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Big BangFrom Wikipedia, the free encyclopediaJump to: navigation, search "Big Bang theory" redirects here. For the American TV sitcom, see The Big Bang Theory. Forother uses, see Big Bang (disambiguation).

According to the Big Bang model, the Universe expanded from an extremely dense and hot stateand continues to expand today. A common analogy explains that space itself is expanding,carrying galaxies with it, like spots on an inflating balloon. The graphic scheme above is anartist's concept illustrating the expansion of a portion of a flat universe.

Part of a series on

Physical cosmology

Universe Big Bang

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of light elements, the cosmic microwave background, large scale structure, and the Hubblediagram.[12] The core ideas of the Big Bang the expansion, the early hot state, the formation oflight elements, and the formation of galaxies are derived from these and other observations. Asthe distance between galaxies increases today, in the past galaxies were closer together. Theconsequence of this is that the characteristics of the universe can be calculated in detail back in

time to extreme densities and temperatures,[13][14][15]

while large particle accelerators replicatesuch conditions, resulting in confirmation and refinement of the details of the Big Bang model.On the other hand, these accelerators can only probe so far into high energy regimes, andastronomers are prevented from seeing the absolute earliest moments in the universe by variouscosmological horizons. The earliest instant of the Big Bang expansion is still an area of openinvestigation. The Big Bang theory does not provide any explanation for the initial conditions ofthe universe; rather, it describes and explains the general evolution of the universe going forwardfrom that point on.

Georges Lematre first proposed what became the Big Bang theory in what he called his"hypothesis of the primeval atom". Over time, scientists built on his initial ideas to form the

modern synthesis. The framework for the Big Bang model relies on Albert Einstein's generalrelativity and on simplifying assumptions such as homogeneity and isotropy of space. Thegoverning equations were first formulated by Alexander Friedmann and similar solutions wereworked on by Willem de Sitter . In 1929, Edwin Hubble discovered that the distances to far awaygalaxies were strongly correlated with their redshifts an idea originally suggested by Lematrein 1927. Hubble's observation was taken to indicate that all very distant galaxies and clustershave an apparent velocity directly away from our vantage point: the farther away, the higher theapparent velocity, regardless of direction.[16] Assuming that we are not at the center of a giantexplosion, the only remaining interpretation is that all observable regions of the universe arereceding from each other .

While the scientific community was once divided between supporters of two different expandinguniverse theories the Big Bang and the Steady State theory,[17] observational confirmation ofthe Big Bang scenario came with the discovery of the cosmic microwave background radiation in1964, and later when its spectrum (i.e., the amount of radiation measured at each wavelength)was found to match that of thermal radiation from a black body. Since then, astrophysicists haveincorporated observational and theoretical additions into the Big Bang model, and its parametrization as the Lambda-CDM model serves as the framework for current investigationsof theoretical cosmology.

Contents

1 Overview o 1.1 Timeline of the Big Bang o 1.2 Underlying assumptions o 1.3 FLRW metric o 1.4 Horizons

2 History o 2.1 Etymology o 2.2 Development

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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 Primordial gas clouds o 3.6 Other lines of evidence 4 Related issues in physics

o 4.1 Baryon asymmetry o 4.2 Dark energy o 4.3 Dark matter o 4.4 Globular cluster age

5 Problems o 5.1 Horizon problem o 5.2 Flatness problem o 5.3 Magnetic monopoles

6 The future according to the Big Bang theory 7 Speculative physics beyond the Big Bang theory 8 Religious and philosophical interpretations 9 Notes 10 References

o 10.1 Books 11 Further reading 12 External links

Overview

Timeline of the Big Bang

Main article: Timeline of the Big Bang

A graphical timeline is available atGraphical timeline of the Big Bang

Extrapolation of the expansion of the Universe backwards in time using general relativity yieldsan infinite density and temperature at a finite time in the past.[18] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity isdebated certainly no closer than the end of the Planck epoch. This singularity is sometimescalled "the Big Bang",[19] but the term can also refer to the early hot, dense phase itself ,[20][notes 1] which can be considered the "birth" of our Universe. Based on measurements of the expansionusing Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the Universe has acalculated age of 13.772 0.059 billion years.[22] The agreement of these three independentmeasurements strongly supports the CDM m odel that describes in detail the contents of theUniverse. In 2013 new Planck data corrected this age to 13.798 0.037 billion years.[2]

