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                        Big Bang
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Navigatio n                This article is about the cosmological model. For other uses, see Big Bang (disambiguation) .
Main page                  "Big Bang theory" redirects here. For other uses, see Big Bang Theory (disambiguation) .
Co ntents               The Big Bang theory is the prevailing cosmological model that explains the early
Featured co ntent       development of the Universe.[1] According to the Big Bang theory, the Universe was once in
Current events          an extremely hot and dense state which expanded rapidly. This rapid expansion caused the
Rando m article         Universe to cool and resulted in its present continuously expanding state. According to the
Do nate to Wikipedia    most recent measurements and observations, the Big Bang occurred approximately 13.75
                        billion years ago, [2][3] which is thus considered the age of the Universe .[4][5] After its initial
Interactio n            expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted
                        into various subatomic particles, including protons, neutrons, and electrons. While protons
                        and neutrons combined to form the first atomic nuclei only a few minutes after the Big Bang, it
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                        would take thousands of years for electrons to combine with them and create electrically
Co mmunity po rtal
                        neutral atoms. The first element produced was hydrogen, along with traces of helium and
Recent changes
                        lithium. Giant clouds of these primordial elements would coalesce through gravity to form
Co ntact Wikipedia
                        stars and galaxies, and the heavier elements would be synthesiz ed either within stars or               According to the Big Bang model, the
                                                                                                                                Universe expanded from an extremely
                        during supernovae.
                                                                                                                                dense and hot state and continues to
To o lbo x
                        The Big Bang is a well- tested scientific theory which is widely accepted within the scientific         expand today. A common analogy explains
What links here                                                                                                                 that space itself is expanding, carrying
                        community because it is the most accurate and comprehensive explanation for the full range
                                                                                                                                galaxies with it, like spots on an inflating
Related changes         of phenomena astronomers observe. Since its conception, abundant evidence has arisen to                 balloon. The graphic scheme above is an
Uplo ad file            further validate the model.[6] Georges Lemaître first proposed what would become the Big                artist's concept illustrating the expansion of
Special pages           Bang theory in what he called his "hypothesis of the primeval atom." Over time, scientists              a portion of a flat universe.
Permanent link          would build on his initial ideas to form the modern synthesis. The framework for the Big Bang
Cite this page          model relies on Albert Einstein's general relativity and on simplifying assumptions                               K e y t o p ics in
                        such as homogeneity and isotropy of space. The governing equations had been                            Physical cosmology
Print/expo rt           formulated by Alexander Friedmann. In 1929 Edwin Hubble discovered that the
                        distances to far away galaxies were generally proportional to their redshifts—an idea
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                     originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate
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                     that all very distant galaxies and clusters have an apparent velocity directly away
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                     from our vantage point: the farther away, the higher the apparent velocity.[7]
                     If the distance between galaxy clusters is increasing today, everything must have
                     been closer together in the past. This idea has been considered in detail back in time                         Universe · B ig B ang
                                                                                                                                     Age of the universe
Afrikaans            to extreme densities and temperatures, [8][9][10] and large particle accelerators have
                                                                                                                                  Timeline of the Big Bang
Alemannisch          been built to experiment on and test such conditions, resulting in significant             Early unive rse
‫اﻟﻌرﺑﯾ ﺔ‬             confirmation and further development of this model. On the other hand, these                             Inflation · Nucleosynthesis
Arago nés            accelerators have limited capabilities to probe into such high energy regimes. There                    GWB · Neutrino background
Asturianu            is little evidence regarding the absolute earliest instant of the expansion. Thus, the                 Cosmic microwave background
Azərbaycanca         Big Bang theory cannot and does not provide any explanation for such an initial            Exp and ing unive rse
বাংলা                condition; rather, it describes and explains the general evolution of the universe going                    Redshift · Hubble's law
Bân-lâm-gú           forward from that point on. The observed abundances of the light elements throughout                       Metric expansion of space
                                                                                                                                  Friedmann equations
Беларуская           the cosmos closely match the calculated predictions for the formation of these
                                                                                                                                       FLRW metric
беларуская           elements from nuclear processes in the rapidly expanding and cooling first minutes of
(тарашкевіца)                                                                                                   St ruct ure f o rmat io n
                     the universe, as logically and quantitatively detailed according to Big Bang
Български                                                                                                                          Shape of the universe
                     nucleosynthesis. After the discovery of the cosmic microwave background radiation in                           Structure formation
Bo arisch            1964, and especially when its spectrum (i.e., the amount of radiation measured at                                 Reioniz ation
Bo sanski            each wavelength) was found to match that of thermal radiation from a black body,                                Galaxy formation
Brezho neg                                                                                                                         Large- scale structure
                     most scientists had become fairly convinced that some version of the Big Bang
                                                                                                                                      Galaxy filament
Català               scenario best fit observations.
Чӑвашла                                                                                                         Fut ure o f unive rse

