Wg modelu Wielkiego Wybuchu, Wszechświat wyłonił się z niesłychanie gęstego i gorącego stanu (na dole). Od tamtej pory sama przestrzeń rozszerzała się z biegiem czasu "unosząc" ze sobą galaktyki.
Wielki Wybuch (ang. Big Bang ) to model powstania Wszechświata uznawany przez współczesną kosmologię za najbardziej prawdopodobny.
Według tego modelu ok. 13,73 mld lat temu dokonał się "Wielki Wybuch" - z nieprawdopodobnie gęstej i gorącej osobliwości początkowej wyłonił się Wszechświat (przestrzeń, czas, materia, energia, oddziaływania, itd.).
1

Teoria ta opiera się na obserwacjach wskazujących na rozszerzanie się przestrzeni zgodnie z metryką Friedmana-Lemaître'a-Robertsona-
Walkera. Przemawia za tym przesunięcie ku czerwieni odległych galaktyk zgodne z prawem Hubble'a w powiązaniu z zasadą kosmologiczną.
Ekstrapolując z przeszłości, obserwacje te wskazują, że Wszechświat rozszerza się od stanu, w którym cała materia oraz energia Wszechświata miała ogromną gęstość i temperaturę. Fizycy nie są zgodni co do tego, co było przedtem ale teoria ogólnej względności przewiduje, że był to stan grawitacyjnej osobliwości.
Termin Wielki Wybuch jest używany zarówno w wąskim znaczeniu na określenie momentu, gdy zaczęło się obserwowane rozszerzanie się
Wszechświata oraz w szerszym - jako określenie dominującego paradygmatu naukowego objaśniającego powstanie Wszechświata oraz uformowanie się przez nukleosyntezę pierwotnej materii (zgodnie z teorią Alphera-Bethe-Gamowa).
Utożsamianie
Wielkiego wybuchu z eksplozją jest o tyle niefortunne, że proces ten, tak jak rozumie i ujmuje go współczesna kosmologia nie polegał na ekspansji w pustej przestrzeni lecz dotyczył "rozdymania" przestrzeni jako takiej (porównaj także model Friedmana).
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
1 Historia teorii Wielkiego Wybuchu
2 Przegląd zagadnienia o 2.1 Model Gorącego Wielkiego Wybuchu o 2.2 Co było przed?
 3 Przeciwnicy


