does not apply beyond motion through space. Hubble’s law describes velocity that results from expansion of space, rather than through space.^[17]

Astronomers often refer to the cosmological redshift as a Doppler shift which can lead to a misconception.^[17] Although similar, the cosmological redshift is not identical to the classically derived Doppler redshift because most elementary derivations of the Doppler redshift do not accommodate the expansion of space. Accurate derivation of the cosmological redshift requires the use of general relativity, and while a treatment using simpler Doppler effect arguments gives nearly identical results for nearby galaxies, interpreting the redshift of more distant galaxies as due to the simplest Doppler redshift treatments can cause confusion.^[17]

Implications

Given current understanding, scientific extrapolations about the future of the universe are only possible for finite durations, albeit for much longer periods than the current age of the universe. Anything beyond that becomes increasingly speculative. Likewise, at present, a proper understanding of the origin of the universe can only be subject to conjecture.^[138]

Pre–Big Bang cosmology

The Big Bang explains the evolution of the universe from a starting density and temperature that is well beyond humanity’s capability to replicate, so extrapolations to the most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the «primeval atom» while Gamow called the material «ylem«. How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, specific laws of nature most likely came to existence in a random way, but as inflation models show, some combinations of these are far more probable.^[139] A flat universe implies a balance between gravitational potential energy and other energy forms, requiring no additional energy to be created.^[134][135]

The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and correcting this will require the development of a correct treatment of quantum gravity.^[20] Certain quantum gravity treatments, such as the Wheeler–DeWitt equation, imply that time itself could be an emergent property.^[140] As such, physics may conclude that time did not exist before the Big Bang.^[141][142]

While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds on the subject of «cosmogony».

Some speculative proposals in this regard, each of which entails untested hypotheses, are:

• The simplest models, in which the Big Bang was caused by quantum fluctuations. That scenario had very little chance of happening, but, according to the totalitarian principle, even the most improbable event will eventually happen. It took place instantly, in our perspective, due to the absence of perceived time before the Big Bang.^{[143][144][145][146]}
• Models in which the whole of spacetime is finite, including the Hartle–Hawking no-boundary condition. For these cases, the Big Bang does represent the limit of time but without a singularity.^[147] In such a case, the universe is self-sufficient.^[148]
• Brane cosmology models, in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the universe cycles from one process to the other.^{[149][150][151][152]}
• Eternal inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe, expanding from its own big bang.^[153][154]

Proposals in the last two categories see the Big Bang as an event in either a much larger and older universe or in a multiverse.

Ultimate fate of the universe

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch.^[18]

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving white dwarfs, neutron stars, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach absolute zero—a Big Freeze.^[155] Moreover, if protons are unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.^[156]

Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.^[157]

Religious and philosophical interpretations

As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy.^[158][159] As a result, it has become one of the liveliest areas in the discourse between science and religion.^[160] Some believe the Big Bang implies a creator,^[161][162] while others argue that Big Bang cosmology makes the notion of a creator superfluous.^[159][163]

See also

• Anthropic principle – philosophical principle about the occurrence of sapient life in the Universe
• Big Bounce – hypothetical cosmological model for the origin of the known universe
• Big Crunch – theoretical scenario for the ultimate fate of the universe
• Cold Big Bang – designation of an absolute zero temperature at the beginning of the Universe
• Cosmic Calendar – method to visualize the chronology of the universe
• Cosmogony – branch of science or a theory concerning the origin of the universe
• Eureka: A Prose Poem – lengthy non-fiction work by American author Edgar Allan Poe, a Big Bang speculation
• Future of an expanding universe – future scenario if the expansion of the universe will continue forever or not
• Heat death of the universe – possible fate of the universe. Also known as the Big Chill and the Big Freeze
• Shape of the universe – the local and global geometry of the universe
• Steady-state model – model of the evolution of the universe, a discredited theory that denied the Big Bang and posited that the universe always existed.

