The Big Bang Model or Theory is the Prevailing

8/6/2019 The Big Bang Model or Theory is the Prevailing

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The Big Bang model or theory is the prevailing[1]

cosmologicaltheory of the earlydevelopment of the universe. According to the Big Bang model, the universe was originally

in an extremely hot and dense state that expanded rapidly. This expansion caused theuniverse to cool and resulted in the present diluted state that continues to expand today.

Based on the best available measurements as of 2010, the original state of the universe

existed around 13.7 billion years ago,[2][3]

which is often referred to as the time when the Big

Bang occurred.[4][5] The theory is the most comprehensive and accurate explanation supportedby scientific evidence and observations.

[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 as

homogeneity and isotropy of space). The governing equations had been formulated by

Alexander Friedmann. In 1929, Edwin Hubble discovered that the distances to far away

galaxies were generallyproportional to theirredshiftsan 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 fartheraway, 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 extreme densitiesand temperatures,

[9][10][11]and largeparticle accelerators have been built to experiment on and

test such conditions, resulting in significant confirmation of the theory, but these acceleratorshave limited capabilities to probe into such high energy regimes. Without any evidence

associated with the earliest instant of the expansion, 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 since that instant. 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 termBig Bangduring a 1949 radio broadcast. It ispopularly 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.[12][13][14] Hoyle later helped

considerably in the effort to understand stellar nucleosynthesis, the nuclear pathway forbuilding certain heavier elements from lighter ones. After the discovery of the cosmic

microwave background radiation in 1964, and especially when its spectrum (i.e., the amount

of radiation measured at each wavelength) was found to match that ofthermal radiation from

ablack body, most scientists were fairly convinced by the evidence that some version of the

Big Bang scenario must have occurred.

Contents

[hide]

y 1 Motivation and developmenty 2 Overview

o 2.1 Timeline of the Big Bang


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o 2.2 Underl ing assumpti nso 2.3 FLRW metri o 2.4 Horizons

y 3 Observational evidenceo 3.1 Hubble's law and t e expansion of spaceo 3.2 Cosmic microwave background radiationo 3.3 Abundance of primordial elementso 3.4 Galactic evolution and distributiono 3.5 Ot erlines of evidence

y 4 Features, issues and problemso 4.1 Horizon problemo 4.2 Flatness/oldness problemo 4.3 Magnetic monopoleso 4.4 Baryon asymmetryo 4.5 Globular cluster ageo 4.6 Dark mattero 4.7 Dark energy

y 5 The future according to the Big Bang theoryy 6 Speculative physics beyond Big Bang theoryy 7 Religious interpretationsy 8 Notesy 9 References

o 9.1 Booksy 10 Further readingy 11 Externallinks

Motiv tion and development

Artist's depiction ofthe WMAP satellite gathering data to help scientists understand the Big

Bang

Mai

arti

l

:Hi

t

ry ofth

Bi

Bangth

orySee al o:Timeline of cosmology andHistory of astronomy

The Big Bang theory developed from observations ofthe structure ofthe Universe and from

theoretical considerations. In 1912Vesto 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 receding from Earth. He did not grasp the cosmological

implications ofthis fact, and indeed atthe time it was highly controversial whether or not

these nebulae were "island universes" outside ourMilky Way.[15][16]

Ten years later,


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Alexander Friedmann, a Russiancosmologist and mathematician, derived the Friedmannequations from Albert Einstein's equations ofgeneral relativity, showing that the Universe

might be expanding in contrast to the static Universe model advocated by Einstein at thattime.[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 theUniverse.

[18]

In 1931 Lematre went further and suggested that the evident expansion in forward time

required that the Universe contracted backwards in time, and would continue to do so until it

could contract no further, bringing all the mass of the Universe into a single point, a

"primeval atom" where and when the fabric of time and space comes 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 as Hubble's law.[8][20]Lematre had already shown that this was expected, given the Cosmological Principle.