4 Przypisy
 5 Zobacz też
6 Linki zewnętrzne
Przed powstaniem teorii Wielkiego Wybuchu obowiązywał powszechnie uznawany pogląd, że Wszechświat jest niezmiennym nieporuszającym się układem galaktyk. Pogląd ten wyrażał także Albert Einstein.
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Teoria Wielkiego Wybuchu została wypracowana na podstawie rozważań teoretycznych próbujących wyjaśnić obserwacje astronomiczne.
Obserwatorzy zauważyli, że większość "mgławic spiralnych" oddala się od Ziemi ale nie byli jeszcze świadomi kosmologicznych implikacji tego faktu (ani tego, że te "mgławice spiralne" to w rzeczywistości galaktyki).
W 1922 roku Aleksander Friedmann wyprowadził równania postulujące rozszerzanie się Wszechświata. Niezależnie od niego, w 1927 roku wyprowadził je również Georges Lemaître, co doprowadziło go do wysunięcia hipotezy pierwotnego atomu. Jego prace zostały w 1929 roku potwierdzone przez obserwacje Edwina Hubble'a. Zaobserwował on, że galaktyki wykazują przesunięcie ku czerwieni wprost proporcjonalne do ich odległości od Ziemi - fakt ten znany jest obecnie jako prawo Hubble'a. Biorąc pod uwagę zasadę kosmologiczną, która stanowi, że
Wszechświat jest jednorodny i izotropowy, z prawa Hubble'a wynika, że cały Wszechświat rozszerza się.
Pojawiły się więc dwie alternatywy. Jedną była teoria stanu stacjonarnego autorstwa Freda Hoyle'a, Thomasa Golda i Hermanna Bondiego, która zakładała, że Wszechświat jest z grubsza niezmienny w czasie. Drugą była teoria Lemaître'a rozwijana dalej przez George'a Gamowa.
Paradoksalnie, została ona nazwana przez jej głównego oponenta - Hoyle'a - który mówił o niej pogardliwie "teoria wielkiego bum" (ang. big bang theory). Nazwa ta jednak się przyjęła.
Przez pewien czas naukowcy byli podzieleni jeśli chodzi o poparcie dla tych teorii. Jednak w latach 60. ubiegłego wieku odkrycie mikrofalowego promieniowania tła przechyliło szalę na korzyść teorii Wielkiego Wybuchu. W tej chwili badania kosmologiczne skupiają się na próbach zrozumienia jak w kontekście teorii Wielkiego Wybuchu formują się galaktyki, co działo się w pierwszych momentach istnienia
Wszechświata i pogodzenia obserwacji z teorią.
Duże postępy w zakresie teorii Wielkiego Wybuchu zostały poczynione w latach 90. XX i w pierwszej dekadzie XXI wieku. Dzięki zaawansowanym teleskopom oraz danym z satelitów COBE, WMAP i kosmicznego teleskopu Hubble'a możliwe stały się pomiary o niespotykanej wcześniej precyzji, które doprowadziły do odkrycia, że tempo rozszerzania się Wszechświata wydaje się przyspieszać.
Opierając się na pomiarach rozszerzania się Wszechświata, do których wykorzystano obserwacje supernowych typu Ia, pomiarach fluktuacji temperatury kosmicznego mikrofalowego promieniowania tła oraz korelacji galaktyk, wiek Wszechświata ocenia się na 1,37 × 10 10 ±2% lat.
Zgodność powyższych trzech niezależnych wyników pomiarów dostarcza silnego potwierdzenia dla tzw. modelu Lambda-CDM, który szczegółowo opisuje naturę zawartości Wszechświata.
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Wszechświat we wczesnym stadium rozwoju był jednolicie i izotropowo wypełniony energią o nieprawdopodobnie wielkiej gęstości, o olbrzymiej temperaturze i ciśnieniu. Jednak w miarę rozszerzania się stygł, przechodząc przez przemiany fazowe odnoszące się do cząstek elementarnych.
Około 10 -35 sekund po erze Plancka przemiana fazowa spowodowała, że Wszechświat wszedł w fazę inflacji kosmologicznej, podczas której rozszerzał się wykładniczo. Kiedy ta inflacja zatrzymała się, materialne składowe Wszechświata były w stanie plazmy kwarkowo-gluonowej, w której cząstki składowe poruszały się relatywistycznie. W miarę jak Wszechwiat dalej rósł stygł, aż w określonej temperaturze zaszła przemiana zwana bariogenezą, podczas której kwarki i gluony połączyły się w bariony takie jak protony i neutrony.
Współcześnie obserwacje przemawiające za prawdziwością modelu
Wielkiego Wybuchu to: zjawisko ucieczki galaktyk, istnienie promieniowania tła, łącznie z mierzonymi ostatnio jego nierównomiernościami. Przemawia także za tą teorią udane wyjaśnienie procentowego udziału lekkich pierwiastków (H, He, Li) w składzie Wszechświata i zgodność modelu z innymi teoriami, w tym z ogólną teorią względności.
Zgodnie z modelem Gorącego Wielkiego Wybuchu, opracowanym jeszcze w 1948 r. przez George'a Gamowa i jego studenta Ralpha Alphera, można przyjąć, że w chwili narodzin Wszechświata, przy ogromnej temperaturze 10 32
kelwinów, był hiperprzestrzennym, dziesięciowymiarowym tworem, w którym zjednoczone były wszystkie oddziaływania i istniała jedna wielka symetria GUT.
Świat ten był jednak niestabilny i po 10 -43 sekundy rozpadł się na cztero- i sześciowymiarowy. Sześciowymiarowy zapadł się do rozmiaru 10 -32 centymetra, a nasz czterowymiarowy zaczął się gwałtownie rozszerzać. Po 10
-35
sekundy silne oddziaływania oddzieliły się od elektrosłabych, a niewielki fragment większego wszechświata rozszerzył się 10 50
razy, stając się ostatecznie naszym widzialnym Wszechświatem. Takie gwałtowne rozszerzenie opisywane jest przez teorię inflacji kosmologicznej. Po upływie dalszego ułamka sekundy oddziaływania elektrosłabe rozpadły się na elektromagnetyczne i słabe, a następnie, gdy temperatura spadła już do 10 14 kelwinów, kwarki zaczęły się łączyć w protony i neutrony.
Teoria Wielkiego Wybuchu nie daje odpowiedzi na pytanie "Co było przed?". W szczególności nie mówi:
1.
Czy Wielki Wybuch jest absolutnym początkiem fizycznego świata (wtedy na gruncie kosmologii pytanie "co było przed Wielkim
Wybuchem?" traci sens).
2.
Czy jest tylko względnym początkiem nowego stadium (tzn. że świat fizyczny istniał już wcześniej, tylko nieznana jest jego postać).
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Tym niemniej, Leon Max Lederman, jeden z największych autorytetów tematu, napisał:
"Na Samym Początku była pustka - zadziwiająca forma próżni - nicość nie zawierająca żadnej przestrzeni, żadnego czasu, żadnej materii, żadnego światła, żadnego dźwięku. Jednak prawa natury były na swoim miejscu i ta dziwna próżnia posiadała potencjał. [...] W logiczny sposób opowieść zaczyna się od początku. Ale ta opowieść dotyczy wszechświata i, niestety, nie mamy żadnych danych na temat Samego Początku.
Żadnych, zero. Nie wiemy nic o wszechświecie, aż do momentu, gdy osiąga on dojrzały wiek bilionowej trylionowej części sekundy - to jest, pewnego bardzo krótkiego czasu po stworzeniu w Wielkim Wybuchu. Kiedy czytasz lub słyszysz cokolwiek na temat narodzin wszechświata, to ktoś to zmyśla. Jesteśmy w sferze filozoficznych rozważań. Tylko Bóg wie co stało się na Samym Początku."
[1]
Do powyższej opinii przychyla się większość czołowych uczonych, jak np. Stephen Hawking. Teoria ta w sposób oczywisty niesie ze sobą kilka nierozwiązanych jak dotąd problemów:
1.
Wybuch nicości przy jednoczesnym samoistnym stworzeniu wszystkiego ex nihilo nie został poparty jakimkolwiek empirycznym doświadczeniem oraz przeczy znanym nam prawom fizyki;
2.
Teoretyczny brak jakiegokolwiek procesu czy zmian prowadzących do wybuchu (z racji braku istnienia czegokolwiek) przeczy przyczynowo-skutkowym zasadom wszelkich zmian we wszechświecie;
3.
Skoro przed Wielkim Wybuchem nie istniała żadna materia, nie zachodziły żadne procesy fizyczno-chemiczne oraz specyficzna próżnia pozostawała w stanie stabilnym (teoretycznie od nieskończenie długiego czasu, którego nie sposób odmierzyć) to dlaczego wybuch miał miejsce 13 miliardów lat temu, a nie tryliony lat wcześniej lub później?
Stephen Hawking, podobnie jak i inni uczeni, porusza te zagadnienia w swoich bestsellerowych publikacjach, nie potrafi jednak udzielić odpowiedzi na powyższe pytania. W jednej ze swoich prac poświęconych kosmologii pisze nawet:
"Uważam, że kiedykolwiek zaczynamy omawiać początki wszechświata dochodzimy do czysto religijnych wniosków."
[2]
Wybitni uczeni odrzucający teorię Wielkiego Wybuchu:
 Hannes Olof Gösta Alfvén
[3]
 Hermann Bondi, Thomas Gold, Sir Fred Hoyle - którzy w 1948 roku stworzyli teorię stanu stacjonarnego, teoria ta została porzucona, jako niezgodna z faktami doświadczalnymi w latach 60. XX w.
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1.
↑ Leon Lederman & Dick Teresi, The God Particle (1993), str. 1
2.
↑ Kitty Ferguson,
Stephen Hawking: Quest for a Theory of Everything (1991), str. 93
3.
↑ "Istnieje stale rosnąca liczba zaobserwowanych faktów, które trudno pogodzić z hipotezą Wielkiego Wybuchu. Establishment Wielkiego
Wybuchu bardzo rzadko o nich wspomina, a kiedy nie-wierzący próbują przyciągnąć do nich uwagę potężny establishment odmawia przedyskutowania ich w uczciwy sposób..." Hannes Alfven, "Cosmology: Myth or Science?" w Journal of Astrophysics and Astronomy 5
/1970/, p. 1203
 podstawowe zagadnienia z zakresu astronomii