Notes

References

Bibliography

Further reading

• Alpher, Ralph A.; Herman, Robert (August 1988). «Reflections on Early Work on ‘Big Bang’ Cosmology». Physics Today. 41 (8): 24–34. Bibcode:1988PhT….41h..24A. doi:10.1063/1.881126.
• Barrow, John D. (1994). The Origin of the Universe. Science Masters. London: Weidenfeld & Nicolson. ISBN 978-0-297-81497-9. LCCN 94006343. OCLC 490957073.
• Davies, Paul (1992). The Mind of God: The Scientific Basis for a Rational World. New York: Simon & Schuster. ISBN 978-0-671-71069-9. LCCN 91028606. OCLC 59940452.
• Lineweaver, Charles H.; Davis, Tamara M. (March 2005). «Misconceptions about the Big Bang» (PDF). Scientific American. Vol. 292, no. 3. pp. 36–45. Archived (PDF) from the original on 9 October 2019. Retrieved 23 December 2019.
• Mather, John C.; Boslough, John (1996). The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe (1st ed.). New York: Basic Books. ISBN 978-0-465-01575-7. LCCN 96010781. OCLC 34357391.
• Riordan, Michael; Zajc, William A. (May 2006). «The First Few Microseconds» (PDF). Scientific American. Vol. 294, no. 5. pp. 34–41. Bibcode:2006SciAm.294e..34R. doi:10.1038/scientificamerican0506-34a. Archived (PDF) from the original on 30 November 2014.
• Singh, Simon (2005) [First U.S. edition published 2004]. Big Bang: The Origin of the Universe (Harper Perennial; illustrated ed.). New York, NY: Harper Perennial. ISBN 978-0007162215.
• Weinberg, Steven (1993) [Originally published 1977]. The First Three Minutes: A Modern View of the Origin of the Universe (Updated ed.). New York: Basic Books. ISBN 978-0-465-02437-7. LCCN 93232406. OCLC 488469247. 1st edition is available from the Internet Archive. Retrieved 23 December 2019.

External links

• Once Upon a Universe Archived 22 June 2020 at the Wayback Machine – STFC funded project explaining the history of the universe in easy-to-understand language
• «Big Bang Cosmology» – NASA/WMAP Science Team
• «The Big Bang» – NASA Science
• «Big Bang, Big Bewilderment» – Big bang model with animated graphics by Johannes Koelman
• Cosmology at Curlie

Нас интересуют события, которые
произошли, по разным оценкам, 13 – 20 млрд.
лет назад (13 млрд. лет в соответствии с
теорией «закрытого мира», а 20 млрд. лет
по теории «открытого мира»). Всё это
время наша Вселенная, согласно теории
Большого взрыва, постоянно расширялась.
В пролом же плотность вещества должна
была быть огромной. Согласно теории
А.Фридмана следует, что плотность могла
быть бесконечно большой, хотя некоторые
учёные называют некий возможный предел
значения плотности вещества, примерно
равный 1097 кг/м3.

Другим важным параметром является
температура. Вопрос о том, «холодной»
или «горячей» была материя в ту эпоху,
долгое время оставался спорным. Решающие
доказательства, что Вселенная была
горячей, удалось получить в середине
60-х годов. В настоящее время большинство
космологов считает, что материя в начале
расширения Вселенной была не только
сверхплотной, но и очень горячей, а
теория рассматривающая физические
процессы в начале расширения Вселенной
получила название «теории горячей
Вселенной».

Согласно этой теории, ранняя Вселенная
представляла собой гигантский ускоритель
«элементарных» частиц. Началом работы
Вселенского ускорителя был Большой
взрыв. Этот термин часто применяют
современные космологи. Наблюдаемый
разлёт галактик и их скоплений –
следствие Большого взрыва. Академик
Я.Б.Зельдович назвал этот взрыв
астрономическим, тем самым, подчеркнув
его отличие от химического взрыва.

У обоих взрывов есть общие черты,
например, в обоих случаях вещество после
взрыва охлаждается при расширении,
падает и его плотность. Но есть и
существенные отличия. Главное состоит
в том, что химический взрыв обусловлен
разностью давлений во взрывающемся
веществе и давлением в окружающей среде
(воздухе). Эта разность давлений создаёт
силу, сообщающую скорость частицам
заряда взрывчатого вещества. В
астрономическом взрыве подобной разности
давлений нет. Астрономический взрыв не
начался из какого-то определённого
центра, распространяясь на всё большие
области, а произошел сразу во всём
существовавшем тогда пространстве.
Представить себе это очень трудно
(возможно?), тем более что «всё пространство»
в начале взрыва могло быть как конечным
(теория замкнутого мира), так и бесконечным
(теория открытого мира).