Česky                                       Co nt e nt s                                                                     Ultimate fate of the universe
                                                                                                                           Future of an expanding universe
Cymraeg               1 Overview
                                                                                                                C o mp o ne nt s
Dansk                     1.1 Timeline o f the Big Bang
                                                                                                                                  Lambda- CDM model
Deutsch                   1.2 Underlying assumptio ns
                                                                                                                                Dark energy · Dark matter
Eesti                     1.3 FLRW metric                                                                                         Dark fluid · Dark flow
Ελληνικά                  1.4 Ho rizo ns                                                                        Hist o ry o f co smo lo g ical t he o rie s
Españo l              2 Histo ry                                                                                         Timeline of cosmological theories
Esperanto                 2.1 Etymo lo gy                                                                                  History of the Big Bang theory
Euskara                   2.2 Develo pment                                                                      Discovery of cosmic microwave background radiation
‫ﻓﺎرﺳﯽ‬                 3 Observatio nal evidence                                                                 Exp e rime nt s
Fiji Hindi                3.1 Hubble's law and the expansio n o f space                                                      Observational cosmology
Français                  3.2 Co smic micro wave backgro und radiatio n                                                            2dF · SDSS
                                                                                                                        COBE · BOOMERanG · WMAP · Planck
Frysk                     3.3 Abundance o f primo rdial elements
Gaeilge                   3.4 Galactic evo lutio n and distributio n                                            Scie nt ist s

Galego                    3.5 Primo rdial gas clo uds                                                               Isaac Newton · Einstein · Hawking · Friedman

Galego                   3.5 Primo rdial gas clo uds
                                                                                                                          Lemaître · Hubble · Penz ias · Wilson
한국어                      3.6 Other lines o f evidence
                                                                                                                          Gamow · Dicke · Z el'dovich · Mather
Հայերեն              4 Features and pro blems                                                                                    Rubin · Smoot · others
िह दी                    4.1 Ho rizo n pro blem                                                                   So cial imp act
Hrvatski                 4.2 Flatness pro blem                                                                                  Religious interpretations
Bahasa Indo nesia        4.3 Dark energy
Interlingua              4.4 Dark matter                                                                                            Ast ro no my Po rt al
                                                                                                                                         C at e g o ry
Íslenska                 4.5 Magnetic mo no po les
Italiano                 4.6 Baryo n asymmetry                                                                                                                    V · T· E ·