 kosmologia
 kosmologia obserwacyjna koniec świata
 "Wielki Wybuch"
Źródło: " http://pl.wikipedia.org/wiki/Wielki_Wybuch "
Kategoria : Kosmologia
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For other uses, see Big Bang (disambiguation).
According to the Big Bang model, the universe expanded from an extremely dense and hot state and continues to expand today. A common and useful analogy explains that space itself is expanding, carrying galaxies with it, like raisins in a rising loaf of bread. General relativistic cosmologies, however, do not actually ascribe any 'physicality' to space.
The Big Bang is a cosmological model of the universe that has become well supported by several independent observations. After Edwin Hubble discovered that galactic distances were generally proportional to their redshifts in 1929, this observation was taken to indicate that the universe is expanding.
[1]
If the universe is seen to be expanding today, then it must have been smaller, denser, and hotter in the past. This idea has been considered in detail all the way back to extreme densities and temperatures, and the resulting conclusions have been found to conform very closely to what is observed.
Ironically, the term 'Big Bang' was first coined by Fred Hoyle in a derisory statement seeking to belittle the credibility of the theory that he did not believe to be true.
[2]
However, the discovery of the cosmic microwave background in 1964 was taken as almost undeniable support for the
Big Bang.
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Analysis of the spectrum of light from distant galaxies reveals a shift towards longer wavelengths proportional to each galaxy's distance in a relationship described by Hubble's law, which is taken to indicate that the universe is undergoing a continuous expansion. Furthermore, the cosmic microwave background radiation discovered in 1964 provides strong evidence that due to the expansion, the universe has naturally cooled from an extremely hot, dense initial state. The discovery of the cosmic microwave background led to almost universal acceptance among physicists, astronomers, and astrophysicists that the Big Bang describes the evolution of the universe quite well, at least in its broad outline.
Further evidence supporting the Big Bang model comes from the relative proportion of light elements in the universe. The observed abundances of hydrogen and helium throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.
However, there are mysteries of the universe that are not explained by the Big Bang model alone. For example, a region of the universe 12 billion lightyears distant in one direction appears little different than a region 12 billion lightyears distant in the opposite direction. But since the universe is 'only' around 13.7 billion years old, it would appear these regions could never have been causally connected . How, then, can they be so similar? Alan Guth's 1981 theory of cosmic inflation, a short, sudden burst of extreme exponential expansion in the very early universe, provided an explanation for this horizon problem and several of the features unaccounted for by the original Big Bang model. The successor to
Guth's original theory has found some circumstantial support, but it is not yet nearly as well supported as the Big Bang model.
Physical cosmology
Universe · Big Bang
Age of the universe
Timeline of the Big Bang
Ultimate fate of the universe
[ show ] Early Universe
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[ show ] Expanding universe
[ show ] Structure formation
[ show ] Components
[ show ] History
[ show ] Experiments
[ show ] Scientists
This box: view • talk • edit
[ hide ]
 1 History
 2 Overview
2.1 Timeline of the Big Bang o o o o
2.2 Big bang theory assumptions
2.3 FLRW metric
2.4 Horizons
 3 Observational evidence o 3.1 Hubble's law and the expansion of space o o
3.2 Cosmic microwave background radiation
3.3 Abundance of primordial elements
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o 3.4 Galactic evolution and distribution o 3.5 Other lines of evidence
 4 Features, issues and problems o o o o o o
4.1 Horizon problem
4.2 Flatness/oldness problem
4.3 Magnetic monopoles
4.4 Baryon asymmetry
4.6 Dark matter
4.5 Globular cluster age o 4.7 Dark energy
 5 The future according to the Big Bang theory


