The Origin of Universe 1

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The Big Bang theory is the prevailingcosmologicalmodelof the early development of the universe.[1]The

major premise of the Big Bang theory is that the universe was once in an extremely hot and dense state

thatexpandedrapidly (a "Big Bang"). This rapid expansion caused the young universe to cool and

resulted in its present continuously expanding state. According to recent measurements, scientific

evidence, and observations,[2][3]the original state happened around 13.7 billion years ago (see age of the

Universe),[4][5]which can be referred to as the time that the Big Bang occurred.[6][7]

Georges Lematre proposed what became known as the Big Bang theory of the origin of the Universe; he

called it his "hypothesis of the primeval atom". The framework for the model relies on Albert

Einstein's general relativity and on simplifying assumptions (such ashomogeneity and isotropy of space).

The governing equations had been formulated by Alexander Friedmann. In 1929, Edwin

Hubblediscovered that the distances to far away galaxieswere generally proportional to theirredshifts

an idea originally suggested by Lematre in 1927. Hubble's observation was taken to indicate that all very

distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther

away, the higher the apparent velocity.[8]

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 to extremedensities and temperatures,[9][10]

[11] and large particle acceleratorshave been built to experiment on and test such conditions, resulting in

significant confirmation of this theory. On the other hand, these accelerators have limited capabilities to

probe into such high energy regimes. There is little evidence regarding the absolute earliest instant of the

expansion. Thus, the Big Bang theory cannotand does notprovide any explanation for such an initial

condition; rather, it describes and explains the general evolution of the universe going forward from that

point on. The observed abundances of the light elements 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.

Fred Hoyle is credited with coining the term Big Bangduring 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.[12][13][14]After the discovery of thecosmic microwave background

radiationin 1964, and especially when its spectrum (i.e., the amount of radiation measured at each

wavelength) was found to match that ofthermal radiation from a black body, most scientists were fairly

convinced by the evidence that some version of the Big Bang scenario must have occurred.

Contents

[hide]

1 Motivation and development

2 Overview

o 2.1 Timeline of the Big Bang

o 2.2 Underlying assumptions

o 2.3 FLRW metric
http://en.wikipedia.org/wiki/Physical_cosmologyhttp://en.wikipedia.org/wiki/Physical_cosmologyhttp://en.wikipedia.org/wiki/Physical_cosmologyhttp://en.wikipedia.org/wiki/Scientific_theoryhttp://en.wikipedia.org/wiki/Scientific_theoryhttp://en.wikipedia.org/wiki/Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-0http://en.wikipedia.org/wiki/Big_Bang#cite_note-0http://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Observational_cosmologyhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-1http://en.wikipedia.org/wiki/Big_Bang#cite_note-1http://en.wikipedia.org/wiki/Big_Bang#cite_note-2http://en.wikipedia.org/wiki/Big_Bang#cite_note-2http://en.wikipedia.org/wiki/Age_of_the_Universehttp://en.wikipedia.org/wiki/Age_of_the_Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-3http://en.wikipedia.org/wiki/Big_Bang#cite_note-3http://en.wikipedia.org/wiki/Big_Bang#cite_note-4http://en.wikipedia.org/wiki/Big_Bang#cite_note-4http://en.wikipedia.org/wiki/Big_Bang#cite_note-5http://en.wikipedia.org/wiki/Big_Bang#cite_note-6http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Hypothesishttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/Homogeneity_(physics)#Translation_invariancehttp://en.wikipedia.org/wiki/Isotropyhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Edwin_Hubblehttp://en.wikipedia.org/wiki/Edwin_Hubblehttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-8http://en.wikipedia.org/wiki/Big_Bang#cite_note-9http://en.wikipedia.org/wiki/Big_Bang#cite_note-10http://en.wikipedia.org/wiki/Particle_acceleratorhttp://en.wikipedia.org/wiki/Particle_acceleratorhttp://en.wikipedia.org/wiki/Scientific_theoryhttp://en.wikipedia.org/wiki/High-energy_physicshttp://en.wikipedia.org/wiki/Big_Bang_nucleosynthesishttp://en.wikipedia.org/wiki/Fred_Hoylehttp://en.wikipedia.org/wiki/Steady_state_universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-11http://en.wikipedia.org/wiki/Big_Bang#cite_note-12http://en.wikipedia.org/wiki/Big_Bang#cite_note-13http://en.wikipedia.org/wiki/Big_Bang#cite_note-13http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Thermal_radiationhttp://en.wikipedia.org/wiki/Black_bodyhttp://en.wikipedia.org/wiki/Big_Banghttp://en.wikipedia.org/wiki/Big_Banghttp://en.wikipedia.org/wiki/Big_Banghttp://en.wikipedia.org/wiki/Big_Bang#Motivation_and_developmenthttp://en.wikipedia.org/wiki/Big_Bang#Motivation_and_developmenthttp://en.wikipedia.org/wiki/Big_Bang#Overviewhttp://en.wikipedia.org/wiki/Big_Bang#Timeline_of_the_Big_Banghttp://en.wikipedia.org/wiki/Big_Bang#Underlying_assumptionshttp://en.wikipedia.org/wiki/Big_Bang#FLRW_metrichttp://en.wikipedia.org/wiki/Scientific_theoryhttp://en.wikipedia.org/wiki/Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-0http://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Scientific_methodhttp://en.wikipedia.org/wiki/Observational_cosmologyhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-1http://en.wikipedia.org/wiki/Big_Bang#cite_note-2http://en.wikipedia.org/wiki/Age_of_the_Universehttp://en.wikipedia.org/wiki/Age_of_the_Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-3http://en.wikipedia.org/wiki/Big_Bang#cite_note-4http://en.wikipedia.org/wiki/Big_Bang#cite_note-5http://en.wikipedia.org/wiki/Big_Bang#cite_note-6http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Hypothesishttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/Homogeneity_(physics)#Translation_invariancehttp://en.wikipedia.org/wiki/Isotropyhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Edwin_Hubblehttp://en.wikipedia.org/wiki/Edwin_Hubblehttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-8http://en.wikipedia.org/wiki/Big_Bang#cite_note-9http://en.wikipedia.org/wiki/Big_Bang#cite_note-10http://en.wikipedia.org/wiki/Particle_acceleratorhttp://en.wikipedia.org/wiki/Scientific_theoryhttp://en.wikipedia.org/wiki/High-energy_physicshttp://en.wikipedia.org/wiki/Big_Bang_nucleosynthesishttp://en.wikipedia.org/wiki/Fred_Hoylehttp://en.wikipedia.org/wiki/Steady_state_universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-11http://en.wikipedia.org/wiki/Big_Bang#cite_note-12http://en.wikipedia.org/wiki/Big_Bang#cite_note-13http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Thermal_radiationhttp://en.wikipedia.org/wiki/Black_bodyhttp://en.wikipedia.org/wiki/Big_Banghttp://en.wikipedia.org/wiki/Big_Bang#Motivation_and_developmenthttp://en.wikipedia.org/wiki/Big_Bang#Overviewhttp://en.wikipedia.org/wiki/Big_Bang#Timeline_of_the_Big_Banghttp://en.wikipedia.org/wiki/Big_Bang#Underlying_assumptionshttp://en.wikipedia.org/wiki/Big_Bang#FLRW_metrichttp://en.wikipedia.org/wiki/Physical_cosmology