‫ע ברית‬                   4.7 Glo bular cluster age
Basa Jawa            5 The future acco rding to the Big Bang theo ry
Къарачай-Малкъар     6 Speculative physics beyo nd Big Bang theo ry
ქართული              7 Religio us and philo so phical implicatio ns
Kerno wek            8 No tes
Kreyò l ayisyen      9 References
Лезги                    9 .1 Bo o ks
Latina               10 Further reading
Latviešu             11 External links
Lietuvių            Overview
Македо нски
                    Timeline of the Big Bang
                       Main article: Timeline of the Big Bang
‫ﻣﺻر ى‬               Extrapolation of the expansion of the Universe backwards in time using general relativity
Bahasa Melayu       yields an infinite density and temperature at a finite time in the past. [11] This singularity            A graphical timeline is available at
Mirandés            signals the breakdown of general relativity. How closely we can extrapolate towards the                   Graphical timeline of the Big Bang
Мо нго л            singularity is debated—certainly no closer than the end of the Planck epoch. This
Nederlands          singularity is sometimes called "the Big Bang", [12] but the term can also refer to the early hot, dense phase itself, [13][notes 1] which can be
नेपाल भाषा          considered the "birth" of our Universe. Based on measurements of the expansion using Type Ia supernovae, measurements of
日本語                 temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the Universe has
no rsk (bo kmål)    a calculated age of 13.75 ± 0.11 billion years.[15] The agreement of these three independent measurements strongly supports the ΛCDM
no rsk (nyno rsk)   model that describes in detail the contents of the Universe.
No uo rmand         The earliest phases of the Big Bang are subject to much speculation. In the most common models the Universe was filled
Occitan             homogeneously and isotropically with an incredibly high energy density and huge temperatures and pressures and was very rapidly
‫ﭘﻧﺟﺎﺑﯽ‬              expanding and cooling. Approximately 10 −37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the
Plattdüütsch        Universe grew exponentially.[16] After inflation stopped, the Universe consisted of a quark–gluon plasma, as well as all other elementary

Po lski             particles.[17] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of
Po rtuguês          all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated
Ro mână             the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order
Runa Simi           of one part in 30 million. This resulted in the predominance of matter over antimatter in the present Universe. [18]
                    The Universe continued to grow in siz e 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. [19]
                    After about 10 −11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle
सं कतम्
                    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
Simple English
                    positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy
Slo venčina
                    density of the Universe was dominated by photons (with a minor contribution from neutrinos).
Slo venščina
‫ﮐورد ی‬              A few minutes into the expansion, when the temperature was about a billion (one thousand million; 10 9 ; SI prefix giga- ) kelvin and the
Српски / srpski     density was about that of air, neutrons combined with protons to form the Universe's deuterium and helium nuclei in a process called Big
Srpsko hrvatski /   Bang nucleosynthesis.[20] Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density
српско хрватски     of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into
Basa Sunda          atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation
Suo mi              is known as the cosmic microwave background radiation.[21]
                                                              Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter
Tagalo g
                                                              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 four possible types of matter are
                                                              known as cold dark matter, warm dark matter, hot dark matter and baryonic matter. The best
                                                              measurements available (from 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
                                                              reioniz ation[22]), and is estimated to make up about 23% of the matter/energy of the universe,
                                                              while baryonic matter makes up about 4.6%.[15] In an "extended model" which includes hot dark
                                                              matter in the form of neutrinos, then if the "physical baryon density" Ω b h2 is estimated at about
Tiếng Việt
                                                              0.023 (this is different from the 'baryon density' Ω b expressed as a fraction of the total
Võ ro
                                                              matter/energy density, which as noted above is about 0.046), and the corresponding cold dark
                     The Hubble Ultra Deep Field              matter density Ω c h2 is about 0.11, the corresponding neutrino density Ω v h2 is estimated to be
                     showcases galaxies from an ancient       less than 0.0062.[15]
‫יי ִדיש‬              era when the Universe was younger,
粵語                   denser, and warmer according to the      Independent lines of evidence from Type Ia supernovae and the CMB imply that the Universe
Žemaitėška           Big Bang theory.                         today is dominated by a mysterious form of energy known as dark energy, which apparently

中文                                               permeates all of space. The observations suggest 73% 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. [23]
     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

     Underlying assumptions
     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 .[24] 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.[notes 2]
     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. [notes 3][citation needed] The Universe has been measured to
     be homogeneous on the largest scales at the 10% level.[25]

     FLRW metric
        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 siz e 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. [26]