6 Speculative physics beyond the Big Bang
 7 Philosophical and religious interpretations
8 References o 8.1 Books
 9 Further reading
 10 External links
Main article: History of the Big Bang theory
See also: Timeline of cosmology and History of astronomy
The Big Bang theory 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 .
[3]
Ten years later, Alexander Friedmann , a Russian cosmologist and mathematician , derived the Friedmann equations from Albert Einstein 's equations of general relativity , showing that the universe might be expanding in contrast to the static universe model advocated by Einstein.
[4]
In 1924, Edwin Hubble 's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies . Independently deriving Friedmann's equations in
1927, Georges Lemaître , a Belgian physicist and Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe.
[5]
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In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the universe into a single point, a "primeval atom ", at a point in time before which time and space did not exist. As such, at this point, the fabric of time and space had not yet come into existence. This perhaps echoed previous speculations about the cosmic egg origin of the universe.
[6]
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder , using the 100-inch
(2,500 mm) 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 recession velocity—now known as Hubble's law .
[1][7]
Lemaître had already shown that this was expected, given the Cosmological Principle .
[8]
Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.
During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model ,
[9]
the oscillatory universe (originally suggested by Friedmann, but advocated by Einstein and Richard Tolman )
[10]
and Fritz Zwicky 's tired light hypothesis.
[11]
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.
[12]
The other was Lemaître's Big Bang theory, advocated and developed by George Gamow , who introduced big bang nucleosynthesis
[13]
and whose associates, Ralph Alpher and Robert
Herman , predicted the cosmic microwave background (CMB).
[14] It is an irony that it was Hoyle who coined the name that would come to be applied to Lemaître's theory, referring to it as "this big bang idea" in derision during a 1950 BBC radio broadcast.
[15][16]
For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts , began to favor the latter. The discovery of the cosmic microwave background radiation in 1964 [17] secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory.
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Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE ,
[18]
the Hubble Space Telescope and WMAP .
[19]
Cosmologists now have fairly precise measurement 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.
See also: Timeline of the Big Bang
A graphical timeline is available here:
Graphical timeline of the Big Bang
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 singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch . The early hot, dense phase is itself referred to as "the Big Bang",
[21]
and is considered the
"birth" of our universe. Based on measurements of the expansion using Type Ia supernovae , measurements of temperature fluctuations in the cosmic microwave background , and measurements of the correlation function of galaxies, the universe has a calculated age of 13.73 ± 0.12 billion years old
[22]
. The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the universe.
The earliest phases of the Big Bang are subject to much speculation. In the most common models, the universe was filled homogeneously and isotropically with an incredibly high energy density , huge temperatures and pressures , and was very rapidly expanding and cooling.
Approximately 10
−35
seconds into the expansion, a phase transition caused a cosmic inflation , during which the universe grew exponentially .
[23]
After inflation stopped, the universe consisted of a quark-gluon plasma , as well as all other elementary particles .
[24]
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. 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 anti-leptons—of the order of 1 part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.
[25]
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The universe continued to grow in size 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.
[26]
After about 10
−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. 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 now no longer high enough to create new proton-antiproton pairs (similarly for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 10
10
of the original protons and neutrons, and none of their antiparticles. 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 (one thousand million; 10
9
; SI prefix giga ) Kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis .
[27]
Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass 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 ); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation .
[28]
The Hubble Ultra Deep Field showcases galaxies from an ancient era when the universe was younger, denser, and warmer according to the Big
Bang theory.
Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars , galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types of matter are known as cold dark matter , hot dark matter and baryonic
13