В теории космологии приято эволюцию
вселенной разделять на 4 эры:

а) адронная эра (начальная фаза,
характеризующаяся высокой температурой
и плотностью вещества, состоящего из
элементарных частиц – «адронов»);

б) лептонная эра (следующая фаза,
характеризующаяся снижением энергии
частиц и температуры вещества, состоящего
из элементарных частиц «лептонов».
Адроны распадаются в мюоны и мюонное
нейтрино – образуется «нейтринное
море»;

в) фотонная эра или эра излучения
(характеризуется снижением температуры
до 10К, аннигиляцией электронов и
позитронов, давление излучения полностью
отделяет вещество от антивещества);

г) звёздная эра (продолжительная эра
вещества, эпоха преобладания частиц,
продолжается со времени завершения
Большого взрыва (примерно 300 000 лет назад)
до наших дней.

В нулевой момент времени Вселенная
возникла из сингулярности, то есть из
точки с нулевым объёмом и бесконечно
высокими плотностью и температурой.
Пытаясь объяснить происхождение
Вселенной, сторонники Большого взрыва
сталкиваются с серьёзной проблемой,
поскольку исходное состояние Вселенной
в разработанной ими модели не поддаётся
математическому описанию. В их описаниях
Вселенная в начале представляла собой
точку пространства бесконечно малого
объёма, имевшую бесконечно большую
плотность и температуру. Такое состояние
вещества в принципе не может быть описано
математически. На языке науки это явление
получило название «сингулярности».

В течение первой миллионной доли
секунды, когда температура значительно
превышала 1012К (по некоторым оценкам до
1014К), а плотность была немыслимо велика,
происходили неимоверно быстро сменяющие
себя экзотические взаимодействия,
недоступные пониманию в рамках современной
физики. Мы можем лишь размышлять, каковы
были эти первые мгновения, например,
возможно, что четыре фундаментальные
силы природы были слиты воедино. Есть
основания полагать, что к концу первой
миллионной доли секунды уже существовал
первичный «бульон» богатых энергией
(«горячих») частиц излучения (фотонов)
и частиц вещества. Иными словами материя
Вселенной представляла собой
электронно-позитронные пары (е– и е+);
мюонами и антимюонами (м– и м+); нейтрино
и антинейтрино, как электронными (v e, v
e), так и мюонными (v m, v m) и тау-нейтрино
(v t, v t); нуклонами (протонами и нейтронами)
и электромагнитным излучением. Эта
самовзаимодействующая масса находилась
в состоянии так называемого теплового
равновесия.

В те первые мгновения все имевшиеся
частицы должны были непрерывно возникать
(парами – частица и античастица) и
аннигилировать. Это взаимное превращение
частиц в излучение и обратно продолжалось
до тех пор, пока плотность энергии
фотонов превышала значение пороговой
энергии образования частиц. Когда
возраст Вселенной достиг одной сотой
доли секунды, её температура упала
примерно до 1011К, став ниже порогового
значения, при котором могут рождаться
протоны и нейтроны, некоторые из этих
частиц избежали аннигиляции – иначе в
современной нам Вселенной не было бы
вещества. Через 1 секунду после Большого
взрыва температура понизилась до 1010К,
и нейтрино перестали взаимодействовать
с веществом.

Вселенная стала практически «прозрачной»
для нейтрино. Электроны и позитроны ещё
продолжали аннигилировать и возникать
снова, но примерно через 10 секунд уровень
плотности энергии излучения упал ниже
и их порога, и огромное число электронов
и позитронов превратилось в излучение
катастрофического процесса взаимной
аннигиляции. По окончанию этого процесса,
однако, осталось определённое количество
электронов, достаточное, чтобы,
объединившись с протонами и нейтронами,
дать начало тому количеству вещества,
которое мы наблюдаем сегодня во Вселенной.

Существует два основных взгляда на
процесс формирования галактик. Первый
состоит в том, что в любой момент времени
в расширяющейся смеси вещества и
излучения могли существовать случайно
распределённые области с плотностью
выше средней. В результате сил тяготения
эти области сначала отделились в виде
очень протяжённых сгустков вещества.
В этих сгустках начался процесс
фрагментации, приведший к образованию
облаков меньших размеров, которые
позднее превратились в скопления и
отдельные галактики, наблюдаемые
сегодня. Далее в этих меньших (по
галактическим размерам) сгустках под
действием сил тяготения в случайных
неоднородностях плотности началось
формирование звёзд. Другая точка зрения
даёт другой сценарий: вначале из
флуктуаций плотности в расширяющемся
первичном шаре сформировались
многочисленные (малые) галактики, которые
с течением времени объединились в
скопления, в сверхскопления и, возможно,
в более крупные иерархические структуры.