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o 2.4 Horizons

3 Observational evidence

o 3.1 Hubble's law and the

expansion of space

o 3.2 Cosmic microwave background

radiation

o 3.3 Abundance of primordial

elements

o 3.4 Galactic evolution and

distribution

o 3.5 Other lines of evidence

4 Features, issues and problems

o 4.1 Horizon problem

o 4.2 Flatness/oldness problem

o 4.3 Magnetic monopoles

o 4.4 Baryon asymmetry

o 4.5 Globular cluster age

o 4.6 Dark matter

o 4.7 Dark energy

5 The future according to the Big Bang

theory

6 Speculative physics beyond Big Bang

theory

7 Religious interpretations

8 Notes

9 References

o 9.1 Books

10 Further reading

11 External links

Motivation and development
http://en.wikipedia.org/wiki/Big_Bang#Horizonshttp://en.wikipedia.org/wiki/Big_Bang#Observational_evidencehttp://en.wikipedia.org/wiki/Big_Bang#Hubble.27s_law_and_the_expansion_of_spacehttp://en.wikipedia.org/wiki/Big_Bang#Hubble.27s_law_and_the_expansion_of_spacehttp://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Big_Bang#Abundance_of_primordial_elementshttp://en.wikipedia.org/wiki/Big_Bang#Abundance_of_primordial_elementshttp://en.wikipedia.org/wiki/Big_Bang#Abundance_of_primordial_elementshttp://en.wikipedia.org/wiki/Big_Bang#Galactic_evolution_and_distributionhttp://en.wikipedia.org/wiki/Big_Bang#Galactic_evolution_and_distributionhttp://en.wikipedia.org/wiki/Big_Bang#Other_lines_of_evidencehttp://en.wikipedia.org/wiki/Big_Bang#Features.2C_issues_and_problemshttp://en.wikipedia.org/wiki/Big_Bang#Horizon_problemhttp://en.wikipedia.org/wiki/Big_Bang#Flatness.2Foldness_problemhttp://en.wikipedia.org/wiki/Big_Bang#Magnetic_monopoleshttp://en.wikipedia.org/wiki/Big_Bang#Baryon_asymmetryhttp://en.wikipedia.org/wiki/Big_Bang#Globular_cluster_agehttp://en.wikipedia.org/wiki/Big_Bang#Dark_matterhttp://en.wikipedia.org/wiki/Big_Bang#Dark_energyhttp://en.wikipedia.org/wiki/Big_Bang#The_future_according_to_the_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#The_future_according_to_the_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Speculative_physics_beyond_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Speculative_physics_beyond_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Religious_interpretationshttp://en.wikipedia.org/wiki/Big_Bang#Noteshttp://en.wikipedia.org/wiki/Big_Bang#Referenceshttp://en.wikipedia.org/wiki/Big_Bang#Bookshttp://en.wikipedia.org/wiki/Big_Bang#Further_readinghttp://en.wikipedia.org/wiki/Big_Bang#External_linkshttp://en.wikipedia.org/wiki/Big_Bang#Horizonshttp://en.wikipedia.org/wiki/Big_Bang#Observational_evidencehttp://en.wikipedia.org/wiki/Big_Bang#Hubble.27s_law_and_the_expansion_of_spacehttp://en.wikipedia.org/wiki/Big_Bang#Hubble.27s_law_and_the_expansion_of_spacehttp://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Big_Bang#Abundance_of_primordial_elementshttp://en.wikipedia.org/wiki/Big_Bang#Abundance_of_primordial_elementshttp://en.wikipedia.org/wiki/Big_Bang#Galactic_evolution_and_distributionhttp://en.wikipedia.org/wiki/Big_Bang#Galactic_evolution_and_distributionhttp://en.wikipedia.org/wiki/Big_Bang#Other_lines_of_evidencehttp://en.wikipedia.org/wiki/Big_Bang#Features.2C_issues_and_problemshttp://en.wikipedia.org/wiki/Big_Bang#Horizon_problemhttp://en.wikipedia.org/wiki/Big_Bang#Flatness.2Foldness_problemhttp://en.wikipedia.org/wiki/Big_Bang#Magnetic_monopoleshttp://en.wikipedia.org/wiki/Big_Bang#Baryon_asymmetryhttp://en.wikipedia.org/wiki/Big_Bang#Globular_cluster_agehttp://en.wikipedia.org/wiki/Big_Bang#Dark_matterhttp://en.wikipedia.org/wiki/Big_Bang#Dark_energyhttp://en.wikipedia.org/wiki/Big_Bang#The_future_according_to_the_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#The_future_according_to_the_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Speculative_physics_beyond_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Speculative_physics_beyond_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Religious_interpretationshttp://en.wikipedia.org/wiki/Big_Bang#Noteshttp://en.wikipedia.org/wiki/Big_Bang#Referenceshttp://en.wikipedia.org/wiki/Big_Bang#Bookshttp://en.wikipedia.org/wiki/Big_Bang#Further_readinghttp://en.wikipedia.org/wiki/Big_Bang#External_links


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Artist's depiction of theWMAP satellite gathering data to help scientists understand the Big Bang

Main article: History of the Big Bang theory

See also:Timeline of cosmologyandHistory of astronomy

The Big Bang theory developed from observations of the structure of the Universe and from theoretical

considerations. In 1912 Vesto Sliphermeasured the firstDoppler shift of a "spiral nebula" (spiral nebula is

the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were recedingfrom 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 ourMilky Way.[15][16] Ten years

later, Alexander Friedmann, aRussiancosmologistand mathematician, derived theFriedmann

equationsfrom Albert Einstein's equationsofgeneral relativity, showing that the Universe might be

expanding in contrast to thestatic Universe model advocated by Einstein at that time.[17]In 1924, Edwin

Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems

were indeed othergalaxies. Independently deriving Friedmann's equations in 1927,Georges Lematre, a

Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due

to the expansion of the Universe.[18]

In 1931 Lematre 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.[19]

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

velocitynow known asHubble's law.[8][20]Lematre had already shown that this was expected, given

the Cosmological Principle.[21]

During the 1930s other ideas were proposed asnon-standard cosmologies to explain Hubble'sobservations, including the Milne model,[22]the oscillatory Universe (originally suggested by Friedmann,

but advocated by Albert Einsteinand Richard Tolman)[23]andFritz Zwicky's tired light hypothesis.[24]