[21]

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's

observations, including the Milne model,[22]

the oscillatory Universe (originally suggested by

Friedmann, but advocated by Albert Einstein and Richard Tolman)[23]

and Frit Zwicky's

tired light hypothesis.[24]

AfterWorld 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.[25]

The other was Lematre's Big

Bang theory,[notes 1]

advocated and developed by George Gamow, who introducedbig bangnucleosynthesis (BBN)

[26]and whose associates, Ralph Alpherand Robert Herman, predicted

the cosmic microwave background radiation (CMB).[27]

Ironically, it was Hoyle who coinedthe phrase that came to be applied to Lematre's theory, referring to it as "this big bangidea"

during a BBCRadio broadcast in March 1949.[28][notes 2]

For a while, support was splitbetween 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 ofthe 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 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.

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of majoradvances in telescope technology as well as the analysis of copious data from satellites such

as COBE,[30]

the Hubble Space Telescope and WMAP.[31]

Cosmologists now have fairlyprecise and accurate measurements of many of the parameters of theBig Bang model, and

have made the unexpected discovery that the expansion of the Universe appears to beaccelerating.


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Overview

Timeline of the Bi Bang

Main article:Timeline ofthe Big Bang

A graphicaltimeline is available

at

Graphicaltimeline ofthe Big

Bang

Extrapolation ofthe expansion ofthe Universe backwards in time usinggeneral relativity

yields an infinite density and temperature at a finite time in the past.[32] This singularity

signals the breakdown of general relativity. How closely we can extrapolate towards the

singularity is debatedcertainly no closerthan the end ofthe Planck epoch. This singularityis sometimes called "the Big Bang",[33] butthe term can also referto the early hot, dense

phase itself,

[34][notes 3]

which can be considered the "birth" of our Universe. Based onmeasurements ofthe expansion usingType Ia supernovae, measurements oftemperature

fluctuations in the cosmic microwave background, and measurements ofthe correlationfunction of galaxies, the Universe has a calculated age of 13.75 0.11billion years.[35] The

agreement ofthese three independent measurements strongly supports the CDM modelthatdescribes in detailthe contents ofthe Universe.

The earliest phases ofthe Big Bang are subjectto much speculation. In the most common

models, the Universe was filled homogeneouslyand isotropically with an incredibly high

energy density, huge temperatures andpressures, and was very rapidly expanding and

cooling. Approximately 1037

seconds into the expansion, aphase transition caused a cosmic

inflation, during which the Universe grew exponentially.[36]

Afterinflation stopped, the

Universe consisted of a quark gluon plasma, as well as all otherelementary particles.[37]

Temperatures were so high thatthe random motions of particles were atrelativisticspeeds,andparticle antiparticle pairs of all kinds were being continuously created and destroyed in

collisions. At some point an unknown reaction calledbaryogenesis violated the conservationofbaryon number, leading to a very small excess ofquarks and leptons over antiquarks and

antileptonsofthe order of one partin 30 million. This resulted in the predominance ofmatteroverantimatterin the present Universe.[38]

The Universe continued to grow in size and fallin temperature, hence the typical energy of

each particle was decreasing. Symmetry breaking phase transitions putthe fundamentalforces of physics and the parameters ofelementary particlesinto their present form.[39] After

about 1011

seconds, the picture becomes less speculative, since particle energies drop to

values that can be attained inparticle physics experiments. At about 106 seconds, quarks andgluons combined to formbaryons 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, leavingjust one in 1010

ofthe

original protons and neutrons, and none oftheir antiparticles. A similar process happened at

about 1 second for electrons and positrons. Afterthese annihilations, the remaining protons,

neutrons and electrons were no longer moving relativistically and the energy density ofthe

Universe was dominated byphotons (with a minor contribution from neutrinos).


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A few minutes into the expansion, when the temperature was about a billion (one thousandmillion; 10

9; SI prefix giga-) kelvin and the density was aboutthat of air, neutrons combined

with protons to form the Universe's deuterium and heliumnucleiin a process called Big Bangnucleosynthesis.[40]Most protons remained uncombined ashydrogen nuclei. As the Universe

cooled, the rest mass energy density of matter came to gravitationally dominate that ofthephoton 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 thecosmic microwave background

radiation.[41]

The Hubble Ultra Deep Field showcases galaxies from an ancient era when the Universe was

younger, denser, and warmer according to the Big Bang theory.

Over a long period oftime, the slightly denser regions ofthe 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 ofthis

process depend on the amount and type of matterin the Universe. The four possible types of

matter are known as cold dark matter, warm dark matter, hot dark matterandbaryonic matter.

The best measurements available (fromWMAP) show thatthe data is well-fit by a Lambda-

CDM modelin which dark matteris assumed to be cold (warm dark matteris ruled out by

early reionization[42]), and is estimated to make up about 23% ofthe matter/energy ofthe

universe, while baryonic matter makes up about 4.6%.[35]

In an "extended model" which

includes hot dark matterin the form ofneutrinos, then ifthe "physical baryon density" bh2

is estimated at about 0.023 (this is different from the 'baryon density' b expressed as a

fraction ofthe total matter/energy density, which as noted above is about 0.046), and thecorresponding cold dark matter density ch

2is about 0.11, the corresponding neutrino density

vh2is estimated to be less than 0.0062.

[35]

Independentlines of evidence fromType Ia supernovae and the CMBimply thatthe Universetoday is dominated by a mysterious form of energy known asdark energy, which apparently

permeates all of space. The observations suggest 73% ofthe total energy density oftoday'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 closertogether, 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 theexpansion ofthe Universeto slowly begin

to accelerate. Dark energy in its simplest formulation takes the form ofthe cosmological

constantterm in Einstein's field equations of general relativity, butits composition and


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mechanism are unknown and, more generally, the details of its equation of state andrelationship with the Standard Model of particle physics continue to be investigated both

observationally and theoretically.[21]

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

quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10

15seconds 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. 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 largestpossible deviation of the fine structure constant over much of the age of the universe is of

order 105

.[43]

Also, general relativity has passed stringent tests on the scale of the solarsystem 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 principlecan 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 Fried annLe atreRobertsonWalker etric andMetric expansion ofspace

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 FriedmannLematreRobertsonWalker metric

(FLRW metric). This metric contains a scale factor, which describes how the si e of theUniverse changes with time. This enables a convenient choice of a coordinate system to bemade, 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 atfixed points on the grid. While theircoordinate distance (comoving distance) remains

constant, thep

ysicaldistance between two such comoving points expands proportionallywith the scale factorof the Universe.

[45]


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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 ofmass 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 Cos ological

orizon

An important feature of the Big Bang spacetime is the presence ofhori ons. Since the

Universe has a finite age, and light travels at a finite speed, there may be events in the pastwhose light has not had time to reach us. This places a limit or a past

orizon on the most

distant objects that can be observed. Conversely, because space is expanding, and moredistant objects are receding ever more quickly, light emitted by us today may never "catch

up" to very distant objects. This defines afuture

orizon, which limits the events in the future

that we will be able to influence. The presence of either type of hori 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 hori 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 hori on recedes in space. If the expansion of the Universe

continues to accelerate, there is a future hori on as well.[46]

Obser ational e idence

The earliest and most direct kinds of observational evidence are the Hubble-type expansionseen in the redshifts of galaxies, the detailed measurements of the cosmic microwave

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

the large scale distribution and apparent evolution of galaxies[47] which are predicted to occurdue to gravitational growth of structure in the standard theory. These are sometimes called

"t

efour pillars oft

e Big Bang t

eory".[48]

Hubble's law and the expansion of space

Main articles

ubble's law andMetric expansion ofspace

See alsoDistance easures (cos ology) andScalefactor (universe)

Observations of distant galaxies and quasars show that these objects are redshiftedthe 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 ofemission lines orabsorption lines corresponding to atoms of the chemical elements interacting with the light.

These redshifts are uniformlyisotropic, 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:[8]

v =

0D,


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where

y vis the recessionalvelocity ofthe galaxy or other distant object,y Dis the comoving distanceto the object, andy H0is Hubble's constant, measured to be 70.4 +1.3

1.4km/s/Mpc by the WMAP probe.[35]

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

galaxieswhich is untenable given the Copernican Principleorthe Universe is uniformlyexpanding everywhere. This universal expansion was predicted fromgeneral relativity by

Alexander Friedmannin 1922[17]

and Georges Lematrein 1927,[18]

well before Hubble madehis 1929 analysis and observations, and it remains the cornerstone ofthe Big Bang theory as

developed by Friedmann, Lematre, Robertson and Walker.