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 horiz ons. 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 horiz on 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 is a past horiz on, 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 horiz on
recedes in space. If the expansion of the Universe continues to accelerate, there is a future horiz on as well.[27]

   Main article: History of the Big Bang theory
   See also: Timeline of cosmology

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an
alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking
image meant to highlight the difference between the two models.[28][29][30]

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.[31][32] 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 at that time. [33] In 1924 Edwin Hubble's
                                                                                                         Artist's depiction of the WMAP

                                                                                                        Artist's depiction of the WMAP
measurement of the great distance to the nearest spiral nebulae showed that these systems               satellite gathering data to help
were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges               scientists understand the Big Bang
Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of
the nebulae was due to the expansion of the Universe.[34]
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.[35]
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.[7][36] Lemaître had already shown that this was expected, given the Cosmological Principle.[23]
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.[37] This perception was enhanced by the fact that the originator of the Big Bang theory, Monsignor Georges
Lemaître, was a Roman Catholic priest.[38] 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. [39][40] Lemaître, however, thought that

      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.[41]

During the 1930s other ideas were proposed as non- standard cosmologies to explain Hubble's observations, including the Milne
model, [42] the oscillatory Universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman) [43] and
Fritz Zwicky's tired light hypothesis.[44]
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.[45] The other was Lemaître's Big
Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN) [46] and whose associates,
Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation (CMB).[47] 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.[48][notes 4] 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 cosmic microwave background
radiation in 1964 [50] 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.
Significant progress in Big Bang cosmology have 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 COBE, [51] the Hubble Space Telescope and WMAP.[52] 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.

Observational evidence
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                    " [T]he big bang picture is too firmly
                                                                                                        grounded in data from every area to be
background, the relative abundances of light elements produced by Big Bang nucleosynthesis,
                                                                                                        proved invalid in its general features."
and today also the large scale distribution and apparent evolution of galaxies [54] predicted to
                                                                                                        Lawrence Krauss [5 3]
occur due to gravitational growth of structure in the standard theory. These are sometimes
called "the four pillars of the Big Bang theory" .[55]

Hubble's law and the expansion of space
   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:[7]
   v = H 0 D,
   v is the recessional velocity of the galaxy or other distant object,
   D is the comoving distance to the object, and
   H 0 is Hubble's constant, measured to be 70.4 −1.4 km/s/Mpc by the WMAP probe.[15]
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 Friedmann in 1922[33] and Georges Lemaître in 1927, [34] 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 comoving distance, v is the recessional velocity, and v , H, and
D vary as the Universe expands (hence we write H 0 to denote the present- day Hubble "constant"). For distances much smaller than the
siz e 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.[56]
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, [7]
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.[57] 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.

Cosmic microwave background radiation
   Main article: Cosmic microwave background radiation
In 1964 Arno Penz ias and Robert Wilson serendipitously discovered the cosmic background
radiation, an omnidirectional signal in the microwave band.[50] Their discovery provided
substantial confirmation of the general CMB predictions: the radiation was found to be consistent
with an almost perfect black body 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 Penz ias and Wilson were awarded
a Nobel Priz e in 1978.
                                                                                                         WMAP image of the cosmic
The surface of last scattering corresponding to emission of the CMB occurs shortly after                 microwave background radiation. The
                                                                                                         radiation is isotropic to roughly one part
recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe
                                                                                                         in 100,000.[5 8 ]
comprised a hot dense photon- baryon plasma sea where photons were quickly scattered from
free charged particles. Peaking at around 372 ± 14 ka, [22] the mean free path for a photon
becomes long enough to reach the present day and the universe becomes transparent.
                                          In 1989 NASA launched the Cosmic Background Explorer satellite (COBE). Its findings were