AfterWorld War II, two distinct possibilities emerged. One wasFred 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.[25]The other was Lematre's Big Bang theory,[notes 1]advocated and

developed by George Gamow, who introduced big bang nucleosynthesis(BBN)[26] and whose

associates, Ralph Alpherand Robert Herman, predicted thecosmic microwave background
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thttp://en.wikipedia.org/wiki/Spiral_nebulahttp://en.wikipedia.org/wiki/Shapley%E2%80%93Curtis_debatehttp://en.wikipedia.org/wiki/Shapley%E2%80%93Curtis_debatehttp://en.wikipedia.org/wiki/Milky_Wayhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-14http://en.wikipedia.org/wiki/Big_Bang#cite_note-15http://en.wikipedia.org/wiki/Alexander_Alexandrovich_Friedmanhttp://en.wikipedia.org/wiki/Russiahttp://en.wikipedia.org/wiki/Physical_cosmologyhttp://en.wikipedia.org/wiki/Mathematicianhttp://en.wikipedia.org/wiki/Friedmann_equationshttp://en.wikipedia.org/wiki/Friedmann_equationshttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/Einstein_equationhttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/Static_Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-af1922-16http://en.wikipedia.org/wiki/Edwin_Hubblehttp://en.wikipedia.org/wiki/Edwin_Hubblehttp://en.wikipedia.org/wiki/Galaxieshttp://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-gl1927-17http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-18http://en.wikipedia.org/wiki/Cosmic_distance_ladderhttp://en.wikipedia.org/wiki/Mount_Wilson_Observatoryhttp://en.wikipedia.org/wiki/Mount_Wilson_Observatoryhttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Hubble's_lawhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Big_Bang#cite_note-christianson-19http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Cosmological_Principlehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-peebles-20http://en.wikipedia.org/wiki/Non-standard_cosmologieshttp://en.wikipedia.org/wiki/Milne_modelhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-21http://en.wikipedia.org/wiki/Oscillatory_Universehttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/Richard_Tolmanhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-22http://en.wikipedia.org/wiki/Fritz_Zwickyhttp://en.wikipedia.org/wiki/Tired_lighthttp://en.wikipedia.org/wiki/Big_Bang#cite_note-23http://en.wikipedia.org/wiki/World_War_IIhttp://en.wikipedia.org/wiki/Fred_Hoylehttp://en.wikipedia.org/wiki/Steady_state_modelhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-24http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-25http://en.wikipedia.org/wiki/George_Gamowhttp://en.wikipedia.org/wiki/Big_bang_nucleosynthesishttp://en.wikipedia.org/wiki/Big_Bang#cite_note-26http://en.wikipedia.org/wiki/Ralph_Alpherhttp://en.wikipedia.org/wiki/Robert_Hermanhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation


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radiation(CMB).[27]Ironically, it was Hoyle who coined the phrase that came to be applied to Lematre's

theory, referring to it as "this big bangidea" during a BBC Radiobroadcast in March 1949.[28][notes 2] 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[29] 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 howgalaxies 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.

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances

intelescopetechnology as well as the analysis of copious data from satellites such as COBE,[30] the Hubble Space Telescope and WMAP.[31] 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.

Overview

Timeline of the Big BangMain article: Timeline of the Big Bang

A graphical timeline is available at

Graphical timeline of the Big Bang

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an

infinite densityand temperature at a finite time in the past.[32] Thissingularitysignals the breakdown of

general relativity. How closely we can extrapolate towards the singularity is debatedcertainly no closer

than the end of the Planck epoch. This singularity is sometimes called "the Big Bang", [33] but the term can

also refer to the early hot, dense phase itself,[34][notes 3]which can be 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 functionof

galaxies, the Universe has a calculated age of 13.75 0.11 billion years.[35]The agreement of these three

independent measurements strongly supports the CDM 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 isotropicallywith an incredibly highenergy density,

huge temperatures and pressures, and was very rapidly expanding and cooling. Approximately

1037 seconds into the expansion, a phase transitioncaused acosmic inflation, during which the Universe

grewexponentially.[36]

After inflation stopped, the Universe consisted of a quarkgluon plasma, as well asall otherelementary particles.[37]Temperatures were so high that the random motions of particles were

at relativisticspeeds, and particleantiparticle pairs of all kinds were being continuously created and

destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation

ofbaryon number, leading to a very small excess ofquarksandleptons over antiquarks and antileptons

of the order of one part in 30 million. This resulted in the predominance ofmatteroverantimatterin the

present Universe.[38]
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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 breakingphase transitions put the fundamental forcesof physics and the

parameters ofelementary particles into their present form.[39]After about 1011 seconds, the picture

becomes less speculative, since particle energies drop to values that can be attained inparticle

physicsexperiments. At about 106 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 overantibaryons. The temperature was now no longer high enough to create new protonantiproton pairs

(similarly for neutronsantineutrons), so a mass annihilation immediately followed, leaving just one in

1010 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

byphotons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI

prefix giga-) kelvinand the density was about that of air, neutrons combined with protons to form the

Universe's deuterium and heliumnucleiin a process called Big Bang nucleosynthesis.[40] Most protons

remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density ofmatter came togravitationally 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 thecosmic microwave

background radiation.[41]

The Hubble Ultra Deep Fieldshowcases 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 four possible types of matter are known as cold dark

matter,warm dark matter,hot dark matterand baryonic matter. The best measurements available