The theory requires the relation v =HDto hold at alltimes, where Dis the comoving

distance, vis the recessional velocity, and v,H, and D vary as the Universe expands (hence

we writeH0to denote the present-day Hubble "constant"). For distances much smallerthan

the size ofthe observable Universe, the Hubble redshift can be thought of as the Doppler shift

corresponding to the recession velocityv. However, the redshiftis not a true Doppler shift,but ratherthe result ofthe expansion ofthe Universe between the time the light was emittedand the time thatit was detected.[49]

Thatspace is undergoing metric expansion is shown by direct observational evidence ofthe

Cosmological Principle and the Copernican Principle, which together with Hubble's law haveno other explanation. Astronomicalredshifts are extremely isotropic and homogenous,[8]

supporting the Cosmological Principle thatthe Universe looks the same in all directions,along with much other evidence. Ifthe redshifts were the result of an explosion from a center

distant from us, they would not be so similarin different directions.

Measurements ofthe effects ofthe cosmic microwave background radiationon the dynamicsof distant astrophysical systems in 2000 proved the Copernican Principle, thatthe Earth is not

in a central position, on a cosmological scale.[50]

Radiation from the Big Bang was

demonstrably warmer at earliertimes throughoutthe Universe. Uniform cooling ofthe

cosmic microwave background over billions of years is explainable only ifthe Universe is

experiencing a metric expansion, and excludes the possibility that we are nearthe unique

center of an explosion.

Cosmi mi rowave background radiation

Main article:Cosmic microwave backgro nd radiation

WMAPimage ofthe cosmic microwave background radiation


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During the first few days ofthe Universe, the Universe was in fullthermal equilibrium, withphotons being continually emitted and absorbed, giving the radiation ablackbody spectrum.

As the Universe expanded, it cooled to a temperature at which photons could no longer becreated or destroyed. The temperature was still high enough for electrons and nucleito

remain unbound, however, and photons were constantly "reflected" from these free electronsthrough a process called Thomson scattering. Because ofthis repeated scattering, the early

Universe was opaque to light.

When the temperature fellto a few thousand Kelvin, electrons and nuclei began to combineto form atoms, a process known as recombination. Since photons scatterinfrequently from

neutral atoms, radiation decoupled from matter when nearly allthe electrons had recombined,atthe epoch oflastscattering, 379,000 years afterthe Big Bang. These photons make up the

CMBthatis observed today, and the observed pattern of fluctuations in the CMBis a direct

picture ofthe Universe atthis early epoch. The energy of photons was subsequently

redshifted by the expansion ofthe Universe, which preserved the blackbody spectrum but

caused its temperature to fall, meaning thatthe photons now fallinto themicrowave region of

the electromagnetic spectrum. The radiation is thoughtto be observable at every pointin 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 newmicrowave receiver owned

by Bell Laboratories.[29]

Their discovery provided substantial confirmation ofthe general

CMB predictionsthe radiation was found to be isotropic and consistent with a blackbody

spectrum of about 3 Kand it pitched the balance of opinion in favor ofthe Big Bang

hypothesis. Penzias and Wilson were awarded aNobel Prize fortheir discovery.

The cosmic microwave background spectrum measured by the FIRAS instrument on theCOBE satelliteis the most-precisely measuredblack body spectrum in nature.[51] The data

points and error bars on this graph are obscured by the theoretical curve.

In 1989

,NASAlaunched the Cosmic Background Explorer satellite(COBE), and the initialfindings, released in 1990, were consistent with the Big Bang's predictionsregarding the

CMB. COBE found a residualtemperature of 2.726 K and in 1992 detected forthe firsttime

the fluctuations (anisotropies) in the CMB, at a level of about one partin 105.[30]John C.

Matherand George Smoot were awarded Nobels fortheirleadership in this work. During the

following decade, CMB anisotropies were furtherinvestigated by a large number of ground-based and balloon experiments. In 2000 2001, several experiments, most notably

BOOMERanG, found the Universe to be almost spatially flat by measuring the typicalangular size (the size on the sky) ofthe anisotropies. (Seeshape ofthe Universe.)