Главным в споре этих двух взглядов
является ответ на вопрос, имел ли процесс
Большого взрыва вихревой (турбулентный)
характер или протекал более гладко.
Признаков турбулентности в крупномасштабной
структуре сегодняшней Вселенной не
наблюдается. Вселенная выглядит
удивительно сглаженной в крупных
масштабах, несмотря на некоторые
отклонения, в целом далёкие галактики
и их скопления распределены по всему
небу равномерно, а степень изотропности
фонового излучения также довольно
высока. Всё это заставляет признать,
что Большой взрыв был безвихревым,
упорядоченным процессом расширения.

В 1978г., пытаясь найти обоснование для
наблюдаемого соотношения фотонов и
барионов (108 : 1) М.Рис высказал предположение,
что фоновое излучение может быть
результатом «эпидемии» образования
массивных звёзд, начавшейся сразу после
отделения излучения от вещества и до
того, как возраст Вселенной достиг 1
млрд. лет. Продолжительность жизни этих
звёзд не могла превышать 10 млн. лет,
многим из них было суждено пройти стадию
сверхновых и выбросить в пространство
тяжёлые химические элементы, которые
частично собрались в крупицы твёрдого
вещества, образовав облака межзвёздной
пыли. Эта пыль, нагретая излучением
догалактических звёзд, могла, в свою
очередь, испускать инфракрасное
излучение, которое в силу его красного
смещения, вызванного расширением
Вселенной, наблюдается сейчас как
микроволновое фоновое излучение.

Эта точка зрения не получила широкого
признания, но в 1979г. Д.П.Вуди и П.Л.Ричардс
из Калифорнийского университета
опубликовали результаты наблюдений,
указывающие на некоторые отклонения
характеристик микроволнового фонового
излучения от кривой излучения абсолютно
чёрного тела. В том же году М.Роуэн-Робинсон,
Дж.Негропонте и Дж.Силк (Колледж королевы
Марии, Лондон) указали, что отклонения
обнаруженные Вуди и Ричардсом, может
быть объяснено излучением пылевых
облаков, образовавшихся вслед за
«эпидемией» массового формирования
звёзд, что соответствует теории М.Риса.
Если эта новая теория соответствует
истине, то это означает, что подавляющее
количество всей массы Вселенной
содержится в невидимых остатках звёзд
первичного, догалактического, поколения
и в настоящее время может находиться в
массивных тёмных гало, окружающих яркие
галактики, которые мы наблюдаем сегодня.

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Нас интересуют события, которые произошли, по разным оценкам, 13 — 20 млрд. лет назад (13 млрд. лет в соответствии с теорией «закрытого мира», а 20 млрд. лет по теории «Открытого мира»). Все это время наша Вселенная, согласно теории Большого взрыва, постоянно расширялась. В пролом же плотность вещества должна было быть огромной. Согласно теории А. Фридмана следует, что плотность могла быть бесконечно большой, хотя некоторые ученые называют некий возможный предел значения плотности вещества, примерно равный 10 97 кг/м 3.

Другим важным параметром является температура. Вопрос о том, холодной» или «горячей» была материя в ту эпоху, долгое время оставался спорным. Решающие доказательства, что Вселенная была горячей, удалось получить в середине 60-х годов. В настоящее время большинство космологов считает, что материя в начале расширения Вселенной была не только сверхплотной, но и очень горячей, а теория рассматривающая физические процессы в начале расширения Вселенной получила название «теории горячей Вселенной».

Согласно этой теории, ранняя Вселенная представляла собой гигантский ускоритель «элементарных» частиц. Началом работы Вселенского ускорителя был Большой взрыв. Этот термин часто применяют современные космологи. Наблюдаемый разлет галактик и их скоплений — следствие Большого взрыва. Академик Я. Б. Зельдович назвал этот взрыв астрономическим, тем самым, подчеркнув его отличие от химического взрыва.