matter . The best measurements available (from WMAP ) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the universe.
[22]
.
Independent lines of evidence from Type Ia supernovae and the CMB imply the universe today is dominated by a mysterious form of energy known as dark energy , which apparently permeates all of space. The observations suggest 72% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.
[8]
All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well supported model describing the action prior to 10
−15
seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier.
Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics .
The Big Bang theory depends on two major assumptions: the universality of physical laws , and the Cosmological Principle . The cosmological principle states that on large scales the universe is homogeneous and isotropic .
These ideas were initially taken as postulates, but today there are efforts 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
.
[29]
Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.
[30]
If the large-scale universe appears isotropic as viewed from Earth, 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 CMB.
[31]
The universe has been measured to be homogeneous on the largest scales at the 10% level.
[32]
14

Main articles: Friedmann-Lemaître-Robertson-Walker metric and Metric expansion of space
General relativity describes spacetime by a metric , which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves 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 metric (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 due to 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 comoving points expands proportionally with the scale factor of the universe.
[33]
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 distance between two comoving points. 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 are gravitationally bound and as such do not experience the large-scale expansion of space.
Main article: Cosmological horizon
An important feature of the Big Bang spacetime is the presence of 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 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. Our understanding of the universe back to very early times suggests that there was a past horizon, though in practice our view is limited by the opacity of the universe at early times. If the expansion of the universe continues to accelerate , there is a future horizon as well.
[34]
The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements (see Big Bang nucleosynthesis ). These are sometimes
15

called the three pillars of the big bang theory. Many other lines of evidence now support the picture, notably various properties of the large-scale structure of the cosmos
[35]
which are predicted to occur due to gravitational growth of structure in the standard Big Bang theory.
Main articles: Hubble's law and metric expansion of space
See also: distance measures (cosmology) and scale factor (universe)
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 lines 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:
[1] where v is the recessional velocity of the galaxy or other distant object
D is the proper distance to the object and
H
0
is Hubble's constant
, measured to be 70.1 ± 1.3 km / s / Mpc by the WMAP probe
[22]
.
Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the Copernican
Principle
—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander
Friedman in 1922
[4]
and
Georges Lemaître
in 1927,
[5]
well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson and Walker .
The theory requires the relation v = HD to hold at all times, where D is the proper distance , v = dD / dt , and v , H , and D all vary as the universe expands (hence we write H
0
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 v . 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.
[36]
16

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 homogenous ,
[1] 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 the Earth is not in a central position, on a cosmological scale.
[37]
Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the cosmic microwave background 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.
Main article: Cosmic microwave background radiation
WMAP image of the cosmic microwave background radiation
During the first few days of the universe, the universe was in full thermal equilibrium , with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. As the universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering . Because of this repeated scattering, the early universe was opaque to light.
When the temperature fell to a few thousand Kelvin , electrons and nuclei began to combine to form atoms, a process known as recombination .
Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering , 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the microwave region of the electromagnetic spectrum . The radiation is thought to be observable at every point in the universe, and comes from all directions with (almost) the same intensity.
17

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories .
[17]
Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a Nobel Prize for their discovery.
In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the
Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations
(anisotropies) in the CMB, at a level of about one part in 10
5
.
[18]
John C. Mather and George Smoot were awarded Nobels for their leadership in this work. 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 universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See shape of the universe .)
This article or section uses citations that are either broken or outdated.
This article or section uses citations that link to broken or outdated sources, and are deemed unreliable. Please improve the article or discuss this issue on the talk page . Help on using footnotes is available. This article has been tagged since April 2008 .
In early 2003, the first results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general
[19]
, it confirms too that a sea of cosmic neutrinos permeates the universe, a clear evidence that the first stars took more than a half-billion years to create a cosmic fog. Another satellite like it will be launched within the next few years, the
Planck Surveyor , which will provide even more accurate measurements of the CMB anisotropies. Many other ground- and balloon-based experiments are also currently running; see Cosmic microwave background experiments .
The background radiation is exceptionally smooth, which presented a problem in that conventional expansion would mean that photons coming from opposite directions in the sky were coming from regions that had never been in contact with each other. The leading explanation for this far reaching equilibrium is that the universe had a brief period of rapid exponential expansion, called inflation . This would have the effect of driving apart regions that had been in equilibrium , so that all the observable universe was from the same equilibrated region.
Main article: Big Bang nucleosynthesis
Using the Big Bang model it is possible to calculate the concentration of helium -4, helium-3, deuterium and lithium -7 in the universe as ratios to the amount of ordinary hydrogen, H.
[27] All the abundances depend on a single parameter, the ratio of photons to baryons , which itself can be
18