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ote-arXiv-20121220-23http://en.wikipedia.org/wiki/Lambda-CDM_modelhttp://en.wikipedia.org/wiki/Lambda-CDM_modelhttp://en.wikipedia.org/wiki/Lambda-CDM_modelhttp://en.wikipedia.org/wiki/Lambda-CDM_modelhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-esa-2http://en.wikipedia.org/wiki/Big_Bang#cite_note-esa-2http://en.wikipedia.org/wiki/Big_Bang#cite_note-esa-2http://en.wikipedia.org/wiki/Big_Bang#cite_note-esa-2http://en.wikipedia.org/wiki/Lambda-CDM_modelhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-arXiv-20121220-23http://en.wikipedia.org/wiki/Correlation_functionhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Type_Ia_supernovahttp://en.wikipedia.org/wiki/Big_Bang#cite_note-20http://en.wikipedia.org/wiki/Big_Bang#cite_note-20http://en.wikipedia.org/wiki/Big_Bang#cite_note-19http://en.wikipedia.org/wiki/Planck_epochhttp://en.wikipedia.org/wiki/Gravitational_singularityhttp://en.wikipedia.org/wiki/Big_Bang#cite_note-18http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/General_relativityhttp://en.wikipedia.org/wiki/Graphical_timeline_of_the_Big_Banghttp://en.wikipedia.org/wiki/Timeline_of_the_Big_Banghttp://en.wikipedia.org/wiki/Big_Bang#External_linkshttp://en.wikipedia.org/wiki/Big_Bang#Further_readinghttp://en.wikipedia.org/wiki/Big_Bang#Bookshttp://en.wikipedia.org/wiki/Big_Bang#Referenceshttp://en.wikipedia.org/wiki/Big_Bang#Noteshttp://en.wikipedia.org/wiki/Big_Bang#Religious_and_philosophical_interpretationshttp://en.wikipedia.org/wiki/Big_Bang#Speculative_physics_beyond_the_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#The_future_according_to_the_Big_Bang_theoryhttp://en.wikipedia.org/wiki/Big_Bang#Magnetic_monopoleshttp://en.wikipedia.org/wiki/Big_Bang#Flatness_problemhttp://en.wikipedia.org/wiki/Big_Bang#Horizon_problemhttp://en.wikipedia.org/wiki/Big_Bang#Problemshttp://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#Baryon_asymmetryhttp://en.wikipedia.org/wiki/Big_Bang#Related_issues_in_physicshttp://en.wikipedia.org/wiki/Big_Bang#Other_lines_of_evidencehttp://en.wikipedia.org/wiki/Big_Bang#Primordial_gas_cloudshttp://en.wikipedia.org/wiki/Big_Bang#Galactic_evolution_and_distributionhttp://en.wikipedia.org/wiki/Big_Bang#Abundance_of_primordial_elementshttp://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiationhttp://en.wikipedia.org/wiki/Big_Bang#Hubble.27s_law_and_the_expansion_of_spacehttp://en.wikipedia.org/wiki/Big_Bang#Observational_evidence

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The earliest phases of the Big Bang are subject to much speculation. In the most common modelsthe Universe was filled homogeneously and isotropically with an incredibly high energy density and huge temperatures and pressures and was very rapidly expanding and cooling.Approximately 1037 seconds into the expansion, a phase transition caused a cosmic inflation, during which the Universe grew exponentially.[23] After inflation stopped, the Universe consisted

of a quark gluon plasma, as well as all other elementary particles.[24]

Temperatures were so highthat the random motions of particles were at relativistic speeds, and particle antiparticle pairs ofall kinds were being continuously created and destroyed in collisions. At some point an unknownreaction called baryogenesis violated the conservation of baryon number , leading to a very smallexcess of quarks and leptons over antiquarks and antileptons of the order of one part in30 million. This resulted in the predominance of matter over antimatter in the presentUniverse.[25]

H ubble eXtr eme Deep Fi eld (XD F )

XDF size compared to the size of the moon - several thousand galaxies, each consisting of

billions of stars, are in this small view.

XDF (2012) view - each light speck is a galaxy - some of these are as old as 13.2 billion year s[26] - the universe is estimated to contain 200 billion galaxies.

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XDF image shows fully mature galaxies in the foreground plane - nearly mature galaxies from 5to 9 billion years ago - protogalaxies, blazing with young stars, beyond 9 billion years.