                                             In 1989 NASA launched the Cosmic Background Explorer satellite (COBE). Its findings were
                                             consistent with predictions regarding the CMB, finding a residual temperature of 2.726 K (more
                                             recent measurements have revised this figure down slightly to 2.725 K) and providing the first
                                             evidence for fluctuations (anisotropies) in the CMB, at a level of about one part in 10 5 .[51] John
                                             C. Mather and George Smoot were awarded the Nobel Priz e 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 shape of the Universe to be spatially almost flat by measuring the
                                             typical angular siz e (the siz e on the sky) of the anisotropies.
                                             In early 2003 the first results of the Wilkinson Microwave Anisotropy Probe (WMAP) were
 The cosmic microwave background             released, yielding what were at the time the most accurate values for some of the cosmological
 spectrum measured by the FIRAS
                                             parameters. The results disproved several specific cosmic inflation models, but are consistent
 instrument on the COBE satellite is the
 most- precisely measured black body         with the inflation theory in general.[52] The Planck space probe was launched in May 2009. Other
 spectrum in nature.[5 9 ] The data points   ground and balloon based cosmic microwave background experiments are ongoing.
 and error bars on this graph are
 obscured by the theoretical curve.          Abundance of primordial elements
                                             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.[20] 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 number)
                     4                       2                     3                          7
are about 0.25 for He/H, about 10 −3 for H/H, about 10 −4 for He/H and about 10 −9 for Li/H.[20]
The measured abundances all agree at least roughly with those predicted from a single value of the baryon- to- photon ratio. The
                                                                           4                                7
agreement is excellent for deuterium, close but formally discrepant for He, and off by a factor of two Li; in the latter two cases there are
substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by Big Bang nucleosynthesis 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.[60] 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 He, and in constant ratios, too.

Galactic evolution and distribution
   Main articles: Galaxy formation and evolution , Large-scale structure of the cosmos , and Structure formation
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
                                                                                                       This panoramic view of the entire
formed at similar distances but shortly after the Big Bang. These observations are strong              near- infrared sky reveals the
arguments against the steady- state model. Observations of star formation, galaxy and quasar           distribution of galaxies beyond the
distributions and larger structures agree well with Big Bang simulations of the formation of           Milky Way. The galaxies are color
                                                                                                       coded by redshift.
structure in the Universe and are helping to complete details of the theory.[61][62]

Primordial gas clouds
In 2011 astronomers have found pristine clouds of the primordial gas that formed in the first few minutes after the Big Bang. The
composition of the gas matches theoretical predictions, providing direct evidence in support of the modern cosmological explanation for
the origins of elements in the universe. The researchers discovered the two clouds of pristine gas by analyz ing the light from distant
quasars, using the HIRES spectrometer on the Keck I Telescope at the W. M. Keck Observatory in Hawaii. They saw absorption lines in
the spectrum where the light was absorbed by the gas, and that allows them to measure the composition of the gas.[63][64]

Other lines of evidence
The age of 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.[65]
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.[66] 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.[67][68]

Features and problems
While the scientific community now supports the Big Bang model over other cosmological models, it 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. 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.[citation needed]
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 observations that are independent from any cosmological model; these include the abundance of light elements , the cosmic
microwave background, large scale structure, and the Hubble diagram for Type Ia supernovae.
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
to the most active laboratory investigations.[69] Remaining issues, such as the cuspy halo problem and the dwarf galaxy problem of cold
dark matter, are not fatal to the dark matter explanation as solutions to such problems exist which involve only further refinements of the
theory. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be
On the other hand, inflation and baryogenesis remain somewhat more speculative features of current Big Bang models: they explain
important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the
theory.[notes 5][citation needed] Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved
problems in physics.

Horizon problem
   Main article: Horizon problem
The horiz on problem results from the premise that information cannot travel faster than light . In a Universe of finite age this sets a limit
—the particle horiz on—on the separation of any two regions of space that are in causal contact.[71] 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 horiz on 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.
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 horiz on 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 horiz on. 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.
If inflation occurred, exponential expansion would push large regions of space well beyond our observable horiz on.