4 calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for
He/H, about 10
−3
for ²H/H, about 10
−4
for ³He/H and about 10
−9
for
7
Li/H.
[27]
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 4 He, and a factor of two off for 7 Li; 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.
[38]
Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., 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.
Main articles: Large-scale structure of the cosmos , Structure formation , and Galaxy formation and evolution
This panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way . The galaxies are color coded by redshift .
Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. 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 . 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.
[39]
19

After some controversy, the age of universe as estimated from the Hubble expansion and the CMB is now in good agreement with (i.e., slightly larger than) 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.
The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of temperature-sensitive emission lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev-Zel'dovich effect in clusters of galaxies does not depend directly on redshift; this seems to be roughly true, but unfortunately the amplitude does depend on cluster properties which do change substantially over cosmic time, so a precise test is impossible.
While very few researchers now doubt the Big Bang occurred, the scientific community was once divided between supporters of the Big Bang and those of alternative cosmological models . Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations (see the history section above). With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter , are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory.
The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many independent observations including Big Bang nucleosynthesis , the cosmic microwave background , large scale structure and Type Ia supernovae , and can hardly be doubted as important and real features of our universe.
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 energy and dark matter are considered the most secure, while inflation and baryogenesis remain speculative: they provide satisfying explanations for important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory.
[40] Explanations for such phenomena remain at the frontiers of inquiry in physics .
Main article: Horizon problem
20

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.
[41]
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 these regions to have the same temperature.
A resolution to this apparent inconsistency is offered by inflationary 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.
Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations , which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian , which has been accurately confirmed by measurements of the CMB.
Main article: Flatness problem
21

The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. From top to bottom: a closed universe with positive curvature, a hyperbolic universe with negative curvature and a flat universe with zero curvature.
The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann-Lemaître-Robertson-Walker metric .
[41] 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 . The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.
timescale for departure from flatness might be the Planck time , 10
−43
[42]
Given that a natural
seconds, the fact that the universe has reached neither a Heat Death nor a
Big Crunch after billions of years requires some explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the universe must have been within one part in 10
14
of the critical density, or it would not exist as it does today.
[43]
A resolution to this problem is offered by inflationary theory . During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is believed that inflation drove the universe to a very nearly spatially flat state, with almost exactly the critical density.
Main article: Magnetic monopole
The magnetic monopole objection was raised in the late 1970s. Grand unification theories 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 searches have never found any monopoles. This problem is also resolved by cosmic inflation , which removes all point defects from the observable universe in the same way that it drives the geometry to flatness.
[41]
A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature hypothesis .
[44][45]
Main article: Baryon asymmetry
It is not yet understood why the universe has more matter than antimatter .
[25]
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 anti-baryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called " baryogenesis " created the asymmetry. For
22

baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CPsymmetry are violated and that the universe depart from thermodynamic equilibrium .
[46]
All these conditions occur in the Standard Model , but the effect is not strong enough to explain the present baryon asymmetry.
In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7billion-year age of the universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds , indicated a much younger age for globular clusters.
[47]
There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the universe.
Main article: Dark matter
A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best
ΛCDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy
23

During the 1970s and 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 was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.
[48]
The evidence for dark matter comes from its gravitational influence on other matter, and 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.
[49]
Main article: Dark energy
Measurements of the redshift
– magnitude relation for type Ia supernovae have revealed 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 ". Dark energy is indicated by several other lines of evidence.
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.
[8] Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses , and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.
Negative pressure is a property of vacuum energy , but the exact nature of dark energy remains one of the great mysteries of the Big Bang.
Possible candidates include a cosmological constant and quintessence . Results from the WMAP team in 2008, which combined data from the
CMB and other sources, indicate that the universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less then 1% of neutrinos.
[22]
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.
In the ΛCDM , the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general theory of relativity . However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas
24

about quantum gravity . Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.
Main article: 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 that was similar to that in which it started—a Big Crunch .
[34]
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 as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs , neutron stars , and black holes . Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero
—a
Big Freeze . Moreover, if the proton were 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 .
Modern observations of accelerated 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, would remain together, and they too would be subject to heat death , as the universe expands and cools. Other explanations of dark energy—so-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 .
[50]
While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the universe's history. The Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time.
However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature , and a correct treatment of quantum gravity may avoid the singularity.
[51]
There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.
25

26

A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. Image from WMAP press release, 2006. ( Detail )
27