The Universe continued to decrease in density and fall in temperature, hence the typical energyof each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form.[27] After about1011 seconds, the picture becomes less speculative, since particle energies drop to values thatcan be attained in particle physics experiments. At about 106 seconds, quarks and gluonscombined to form baryons such as protons and neutrons. The small excess of quarks overantiquarks led to a small excess of baryons over antibaryons. The temperature was now no longerhigh enough to create new proton antiproton pairs (similarly for neutrons antineutrons), so amass annihilation immediately followed, leaving just one in 1010 of the original protons andneutrons, and none of their antiparticles. A similar process happened at about 1 second forelectrons and positrons. After these annihilations, the remaining protons, neutrons and electronswere no longer moving relativistically and the energy density of the Universe was dominated by photons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion (one thousandmillion; 109; SI prefix giga-) kelvin and the density was about that of air, neutrons combinedwith protons to form the Universe's deuterium and helium nuclei in a process called Big Bangnucleosynthesis.[28] Most protons remained uncombined as hydrogen nuclei. As the Universecooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms(mostly hydrogen); hence the radiation decoupled from matter and continued through spacelargely unimpeded. This relic radiation is known as the cosmic microwave backgroundradiation.[29]

Over a long period of time, the slightly denser regions of the nearly uniformly distributed mattergravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this processdepend on the amount and type of matter in the Universe. The four possible types of matter areknown as cold dark matter , warm dark matter , hot dark matter , and baryonic matter . The bestmeasurements 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 earlyreionization[30]), and is estimated to make up about 23% of the matter/energy of the universe,while baryonic matter makes up about 4.6%.[31] In an "extended model" which includes hot darkmatter in the form of neutrinos, then if the "physical baryon density" bh2 is estimated at about0.023 (this is different from the 'baryon density' b expressed as a fraction of the totalmatter/energy density, which as noted above is about 0.046), and the corresponding cold dark

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matter density ch2 is about 0.11, the corresponding neutrino density vh2 is estimated to be lessthan 0.0062.[31]

Independent lines of evidence from Type Ia supernovae and the CMB imply that the Universetoday is dominated by a mysterious form of energy known as dark energy, which apparently

permeates all of space. The observations suggest 73% of the total energy density of today'sUniverse is in this form. When the Universe was very young, it was likely infused with darkenergy, but with less space and everything closer together, gravity had the upper hand, and it wasslowly braking the expansion. But eventually, after numerous billion years of expansion, thegrowing abundance of dark energy caused the expansion of the Universe to slowly begin toaccelerate. 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 areunknown and, more generally, the details of its equation of state and relationship with theStandard Model of particle physics continue to be investigated both observationally andtheoretically.[32]

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the CDM model of cosmology, which uses the independent frameworks of quantummechanics and Einstein's General Relativity. As noted above, there is no well-supported modeldescribing the action prior to 1015 seconds or so. Apparently a new unified theory of quantumgravitation is needed to break this barrier. Understanding this earliest of eras in the history of theUniverse is currently one of the greatest unsolved problems in physics.

Underlying assumptions

The Big Bang theory depends on two major assumptions: the universality of physical laws andthe 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. Forexample, the first assumption has been tested by observations showing that largest possibledeviation of the fine structure constant over much of the age of the universe is of order 105.[33] Also, general relativity has passed stringent tests on the scale of the Solar System and binarystars.[notes 2]

If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principlecan be derived from the simpler Copernican principle, which states that there is no preferred (orspecial) observer or vantage point. To this end, the cosmological principle has been confirmed to

a level of 105

via observations of the CMB.[notes 3][citation needed ]

The Universe has been measuredto be homogeneous on the largest scales at the 10% level.[34]

FLRW metric

Main articles: Friedmann Lematre Robertson Walker metric and Metric expansion of space

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General relativity describes spacetime by a metric, which determines the distances that separatenearby points. The points, which can be galaxies, stars, or other objects, themselves are specifiedusing 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, whichuniquely singles out the Friedmann Lematre Robertson Walker metric (FLRW metric). This

metric contains a scale factor , which describes how the size of the Universe changes with time.This enables a convenient choice of a coordinate system to be made, called comovingcoordinates. In this coordinate system the grid expands along with the Universe, and objects thatare moving only due to the expansion of the Universe remain at fixed points on the grid. Whiletheir coordinate distance(comoving distance) remains constant, the physical distance betweentwo such comoving points expands proportionally with the scale factor of the Universe.[35]

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 twocomoving 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.Horizons

Main article: Cosmological horizon

An important feature of the Big Bang spacetime is the presence of horizons. Since the Universehas a finite age, and light travels at a finite speed, there may be events in the past whose light hasnot 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 evermore 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 describesour 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 earlytimes. 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.[36]

History

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

Etymology

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 strikingimage meant to highlight the difference between the two models.[37][38][39]

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Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was "repugnant" to him.[48][49] Lematre, however, thought that