У обоих взрывов есть общие черты, например, в обоих случаях вещество после взрыва охлаждается при расширении, падает и его плотность. Но есть и существенный отличия. Главное состоит в том, что химический взрыв обусловлен разностью давлений во взрывающемся веществе и давлением в окружающей среде (воздухе). Эта разность давлений создает силу, сообщающую скорость частицам заряда взрывчатого вещества. В астрономическом взрыве подобной разности давлений нет. Астрономический взрыв не начался из какого-то определенного центра, распространяясь на все большие области, а произошел сразу во всем существовавшем тогда пространстве. Представить себе это очень трудно, тем более что «все пространство» в начале взрыва могло быть как конечным (теория замкнутого мира), так и бесконечным (теория открытого мира).

В теории космологии приято эволюцию вселенной разделять на 4 эры:

а) адронная эра (начальная фаза, характеризующаяся высокой температурой и плотностью вещества, состоящего из элементарных частиц — «адронов»);

б) лептонная эра (следующая фаза, характеризующаяся снижением энергии частиц и температуры вещества, состоящего из элементарных частиц «лептонов». Адроны распадаются в мюоны и мюонное нейтрино — образуется «нейтринное море»;

в) фотонная эра или эра излучения (характеризуется снижением температуры до 10 К, аннигиляцией электронов и позитронов, давление излучения полностью отделяет вещество от антивещества);

г) звездная эра (продолжительная эра вещества, эпоха преобладания частиц, продолжается со времени завершения Большого взрыва (примерно 300 000 лет назад) до наших дней.

В нулевой момент времени Вселенная возникла из сингулярности, то есть из точки с нулевым объемом и бесконечно высокими плотностью и температурой. Пытаясь объяснить происхождение Вселенной, сторонники Большого взрыва сталкиваются с серьезной проблемой, поскольку исходное состояние Вселенной в разработанной ими модели не поддается математическому описанию. В их описаниях Вселенная в начале представляла собой точку пространства бесконечно малого объема, имевшую бесконечно большую плотность и температуру. Такое состояние вещества в принципе не может быть описано математически. На языке науки это явление получило название «сингулярности».

В течение первой миллионной доли секунды, когда температура значительно превышала 10 12 К (по некоторым оценкам до 10 14 К), а плотность была немыслимо велика, происходили неимоверно быстро сменяющие себя экзотические взаимодействия, недоступные пониманию в рамках современной физики. Мы можем лишь размышлять, каковы были эти первые мгновения, например, возможно, что четыре фундаментальные силы природы были слиты воедино. Есть основания полагать, что к концу первой миллионной доли секунды уже существовал первичный «бульон» богатых энергией («горячих») частиц излучения (фотонов) и частиц вещества. Иными словами материя Вселенной представляла собой электронно-позитронные пары (еи е+); мюонами и антимюонами (м — и м +); нейтрино и антинейтрино, как электронными (v e, v e), так и мюонными (v m, v m) и тау-нейтрино (v t, v t); нуклонами (протонами и нейтронами) и электромагнитным излучением. Эта самовзаимодействующая масса находилась в состоянии так называемого теплового равновесия.

В те первые мгновения все имевшиеся частицы должны были непрерывно возникать (парами — частица и античастица) и аннигилировать. Это взаимное превращение частиц в излучение и обратно продолжалось до тез пор, пока плотность энергии фотонов превышала значение пороговой энергии образования частиц. Когда возраст Вселенной достиг одной сотой доли секунды, ее температура упала примерно до 10 11 К, став ниже порогового значения, при котором могут рождаться протоны и нейтроны, некоторые из этих частиц избежали аннигиляции — иначе в современной нам Вселенной не было бы вещества. Через 1 секунду после Большого взрыва температура понизилась до 10 10 К, и нейтрино перестали взаимодействовать с веществом. Вселенная стала практически «прозрачной» для нейтрино. Электроны и позитроны еще продолжали аннигилировать и возникать снова, но примерно через 10 секунд уровень плотности энергии излучения упал ниже и их порога, и огромное число электронов и позитронов превратилось в излучение катастрофического процесса взаимной аннигиляции. По окончанию этого процесса, однако, осталось определенное количество электронов, достаточное, чтобы, объединившись с протонами и нейтронами, дать начало тому количеству вещества, которое мы наблюдаем сегодня во Вселенной.