Flatness problem
   Main article: Flatness problem
The flatness problem (also known as the oldness problem) is an observational problem

The flatness problem (also known as the oldness problem) is an observational problem
associated with a Friedmann–Lemaître–Robertson–Walker metric .[71] The Universe may
have positive, negative or z ero spatial curvature depending on its total energy density.
Curvature is negative if its density is less than the critical density, positive if greater, and
z ero 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.[notes 6] Given that a natural timescale for departure from
flatness might be the Planck time, 10 −43 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 density must have been within one part in 10 14 of its
critical value, or it would not exist as it does today.[72]
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                     The overall geometry of the Universe is
smoothed out. Thus, it is theoriz ed that inflation drove the Universe to a very nearly             determined by whether the Omega cosmological
spatially flat state, with almost exactly the critical density.                                     parameter is less than, equal to or greater than 1.
                                                                                                    Shown from top to bottom are a closed Universe
                                                                                                    with positive curvature, a hyperbolic Universe with
Dark energy                                                                                         negative curvature and a flat Universe with z ero
   Main article: Dark energy                                                                        curvature.

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". 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.[23] 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 contributions to mass/energy density in the Universe today are approximately 73%
dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.[15] 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, a leading model of the Big Bang, [citation needed] dark energy is explained by the presence of a cosmological constant in the
general theory of relativity .[citation needed] However, the siz e of the constant that properly explains dark energy is surprisingly small relative
to naive estimates based on ideas about quantum gravity.[citation needed] Distinguishing between the cosmological constant and other
explanations of dark energy is an active area of current research.[citation needed]

Dark matter
   Main article: Dark matter
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              A pie chart indicating the proportional composition of different
observations: the anisotropies in the CMB, galaxy cluster velocity               energy- density components of the Universe, according to the best
                                                                                 ΛCDM model fits – roughly 95% is in the exotic forms of dark matter
dispersions, large- scale structure distributions, gravitational lensing         and dark energy
studies, and X- ray measurements of galaxy clusters. [73]
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.[74]

Magnetic monopoles
   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. [71]
A resolution to the horiz on, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature

Baryon asymmetry
   Main article: Baryon asymmetry
It is not yet understood why the Universe has more matter than antimatter.[18] 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 hypothesiz ed 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.[77] All
these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.

Globular cluster age
In the mid- 1990s observations of globular clusters appeared to be inconsistent with the Big Bang theory. 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 13.7 billion year age of the Universe. This issue was partially 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. [78] There
remain some questions as to how accurately the ages of the clusters are measured, but it is clear that observations of globular clusters
no longer appear inconsistent with the Big Bang theory.

The future according to the Big Bang theory
   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 siz e 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.[27] 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. 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 z ero—a Big Freez e. 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 organiz ed form of energy could be extracted from it, a scenario known as heat death.
Modern observations of accelerating expansion imply that more and more of the currently visible Universe will pass beyond our event
horiz on 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.[79]

Speculative physics beyond Big Bang theory
While the Big Bang model is well established in cosmology, it is likely to

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 equations of classical general relativity indicate a
singularity at the origin of cosmic time, although this conclusion depends
on several assumptions. Moreover, general relativity must break down
before the Universe reaches the Planck temperature, and a correct
treatment of quantum gravity may avoid the would- be singularity.[80]
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.[81]
   Big Bang lattice model [82] states that the Universe at the moment of
   the Big Bang consists of an infinite lattice of fermions which is smeared                This is an artist's concept of the Universe expansion, where
                                                                                            space (including hypothetical non- observable portions of the
   over the fundamental domain so it has both rotational, translational and
                                                                                            Universe) is represented at each time by the circular sections.
   gauge symmetry. The symmetry is the largest symmetry possible and                        Note on the left the dramatic expansion (not to scale) occurring
   hence the lowest entropy of any state.                                                   in the inflationary epoch, and at the center the expansion
                                                                                            acceleration. The scheme is decorated with WMAP images on
   brane cosmology models [83] in which inflation is due to the movement
                                                                                            the left and with the representation of stars at the appropriate
   of branes in string theory; the pre- Big Bang model; the ekpyrotic model,                level of development.
   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 endlessly cycles from one
   process to the other.[84][85][86]
   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.[87][88]
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