(from WMAP) show that the data is well-fit by aLambda-CDM model in which dark matter is assumed to

be cold (warm dark matteris ruled out by earlyreionization[42] ), and is estimated to make up about 23% of

the matter/energy of the universe, while baryonic matter makes up about 4.6%.[35] In an "extended model"
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which includes hot dark matter in the form ofneutrinos, then if the "physical baryon density" bh2 is

estimated at about 0.023 (this is different from the 'baryon density' bexpressed as a fraction of the total

matter/energy density, which as noted above is about 0.046), and the corresponding cold dark matter

density ch2 is about 0.11, the corresponding neutrino density vh

2 is estimated to be less than 0.0062.[35]

Independent lines of evidence fromType Ia supernovae and the CMB imply that the Universe today is

dominated by a mysterious form of energy known asdark energy, which apparently 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

Universeto 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 Modelof particle physics continue to be investigated both observationally and theoretically.[21]

All of this cosmic evolution after theinflationary epoch can be rigorously described and modeled bythe CDM modelof 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 1015 seconds or so. Apparently a new unified theory ofquantum 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.

Underlying assumptions

The Big Bang theory depends on two major assumptions: the universality ofphysical laws, and

the cosmological principle.[citation needed] The cosmological principle states that on large scales the Universe

is homogeneousand 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 105.[43]Also, general

relativity has passed stringenttests 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 4]

If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be

derived from the simplerCopernican 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 105 via

observations of the CMB.[notes 5]The Universe has been measured to be homogeneous on the largest

scales at the 10% level.[44]

FLRW metric

Main articles: FriedmannLematreRobertsonWalker metricandMetric expansion of space

General relativity describes spacetime by ametric, which determines the distances that separate nearby

points. The points, which can be galaxies, stars, or other objects, themselves are specified using

acoordinate chart or "grid" that is laid down over allspacetime. The cosmological principle implies that
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the metric should be homogeneous andisotropic on large scales, which uniquely singles out

the FriedmannLematreRobertsonWalker 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

acoordinate systemto 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 theircoordinate distance (comoving distance) remains constant,thephysicaldistance between two such comoving points expands proportionally with the scale factorof

the Universe.[45]

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 scaleslocal concentrations of matter such as our galaxy are gravitationally bound and as

such do not experience the large-scale expansion of space.

Horizons

Main article: Cosmological horizon

An important feature of the Big Bang spacetime is the presence ofhorizons. 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 apast 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 is a past horizon, though in practice our view is also

limited by the opacity of the Universe at early times. So our view cannot extend further backward in time,

though the horizon recedes in space. If the expansion of the Universe continues to accelerate, there is a

future horizon as well.[46]

Observational evidence

The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in

the redshiftsof galaxies, the detailed measurements of the cosmic microwave background, the

abundance of light elements (see Big Bang nucleosynthesis), and today also the large scale

distributionand apparent evolution of galaxies[47] which are predicted to occur due to gravitational growth

of structure in the standard theory. These are sometimes called "the four pillars of the Big Bang theory".[48]

Hubble's law and the expansion of space

Main articles: Hubble's lawandMetric expansion of space

See also:Distance measures (cosmology)andScale factor (universe)

Observations of distant galaxies and quasarsshow that these objects are redshiftedthelight emitted

from them has been shifted to longer wavelengths. This can be seen by taking afrequency spectrum of an

object and matching the spectroscopic pattern ofemission lines orabsorption linescorresponding

to atomsof the chemical elements interacting with the light. These redshifts are uniformlyisotropic,

distributed evenly among the observed objects in all directions. If the redshiftis interpreted as aDoppler

shift, the recessional velocityof the object can be calculated. For some galaxies, it is possible to estimate
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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:[8]

v= H0D,

where

vis the recessional velocityof the galaxy or other distant object,

D is the comoving distance to the object, and

H0 isHubble's constant, measured to be 70.4 +1.3

1.4km/s/Mpc by theWMAP probe.[35]

Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies

which is untenable given theCopernican Principleor the Universe isuniformly

expandingeverywhere. This universal expansion was predicted fromgeneral relativitybyAlexander

Friedmann in 1922[17]andGeorges Lematre in 1927,[18] well before Hubble made his 1929 analysis

and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann,

Lematre, Robertson and Walker.