Some proposals, each of which entails untested hypotheses, are:
 models including the Hartle-Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the
 limit of time, but without the need for a singularity.
[52] brane cosmology models [53] 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.
[54][55][56] chaotic inflation , in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.
[57]
Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse , and not the literal beginning.
Main article: Philosophical and religious interpretations of the Big Bang theory
The Big Bang is a scientific theory, and as such stands or falls by its agreement with observations. But as a theory which addresses, or at least seems to address, the origins of reality, it has always been entangled with theological and philosophical implications. In the 1920s and '30s almost every major cosmologist preferred an eternal 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 .
[58] enhanced by the fact that
Georges Lemaître
, who put the theory forth, was a Roman Catholic priest .
This perception was
1.
^ a b c d Hubble, Edwin (1929). "A relation between distance and radial velocity among extra-galactic nebulae". PNAS 15 : 168–173. doi : 0.1073/pnas.15.3.168
.
2.
^ BBC News - 'Big bang' astronomer dies
3.
^ Slipher, V. M.
. " The radial velocity of the Andromeda nebula ". Lowell Observatory Bulletin 1 : 56–57.
Slipher, V. M.
. " Spectrographic observations of nebulae ". Popular Astronomy 23 : 21–24.
4.
^ a b Friedman, A (1922). "Über die Krümmung des Raumes". Z. Phys.
10 : 377–386. (German) (English translation in: Friedman, A
(1999). " On the Curvature of Space ". General Relativity and Gravitation 31 : 1991–2000. doi : 10.1023/A:1026751225741 .)
5.
^ a b Lemaître, G. (1927). "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques".
Annals of the Scientific Society of Brussels 47A : 41. (French) Translated in: " Expansion of the universe, A
28

homogeneous universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae " (1931). Monthly
Notices of the Royal Astronomical Society 91 : 483–490.
6.
^ Lemaître, G. (1931). "The evolution of the universe: discussion". Nature 128 : suppl.: 704.
7.
^ E. Christianson (1995). Edwin Hubble: Mariner of the Nebulae . Farrar Straus & Giroux. ISBN 0374146608 .
8.
^ a b c P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy". Reviews of Modern Physics 75 : 559–606. doi : 10.1103/RevModPhys.75.559
. arXiv : astro-ph/0207347 .
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^ E. A. Milne (1935). Relativity, Gravitation and World Structure . Oxford University Press.
10.
^ R. C. Tolman (1934). Relativity, Thermodynamics, and Cosmology . Oxford: Clarendon Press. LCCN 340-32023. Reissued (1987) New
York: Dover ISBN 0-486-65383-8 .
11.
^ Zwicky, F (1929). " On the Red Shift of Spectral Lines through Interstellar Space ". Proceedings of the National Academy of Sciences
15 : 773–779. Full article PDF (672 KiB ) .
12.
^ Hoyle, Fred (1948). " A New Model for the Expanding universe ". Monthly Notices of the Royal Astronomical Society 108 : 372.
13.
^ R. A. Alpher , H. Bethe , G. Gamow (1948). " The Origin of Chemical Elements ". Physical Review 73 : 803. doi : 10.1103/PhysRev.73.803
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14.
^ R. A. Alpher and R. Herman (1948). "Evolution of the Universe". Nature 162 : 774. doi : 10.1045/march2004-featured.collection
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15.
^ Simon Singh . Big Bang . Retrieved on 2007 05-28 .
16.
^ It is popularly reported that Hoyle intended this to be pejorative. However, Hoyle denied that and said it was just a striking image meant to emphasize the difference between the two theories for radio listeners. See chapter 9 of The Alchemy of the Heavens by Ken Croswell,
Anchor Books, 1995.
17.
^ a b A. A. Penzias and R. W. Wilson (1965). " A Measurement of Excess Antenna Temperature at 4080 Mc/s ". Astrophysical Journal
142 : 419.
18.
^ a b Boggess, N.W., et al. ( COBE collaboration) (1992). "The COBE Mission: Its Design and Performance Two Years after the launch".
Astrophysical Journal 397 : 420, Preprint No. 92-02. doi : 10.1086/171797 .
19.
^ a b D. N. Spergel et al. ( WMAP collaboration) (2006). " Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results:
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20.
^ S. W. Hawking and G. F. R. Ellis (1973). The large-scale structure of space-time . Cambridge: Cambridge University Press. ISBN 0-
521-20016-4 .
21.
^ There is no consensus about how long the Big Bang phase lasted: for some writers this denotes only the initial singularity, for others the whole history of the universe. Usually at least the first few minutes, during which helium is synthesised, are said to occur "during the Big
Bang".
22.
^ a b c d G. Hinshaw, J. L. Weiland, R. S. Hill, N. Odegard, D. Larson, C. L. Bennett, J. Dunkley, B. Gold, M. R. Greason, N. Jarosik, E.
Komatsu, M. R. Nolta, L. Page, D. N. Spergel, E. Wollack, M. Halpern, A. Kogut, M. Limon, S. S. Meyer, G. S. Tucker, E. L. Wright
(2008). " Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results ".
Astrophys. J .
29