Religious and philosophical implications
   Main article: Religious interpretations of the Big Bang theory
As a theory relevant to the origin of the universe, the Big Bang has significant bearing on religion and philosophy. [89][90] As a result, it has
become one of the liveliest areas in the discourse between science and religion.[91] Some believe the Big Bang implies a creator, [92]
while others argue that Big Bang cosmology makes the notion of a creator superfluous.[90][93]

   1. ^ There is no co nsensus abo ut ho w lo ng the Big Bang phase lasted. Fo r so me writers this deno tes o nly the initial singularity, fo r o thers the
  1. ^ There is no co nsensus abo ut ho w lo ng the Big Bang phase lasted. Fo r so me writers this deno tes o nly the initial singularity, fo r o thers the
     who le histo ry o f the Universe. Usually, at least the first few minutes (during which helium is synthesized) are said to o ccur "during the Big
     Bang".[ 14] (see also Big Bang nucleo synthesis)
  2. ^ Detailed info rmatio n o f and references fo r tests o f general relativity are given in the article tests o f general relativity.
  3. ^ This igno res the dipo le aniso tro py at a level o f 0 .1% due to the peculiar velo city o f the so lar system thro ugh the radiatio n field.
  4. ^ It is co mmo nly repo rted that Ho yle intended this to be pejo rative. Ho wever, Ho yle later denied that, saying that it was just a striking image
     meant to emphasize the difference between the two theo ries fo r radio listeners.[ 49 ]
  5. ^ If inflatio n is true, baryo genesis must have o ccurred, but no t vice versa.
  6 . ^ Strictly, dark energy in the fo rm o f a co smo lo gical co nstant drives the Universe to wards a flat state; ho wever, o ur Universe remained clo se to
      flat fo r several billio n years, befo re the dark energy density became significant.

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  Kolb, E.; Turner, M. (1988). The Early Universe. Addison–Wesley. ISBN 0- 201- 11604- 9.
  Peacock, J. (1999). Cosmological Physics . Cambridge University Press. ISBN 0- 521- 42270- 1.

Further reading
  For an annotated list of textbooks and monographs, see physical cosmology .
  Barrow, J.D. (1994). The Origin of the Universe: To the Edge of Space and Time . New York: Phoenix. ISBN 0- 465- 05354- 8.
  Alpher, R.A.; Herman, R. (1988). "Reflections on early work on 'big bang' cosmology". Physics Today 8: 24–34.
  Mather, J.C.; Boslough, J. (1996). The very first light: the true inside story of the scientific journey back to the dawn of the Universe .
  Basic Books. p. 300. ISBN 0- 465- 01575- 1.
  Singh, S. (2004). Big Bang: The origins of the universe . Fourth Estate. ISBN 0- 00- 716220- 0.
  Davies, P.C.W. (1992). The Mind of God: The scientific basis for a rational world . Simon & Schuster . ISBN 0- 671- 71069- 9.
  "Cosmic Journey: A History of Scientific Cosmology"                . American Institute of Physics .
  Feuerbacher, B.; Scranton, R. (2006). "Evidence for the Big Bang"                  . TalkOrigins.
  "Misconceptions about the Big Bang"              . Scientific American. March 2005.
  "The First Few Microseconds"            . Scientific American. May 2006.

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   V · T· E ·                                                         T im e line o f t he Big Bang
  B ig B ang · Planck epoch · Grand unification epoch · Electroweak epoch (Inflationary epoch, Reheating, Baryogenesis) · Quark epoch · Hadron epoch ·
                  Lepton epoch · Photon epoch (Big Bang nucleosynthesis, Matter domination, Recombination) · Dark ages · Reioniz ation ·
                                                        See also: Graphical timeline of the Big Bang ·

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