The theory requires the relation v= HD to hold at all times, where D is thecomoving distance,vis

the recessional velocity, and v, H, and D vary as the Universe expands (hence we writeH0 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.[49]

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 isotropicandhomogenous,[8] 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 theCopernican Principle, that the Earth is not in a

central position, on a cosmological scale.[50] 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 radiationMain article: Cosmic microwave background radiation
http://en.wikipedia.org/wiki/Cosmic_distance_ladderhttp://en.wikipedia.org/wiki/Hubble's_lawhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Comoving_distancehttp://en.wikipedia.org/wiki/Hubble's_constanthttp://en.wikipedia.org/wiki/Hubble's_constanthttp://en.wikipedia.org/wiki/Hubble's_constanthttp://en.wikipedia.org/wiki/Kilometershttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Megaparsechttp://en.wikipedia.org/wiki/WMAPhttp://en.wikipedia.org/wiki/WMAPhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-wmap7year-37http://en.wikipedia.org/wiki/Hubble's_lawhttp://en.wikipedia.org/wiki/Copernican_Principlehttp://en.wikipedia.org/wiki/Copernican_Principlehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-af1922-16http://en.wikipedia.org/wiki/Big_Bang#cite_note-af1922-16http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-gl1927-17http://en.wikipedia.org/wiki/Big_Bang#cite_note-gl1927-17http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metrichttp://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metrichttp://en.wikipedia.org/wiki/Comoving_distancehttp://en.wikipedia.org/wiki/Comoving_distancehttp://en.wikipedia.org/wiki/Comoving_distancehttp://en.wikipedia.org/wiki/Recessional_velocityhttp://en.wikipedia.org/wiki/Observable_Universehttp://en.wikipedia.org/wiki/Observable_Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-peacock_c3-53http://en.wikipedia.org/wiki/Big_Bang#cite_note-peacock_c3-53http://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Cosmological_Principlehttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Homogeneity_(physics)#Translation_invariancehttp://en.wikipedia.org/wiki/Homogeneity_(physics)#Translation_invariancehttp://en.wikipedia.org/wiki/Homogeneity_(physics)#Translation_invariancehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Copernican_Principlehttp://en.wikipedia.org/wiki/Copernican_Principlehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-54http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/File:WMAP_2010.pnghttp://en.wikipedia.org/wiki/Cosmic_distance_ladderhttp://en.wikipedia.org/wiki/Hubble's_lawhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Comoving_distancehttp://en.wikipedia.org/wiki/Hubble's_constanthttp://en.wikipedia.org/wiki/Kilometershttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Megaparsechttp://en.wikipedia.org/wiki/WMAPhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-wmap7year-37http://en.wikipedia.org/wiki/Hubble's_lawhttp://en.wikipedia.org/wiki/Copernican_Principlehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Alexander_Friedmannhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-af1922-16http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtrehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-gl1927-17http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metrichttp://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metrichttp://en.wikipedia.org/wiki/Comoving_distancehttp://en.wikipedia.org/wiki/Recessional_velocityhttp://en.wikipedia.org/wiki/Observable_Universehttp://en.wikipedia.org/wiki/Observable_Universehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-peacock_c3-53http://en.wikipedia.org/wiki/Metric_expansion_of_spacehttp://en.wikipedia.org/wiki/Cosmological_Principlehttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Homogeneity_(physics)#Translation_invariancehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-hubble-7http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.o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WMAPimage of the cosmic microwave background radiation

During the first few days of the Universe, the Universe was in fullthermal 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. Thetemperature 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 theepoch 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 photonsnow fall into themicrowaveregion of theelectromagnetic spectrum. The radiation is thought to be

observable at every point in the Universe, and comes from all directions with (almost) the same

intensity.

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation

while conducting diagnostic observations using a new microwave receiver owned byBell

Laboratories.[29]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 Kand it

pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were

awarded a Nobel Prizefor their discovery.

The cosmic microwave background spectrum measured by the FIRAS instrument on theCOBE satelliteis the most-

precisely measuredblack body spectrum in nature.[51]Thedata pointsanderror barson this graph are obscured by the

theoretical curve.