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^ Guth, Alan H.
(1998). The Inflationary Universe: Quest for a New Theory of Cosmic Origins . Vintage. ISBN 978-0099959502 .
24.
^ Schewe, Phil, and Ben Stein (2005). " An Ocean of Quarks ". Physics News Update, American Institute of Physics 728 (#1). Retrieved on 2007 05-27 .
25.
^ a b Kolb and Turner (1988), chapter 6
26.
^ Kolb and Turner (1988), chapter 7
27.
^ a b c Kolb and Turner (1988), chapter 4
28.
^ Peacock (1999), chapter 9
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^ Ivanchik, A. V.; A. Y. Potekhin and D. A. Varshalovich (1999). " The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences ". Astronomy and Astrophysics 343 : 459.
30.
^ Detailed information of and references for tests of general relativity are given at Tests of general relativity .
31.
^ This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solar system through the radiation field.
32.
^ Goodman, J. (1995). "Geocentrism reexamined". Physical Review D 52 : 1821. doi : 10.1103/PhysRevD.52.1821
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33.
^ d'Inverno, Ray (1992). Introducing Einstein's Relativity . Oxford: Oxford University Press. ISBN 0-19-859686-3 . Chapter 23
34.
^ a b Kolb and Turner (1988), chapter 3
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^ Gladders, Michael D.; Yee, H. K. C.; Majumdar, Subhabrata; Barrientos, L. Felipe; Hoekstra, Henk; Hall, Patrick B.; Infante, Leopoldo
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^ Peacock (1999), chapter 3
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^ Astronomers reported their measurement in a paper published in the December 2000 issue of Nature titled The microwave background temperature at the redshift of 2.33771
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^ Steigman, Gary. "Primordial Nucleosynthesis: Successes And Challenges". arXiv : astro-ph/0511534 .
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Edmund Bertschinger (1998). " Simulations of structure formation in the universe ". Annual Review of Astronomy and Astrophysics 36 :
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^ If inflation is true, baryogenesis must have occurred, but not vice versa.
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^ a b c Kolb and Turner (1988), chapter 8
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^ Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; but our universe remained close to flat for several billion years, before the dark energy density became significant.
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^ R. H. Dicke and P. J. E. Peebles. "The big bang cosmology — enigmas and nostrums". S. W. Hawking and W. Israel (eds) General
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^ R. Penrose (1979). "Singularities and Time-Asymmetry". S. W. Hawking and W. Israel General Relativity: An Einstein Centenary
Survey : 581–638, Cambridge University Press.
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^ R. Penrose (1989). "Difficulties with Inflationary Cosmology". E. J. Fergus Proc. 14th Texas Symp. on Relativistic Astrophysics : 249-
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^ Navabi, A. A.; N. Riazi (2003). "Is the Age Problem Resolved?". Journal of Astrophysics and Astronomy 24 : 3.
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^ Yao, W. M.; et al. (2006). "Review of Particle Physics". J. Phys. G: Nucl. Part. Phys.
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^ J. Hartle and S. W. Hawking (1983). "Wave function of the universe". Phys. Rev. D 28 : 2960. doi : 10.1088/1126-6708/2005/09/063 .
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^ Linde, Andre (2002). "Inflationary Theory versus Ekpyrotic/Cyclic Scenario". arXiv : hep-th/0205259 .
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^ " Recycled Universe: Theory Could Solve Cosmic Mystery ", Space.com
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^ Kragh, Helge (1996). Cosmology and Controversy . Princeton University Press. ISBN 069100546X .
 Kolb, Edward; Michael Turner (1988). The Early Universe . Addison-Wesley. ISBN 0-201-11604-9 .
 Peacock, John (1999). Cosmological Physics . Cambridge University Press. ISBN 0521422701 .
For an annotated list of textbooks and monographs, see physical cosmology .


Barrow, John D. (1994). The Origin of the Universe: To the Edge of Space and Time . Phoenix, 150.
 Alpher, R. A.; R. Herman (August 1988). Reflections on early work on 'big bang' cosmology . Physics Today , 24–34.
Mather, John C.; John Boslough (1996). The very first light: the true inside story of the scientific journey back to the dawn of the universe , 300. ISBN 0-465-01575-1 .
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


Singh, Simon (2004). Big Bang: The most important scientific discovery of all time and why you need to know about it . Fourth Estate.
 Davies, Paul (1992). The Mind of God . Simon & Schuster UK. ISBN 0-671-71069-9 .
Cosmic Journey: A History of Scientific Cosmology . American Institute of Physics
Feuerbacher, Björn; Ryan Scranton (2006).
Evidence for the Big Bang .
.


Misconceptions about the Big Bang . Scientific American (March 2005).
 The First Few Microseconds . Scientific American (May 2006).
Expansion of the Universe - Standard Big Bang Model . University of Helsinki, astro-ph (February 2008).
 Cosmology at the Open Directory Project
Retrieved from " http://en.wikipedia.org/wiki/Big_Bang "
Categories : Physical cosmology | Theories
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