Существует два основных взгляда на процесс формирования галактик. Первый состоит в том, что в любой момент времени в расширяющейся смеси вещества и излучения могли существовать случайно распределенные области с плотностью выше средней. В результате сил тяготения эти области сначала отделились в виде очень протяженных сгустков вещества. В этих сгустках начался процесс фрагментации, приведший к образованию облаков меньших размеров, которые позднее превратились в скопления и отдельные галактики, наблюдаемые сегодня. Далее в этих меньших (по галактическим размерам) сгустках под действием сил тяготения в случайных неоднородностях плотности началось формирование звезд. Другая точка зрения дает другой сценарий: вначале из флуктуаций плотности в расширяющемся первичном шаре сформировались многочисленные (малые) галактики, которые с течением времени объединились в скопления, в сверхскопления и, возможно, в более крупные иерархические структуры.

Главным в споре этих двух взглядов является ответ на вопрос, имел ли процесс Большого взрыва вихревой (турбулентный) характер или протекал более гладко. Признаков турбулентности в крупномасштабной структуре сегодняшней Вселенной не наблюдается. Вселенная выглядит удивительно сглаженной в крупных масштабах, несмотря на некоторые отклонения, в целом далекие галактики и их скопления галактики распределены по всему небу равномерно, а степень изотропности фонового излучения также довольно высока. Все это заставляет признать, что Большой взрыв был безвихревым, упорядоченным процессом расширения.

В 1978 г., пытаясь найти обоснование для наблюдаемого соотношения фотонов и барионов (10 8: 1) М. Рис высказал предположение, что фоновое излучение может быть результатом «эпидемии» образования массивных звезд, начавшейся сразу после отделения излучения от вещества и до того, как возраст Вселенной достиг 1 млрд. лет. Продолжительность жизни этих звезд не могла превышать 10 млн. лет, многим из них было суждено пройти стадию сверхновых и выбросить в пространство тяжелые химические элементы, которые частично собрались в крупицы твердого вещества, образовав облака межзвездной пыли. Эта пыль, нагретая излучением догалактических звезд, могла, в свою очередь, испускать инфракрасное излучение, которое в силу его красного смещения, вызванного расширением Вселенной, наблюдается сейчас как микроволновое фоновое излучение.

Эта точка зрения не получила широкого признания, но в 1979 г. Д. П. Вуди и П. Л. Ричардс из Калифорнийского университета опубликовали результаты наблюдений, указывающие на некоторые отклонения характеристик микроволнового фонового излучения от кривой излучения абсолютно черного тела. В том же году М. Роуэн-Робинсон, Дж. Негропонте и Дж. Силк (Колледж королевы Марии, Лондон) указали, что отклонения обнаруженные Вуди и Ричардсом, может быть объяснено излучением пылевых облаков, образовавшихся вслед за «эпидемией» массового формирования звезд, что соответствует теории М. Риса. Если эта новая теория соответствует истине, то это означает, что подавляющее количество всей массы Вселенной содержится в невидимых остатках звезд первичного, догалактического, поколения и в настоящее время может находиться в массивных темных гало, окружающих яркие галактики, которые мы наблюдаем сегодня.

Как появилась Вселенная, которую мы знаем? И как мы объясним ее происхождение? Несомненно, все остальные свидетельства и данные, собранные за эти годы космологами, указывают на то, что все это могло начаться с «большого взрыва». Но что, если есть еще?

В 1927 году бельгийский астроном Жорж Леметр стал первым, кто предложил теорию расширяющейся Вселенной (позже подтвержденную Эдвином Хабблом). Он предположил, что расширяющаяся Вселенная может быть прослежена до особой точки, которую он назвал «первичным атомом», назад во времени. Это заложило основу современной теории Большого Взрыва.

Что такое теория большого взрыва?

Теория Большого взрыва — это объяснение, основанное в основном на математических моделях, того, как и когда возникла Вселенная.

Космологическая модель Вселенной, описанная в теории Большого взрыва, объясняет, как она первоначально расширилась из состояния бесконечной плотности и температуры, известного как изначальная (или гравитационная) сингулярность. За этим расширением последовала космическая инфляция и резкое падение температуры. Во время этой фазы Вселенная раздувалась с гораздо большей скоростью, чем скорость света (в 10²⁶раз).