In 1989, NASAlaunched 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 105.[30]John C. Matherand George Smootwere awarded
http://en.wikipedia.org/wiki/WMAPhttp://en.wikipedia.org/wiki/WMAPhttp://en.wikipedia.org/wiki/Thermodynamic_equilibriumhttp://en.wikipedia.org/wiki/Thermodynamic_equilibriumhttp://en.wikipedia.org/wiki/Blackbodyhttp://en.wikipedia.org/wiki/Thomson_scatteringhttp://en.wikipedia.org/wiki/Thomson_scatteringhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Recombination_(cosmology)http://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Arno_Penziashttp://en.wikipedia.org/wiki/Robert_Woodrow_Wilsonhttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Bell_Laboratorieshttp://en.wikipedia.org/wiki/Bell_Laboratorieshttp://en.wikipedia.org/wiki/Big_Bang#cite_note-penzias-30http://en.wikipedia.org/wiki/Big_Bang#cite_note-penzias-30http://en.wikipedia.org/wiki/Big_Bang#cite_note-penzias-30http://en.wikipedia.org/wiki/Nobel_Prizehttp://en.wikipedia.org/wiki/Nobel_Prizehttp://en.wikipedia.org/wiki/COBEhttp://en.wikipedia.org/wiki/COBEhttp://en.wikipedia.org/wiki/COBEhttp://en.wikipedia.org/wiki/Black_bodyhttp://en.wikipedia.org/wiki/Black_bodyhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-dpf99-55http://en.wikipedia.org/wiki/Data_pointhttp://en.wikipedia.org/wiki/Data_pointhttp://en.wikipedia.org/wiki/Data_pointhttp://en.wikipedia.org/wiki/Standard_error_of_estimationhttp://en.wikipedia.org/wiki/Standard_error_of_estimationhttp://en.wikipedia.org/wiki/Standard_error_of_estimationhttp://en.wikipedia.org/wiki/National_Aeronautics_and_Space_Administrationhttp://en.wikipedia.org/wiki/National_Aeronautics_and_Space_Administrationhttp://en.wikipedia.org/wiki/Cosmic_Background_Explorer_satellitehttp://en.wikipedia.org/wiki/Cosmic_Background_Explorer_satellitehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-cobe-31http://en.wikipedia.org/wiki/John_C._Matherhttp://en.wikipedia.org/wiki/John_C._Matherhttp://en.wikipedia.org/wiki/George_Smoothttp://en.wikipedia.org/wiki/George_Smoothttp://en.wikipedia.org/wiki/File:Firas_spectrum.jpghttp://en.wikipedia.org/wiki/File:Firas_spectrum.jpghttp://en.wikipedia.org/wiki/File:WMAP_2010.pnghttp://en.wikipedia.org/wiki/WMAPhttp://en.wikipedia.org/wiki/Thermodynamic_equilibriumhttp://en.wikipedia.org/wiki/Blackbodyhttp://en.wikipedia.org/wiki/Thomson_scatteringhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Recombination_(cosmology)http://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Arno_Penziashttp://en.wikipedia.org/wiki/Robert_Woodrow_Wilsonhttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Bell_Laboratorieshttp://en.wikipedia.org/wiki/Bell_Laboratorieshttp://en.wikipedia.org/wiki/Big_Bang#cite_note-penzias-30http://en.wikipedia.org/wiki/Nobel_Prizehttp://en.wikipedia.org/wiki/COBEhttp://en.wikipedia.org/wiki/Black_bodyhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-dpf99-55http://en.wikipedia.org/wiki/Data_pointhttp://en.wikipedia.org/wiki/Standard_error_of_estimationhttp://en.wikipedia.org/wiki/National_Aeronautics_and_Space_Administrationhttp://en.wikipedia.org/wiki/Cosmic_Background_Explorer_satellitehttp://en.wikipedia.org/wiki/Big_Bang#cite_note-cobe-31http://en.wikipedia.org/wiki/John_C._Matherhttp://en.wikipedia.org/wiki/George_Smoot


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the Nobel Prize 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 20002001,

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. (Seeshape of the

Universe.)

In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe (WMAP) were released,

yielding what were at the time the most accurate values for some of the cosmological parameters.

This spacecraft also disproved several specific cosmic inflationmodels, but the results were

consistent with the inflation theory in general,[31]it confirms too that a sea ofcosmic

neutrinospermeates the Universe, a clear evidence that the first stars took more than a half-billion

years to create a cosmic fog. A new space probe namedPlanck, with goals similar to WMAP, was

launched in May 2009. It is anticipated to soon 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 fromregions 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, calledinflation.

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.

Abundance of primordial elements

Main article: Big Bang nucleosynthesis

Using the Big Bang model it is possible to calculate the concentration ofhelium-4, helium-

3,deuterium and lithium-7 in the Universe as ratios to the amount of ordinary hydrogen, H.[40]All the

abundances depend on a single parameter, the ratio ofphotonstobaryons, which itself can be

calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by

mass, not by number) are about 0.25 for4

He/H, about 103 for2

H/H, about 104 for3

He/H and about 109 for7

Li/H.[40]

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 for4

He, and a factor of two off for7

Li; in the latter two cases there are substantialsystematic uncertainties. Nonetheless, the general

consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory isthe 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 2030% helium.[52]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 ofstellar nucleosynthesis products)

should have more helium than deuterium or more deuterium than 3

He, and in constant ratios, too.

Galactic evolution and distribution
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Main articles: Large-scale structure of the cosmos,Structure formation, andGalaxy formation and

evolution

This panoramic view of the entire near-infraredsky reveals the distribution of galaxies beyond the Milky Way. The

galaxies are color coded by redshift.