If the world has begun with a single quantum, the notions of space and time would altogether failto 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 iscorrect, the beginning of the world happened a little before the beginning of space and time.[50]

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble'sobservations, including the Milne model,[51] the oscillatory Universe (originally suggested byFriedmann, but advocated by Albert Einstein and Richard Tolman)[52] and Fritz Zwicky's tiredlight hypothesis.[53]

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 theUniverse is roughly the same at any point in time.[54] The other was Lematre's Big Bang theory,

advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN)[55] and whose associates, Ralph Alpher and Robert Herman, predicted the cosmicmicrowave background radiation (CMB).[56] Ironically, it was Hoyle who coined the phrase thatcame to be applied to Lematre's theory, referring to it as "thisbig bang idea" during a BBCRadio broadcast in March 1949.[57][notes 4] For a while, support was split between these twotheories. Eventually, the observational evidence, most notably from radio source counts, began tofavor Big Bang over Steady State. The discovery and confirmation of the cosmic microwave background radiation in 1964[59] secured the Big Bang as the best theory of the origin andevolution 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 earlierand 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 ofadvances in telescope technology as well as the analysis of data from satellites such as COBE,[60] the Hubble Space Telescope and WMAP.[61] Cosmologists now have fairly precise and accuratemeasurements of many of the parameters of the Big Bang model, and have made the unexpecteddiscovery that the expansion of the Universe appears to be accelerating.

Observational evidence

"[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its generalfeatures."

Lawrence Krauss[62]

The earliest and most direct observational evidence of the validity of the theory are theexpansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies),discovery and measurement of the cosmic microwave background and the relative abundances oflight elements produced by Big Bang nucleosynthesis. More recent evidence includes

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observations of galaxy formation and evolution, and the distribution of large-scale cosmicstructures,[63] These are sometimes called the "four pillars" of the Big Bang theory.[64]

Precise modern models of the Big Bang appeal to various exotic physical phenomena that havenot been observed in terrestrial laboratory experiments or incorporated into the Standard Model

of particle physics. Of these features, dark matter is currently subjected to the most activelaboratory investigations.[65] Remaining issues include the cuspy halo problem and the dwarfgalaxy problem of cold dark matter . Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.[66] Inflation and baryogenesis remain more speculative features of current Big Bang models.[notes 5][citation needed ] Viable, quantitative explanations for such phenomena are still being sought. These are currentlyunsolved problems in physics.

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 afrequency spectrum of an object and matching the spectroscopic pattern of emission lines orabsorption lines corresponding to atoms of the chemical elements interacting with the light.These redshifts are uniformly isotropic, distributed evenly among the observed objects in alldirections. If the redshift is interpreted as a Doppler shift, the recessional velocity of the objectcan be calculated. For some galaxies, it is possible to estimate distances via the cosmic distanceladder . When the recessional velocities are plotted against these distances, a linear relationshipknown as Hubble's law is observed:[16]

v = H 0 D ,

where

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+1.3

1.4 km/s/Mpc by the WMAP probe.[31]

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 uniformlyexpanding everywhere. This universal expansion was predicted from general relativity byAlexander Friedmann in 1922[42] and Georges Lematre in 1927,[43] well before Hubble made his1929 analysis and observations, and it remains the cornerstone of the Big Bang theory asdeveloped byFriedmann, Lematre, Robertson, and Walker .

The theory requires the relationv = 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

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The cosmic microwave background spectrum measured by the FIRAS instrument on the COBEsatellite is the most-precisely measured black body spectrum in nature.[71] The data points anderror bars on this graph are obscured by the theoretical curve.

In 1989 NASA launched the Cosmic Background Explorer satellite (COBE). Its findings wereconsistent with predictions regarding the CMB, finding a residual temperature of 2.726 K (morerecent measurements have revised this figure down slightly to 2.725 K) and providing the firstevidence for fluctuations (anisotropies) in the CMB, at a level of about one part in 105.[60] JohnC. Mather and George Smoot were awarded the Nobel Prize for their leadership in this work.During the following decade, CMB anisotropies were further investigated by a large number ofground-based and balloon experiments. In 2000 2001 several experiments, most notablyBOOMERanG, found the shape of the Universe to be spatially almost flat by measuring thetypical angular size (the size on the sky) of the anisotropies.

In early 2003 the first results of the Wilkinson Microwave Anisotropy Probe (WMAP) werereleased, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistentwith the inflation theory in general.[61] The Planck space probe was launched in May 2009. Otherground and balloon based cosmic microwave background experiments are ongoing.

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