Впоследствии Вселенная была разогрета до такой степени, что элементарные частицы (кварки, лептоны и так далее) до постепенного понижения температуры (и плотности) привели к образованию первых протонов и нейтронов.

Через несколько минут после расширения протоны и нейтроны объединяются, образуя первичные ядра водорода и гелия-4. Предполагаемый радиус наблюдаемой Вселенной в течение этой фазы составлял 300 световых лет. Первые звезды и галактики появились примерно через 400 миллионов лет после этого события.

Важнейшим элементом модели Большого Взрыва является космическое сверхвысокочастотное фоновое излучение (Реликтовое излучение), представляющий собой электромагнитное излучение, оставшееся со времен зарождения Вселенной. Реликтовое излучение остается самым убедительным доказательством большого взрыва.

Хотя теория остается широко признанной во всем научном спектре, несколько альтернативных объяснений — таких, как стационарная Вселенная и вечная инфляция, приобрели привлекательность с годами.

Понятие инфляции было введено космологом Аланом Гутом в 1979 году, чтобы объяснить, почему Вселенная плоская, чего не хватало в первоначальной теории Большого взрыва.

Хотя идея Гута об инфляции объясняет плоскую Вселенную, она создала сценарий, который не позволяет Вселенной избежать этой инфляции. Если бы это было так, не произошло бы повторного нагрева Вселенной, равно как и образования звезд и галактик.

Эта конкретная проблема была решена Андреасом Альбрехтом и Полем Штайнхардтом в их «новой инфляции». Они утверждали, что быстрое расширение Вселенной произошло всего за несколько секунд, прежде чем прекратиться. Он продемонстрировал, как Вселенная может быстро раздуваться и при этом нагреваться.

Концепция «вечной инфляции», или теория хаотической инфляции, была введена Андреем Линде, профессором Стэнфордского университета. Он был основан на предыдущих работах Штейнхардта и Александра Виленкина.

Теория утверждает, что инфляционная фаза Вселенной продолжается вечно; это не конец для Вселенной в целом. Другими словами, космическая инфляция продолжается в одних частях Вселенной и прекращается в других. Это приводит к сценарию мультивселенной, в котором пространство разбивается на пузыри. Это как вселенная внутри вселенной.

В мультивселенной в разных вселенных могут действовать разные законы природы, физики. Итак, вместо единого расширяющегося космоса наша Вселенная могла бы быть инфляционной мультивселенной с множеством маленьких вселенных с различными свойствами.

Однако Пол Стейнхардт считает, что его теория «новой инфляции» ни к чему не приводит и не предсказывает, и утверждает, что понятие мультивселенной является «фатальным недостатком» и неестественным.

6. Конформная циклическая модель

Модель конформной циклической космологии (англ. conformal cyclic cosmology или CCC) предполагает, что Вселенная проходит через повторяющиеся циклы большого взрыва и последующих расширений. Общая идея состоит в том, что «большой взрыв» был не началом Вселенной, а скорее переходной фазой. Его разработал физик-теоретик и математик Роджер Пенроуз.

В качестве основы для своей модели Пенроуз использовал множественные метрические последовательности FLRW (Фридмана – Лемэтра – Робертсона – Уокера). Он утверждал, что конформная граница одной последовательности FLRW может быть присоединена к границе другой.

Метрика FLRW — это наиболее близкое приближение к природе Вселенной и часть модели Лямбда-CDM. Каждая последовательность начинается с большого взрыва, за которым следует инфляция и последующее расширение.

Циклическая или осциллирующая модель, в которой Вселенная повторяется снова и снова в неопределенном цикле, впервые оказалась в центре внимания в 1930-х годах, когда Альберт Эйнштейн исследовал идею «вечной» Вселенной. Он считал, что по достижении определенной точки Вселенная начинает коллапсировать и заканчивается Большим хрустом перед тем, как пройти через Большой отскок.

Прямо сейчас существует четыре различных варианта циклической модели Вселенной, одна из которых — конформная циклическая космология.

5. Мираж четырехмерной черной дыры

Исследование, проведенное группой исследователей в 2013 году, предположило, что наша Вселенная могла возникнуть из обломков, выброшенных из коллапсировавшей четырехмерной звезды или черной дыры.

По мнению космологов, участвовавших в исследовании, одно из ограничений теории Большого взрыва — объяснение температурного равновесия, обнаруженного во Вселенной.