Detailed observations of the morphology anddistribution of galaxies and quasarsprovide 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 asgalaxy clustersand superclusters. Populations of stars have been aging andevolving, 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 ofstar 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.[53][54]

Other lines of evidence

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 measuredby applying the theory ofstellar evolutionto globular clusters and through radiometric datingof

individual Population IIstars.[citation needed]

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 SunyaevZel'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.[citation needed]

Features, issues and problems

While scientists now prefer the Big Bang model over other cosmological models, the scientific

community was once divided between supporters of the Big Bang and those ofalternative

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 thecosmological observations. With the overwhelming consensus in the community today

supporting the Big Bang model, many of these problems are remembered as being mainly of
http://en.wikipedia.org/wiki/Large-scale_structure_of_the_cosmoshttp://en.wikipedia.org/wiki/Structure_formationhttp://en.wikipedia.org/wiki/Structure_formationhttp://en.wikipedia.org/wiki/Structure_formationhttp://en.wikipedia.org/wiki/Galaxy_formation_and_evolutionhttp://en.wikipedia.org/wiki/Galaxy_formation_and_evolutionhttp://en.wikipedia.org/wiki/Galaxy_formation_and_evolutionhttp://en.wikipedia.org/wiki/Near-infraredhttp://en.wikipedia.org/wiki/Near-infraredhttp://en.wikipedia.org/wiki/Milky_Wayhttp://en.wikipedia.org/wiki/Milky_Wayhttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Galaxy_morphological_classificationhttp://en.wikipedia.org/wiki/Large-scale_structure_of_the_cosmoshttp://en.wikipedia.org/wiki/Large-scale_structure_of_the_cosmoshttp://en.wikipedia.org/wiki/Quasarshttp://en.wikipedia.org/wiki/Quasarshttp://en.wikipedia.org/wiki/Galaxy_groups_and_clustershttp://en.wikipedia.org/wiki/Galaxy_groups_and_clustershttp://en.wikipedia.org/wiki/Galaxy_groups_and_clustershttp://en.wikipedia.org/wiki/Superclusterhttp://en.wikipedia.org/wiki/Superclusterhttp://en.wikipedia.org/wiki/Star_formationhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-57http://en.wikipedia.org/wiki/Big_Bang#cite_note-57http://en.wikipedia.org/wiki/Big_Bang#cite_note-58http://en.wikipedia.org/wiki/Stellar_evolutionhttp://en.wikipedia.org/wiki/Stellar_evolutionhttp://en.wikipedia.org/wiki/Globular_clustershttp://en.wikipedia.org/wiki/Radiometric_datinghttp://en.wikipedia.org/wiki/Radiometric_datinghttp://en.wikipedia.org/wiki/Population_IIhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel'dovich_effecthttp://en.wikipedia.org/wiki/Clusters_of_galaxieshttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Non-standard_cosmologieshttp://en.wikipedia.org/wiki/Non-standard_cosmologieshttp://en.wikipedia.org/wiki/Observational_cosmologyhttp://en.wikipedia.org/wiki/Observational_cosmologyhttp://en.wikipedia.org/wiki/Scientific_consensushttp://en.wikipedia.org/wiki/File:2MASS_LSS_chart-NEW_Nasa.jpghttp://en.wikipedia.org/wiki/File:2MASS_LSS_chart-NEW_Nasa.jpghttp://en.wikipedia.org/wiki/Large-scale_structure_of_the_cosmoshttp://en.wikipedia.org/wiki/Structure_formationhttp://en.wikipedia.org/wiki/Galaxy_formation_and_evolutionhttp://en.wikipedia.org/wiki/Galaxy_formation_and_evolutionhttp://en.wikipedia.org/wiki/Near-infraredhttp://en.wikipedia.org/wiki/Milky_Wayhttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Galaxy_morphological_classificationhttp://en.wikipedia.org/wiki/Large-scale_structure_of_the_cosmoshttp://en.wikipedia.org/wiki/Quasarshttp://en.wikipedia.org/wiki/Galaxy_groups_and_clustershttp://en.wikipedia.org/wiki/Superclusterhttp://en.wikipedia.org/wiki/Star_formationhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-57http://en.wikipedia.org/wiki/Big_Bang#cite_note-58http://en.wikipedia.org/wiki/Stellar_evolutionhttp://en.wikipedia.org/wiki/Globular_clustershttp://en.wikipedia.org/wiki/Radiometric_datinghttp://en.wikipedia.org/wiki/Population_IIhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel'dovich_effecthttp://en.wikipedia.org/wiki/Clusters_of_galaxieshttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Non-standard_cosmologieshttp://en.wikipedia.org/wiki/Non-standard_cosmologieshttp://en.wikipedia.org/wiki/Observational_cosmologyhttp://en.wikipedia.org/wiki/Scientific_consensus


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historical interest; the solutions to them have been obtained either through modifications to the theory

or as the result of better observations.

The core ideas of the Big Bangthe expansion, the early hot state, the formation of helium, the

formation of galaxiesare 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 diagramforType 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

Modelofparticle physics. Of these features,dark matteris currently the subject to the most active

laboratory investigations.[55]Remaining issues, such as the cuspy halo problemand the dwarf galaxy

problemofcold 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

The Origin of Universe 1

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