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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. 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 neutrinos permeates 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 named Planck,

with goals similar to WMAP, was launched in May 2009. It is anticipated to soon provideeven more accurate measurements of the CMB anisotropies. Many other ground- and

balloon-based experiments are also currently running; see Cosmic microwave background

experiments.

The background radiation is exceptionally smooth, which presented a problem in that

conventional expansion would mean that photons coming from opposite directions in the sky

were coming from regions that had never been in contact with each other. The leading

explanation for this far reaching equilibrium is that the Universe had a brief period of rapid

exponential expansion, called inflation. This would have the effect of driving apart regions

that had been in equilibrium, so that all the observable Universe was from the sameequilibrated region.

Abundance ofprimordial elements

Main article Big Bang nucleosynt

esis

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 ofphotons tobaryons, which itselfcan be calculated independently from the detailed structure ofCMB fluctuations. The ratios

predicted (by mass, not by number) are about 0.25 for4

He/H, about 103

for2

H/H, about 10

4

for3He/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 substantial systematic uncertainties. Nonetheless, the

general consistency with abundances predicted by BBN is strong evidence for the Big Bang,

as the theory is the only known explanation for the relative abundances of light elements, andit 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, theyoung Universe (i.e., before star formation, as determined by studying matter supposedly free

ofstellar nucleosynthesis products) should have more helium than deuterium or moredeuterium than 3

He, and in constant ratios, too.

Galactic e olution and distribution


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Main articles:Large-scale str cture ofthe cosmos, Structure formation, andGalaxyformation and evolution

This panoramic view ofthe entire near-infrared sky reveals the distribution of galaxies

beyond the Milky Way. The galaxies are color coded by redshift.

Detailed observations ofthe morphology and distribution of galaxies and quasars provide

strong evidence forthe Big Bang. A combination of observations and theory suggestthatthe

first quasars and galaxies formed about a billion years afterthe Big Bang, and since then

larger structures have been forming, such asgalaxy 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 recentstate). Moreover, galaxies that formed relatively recently appear markedly different fromgalaxies formed at similar distances but shortly afterthe Big Bang. These observations are

strong arguments againstthe steady-state model. Observations ofstar formation, galaxy andquasar distributions and larger structures agree well with Big Bang simulations ofthe

formation of structure in the Universe and are helping to complete details ofthe theory.[53][54]

Other lines of evidence

After some controversy, the age of Universe as estimated from the Hubble expansion and the

CMBis now in good agreement with (i.e., slightly largerthan) the ages ofthe oldest stars,both as measured by applying the theory ofstellar evolutionto globular clusters and through

radiometric dating ofindividualPopulation II stars.[citation needed]

The prediction thatthe CMBtemperature was higherin the past has been experimentally

supported by observations oftemperature-sensitive emission lines in gas clouds at high

redshift. This prediction also implies thatthe amplitude oftheSunyaev Zel'dovich effectin

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 testis impossible.[citation needed]

Features, issues and problems

While scientists now preferthe Big Bang model over other cosmological models, thescientific community was once divided between supporters ofthe Big Bang and those of

alternative cosmological models. Throughoutthe historical development ofthe 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 consensusin the community today supporting the Big Bang model, many of

these problems are remembered as being mainly of historicalinterest;the solutions to them

have been obtained eitherthrough modifications to the theory or as the result of better

observations.


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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 cosmicmicrowave background, large scale structure, and the Hubble diagram forType Ia

supernovae.

Precise modern models of the Big Bang appeal to various exotic physical phenomena thathave not been observed in terrestrial laboratory experiments or incorporated into the Standard

Model ofparticle physics. Of these features, dark matteris currently the subject to the most

active laboratory investigations.[55]

Remaining issues, such as the cuspy halo problem and the

dwarf galaxy problem ofcold 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 possible.[56]

On the other hand, inflation andbaryogenesis 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 6]

Discovering the

correct explanations for such phenomena are some of the remaining unsolved problems inphysics.

Horizon problem

Main article ! "

orizon proble#

The hori on problem results from the premise that information cannot travel faster than light.

In a Universe of finite age, this sets a limittheparticle hori onon the separation of any

two regions of space that are in causal contact.[57]

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 hori on at that time would correspond toabout 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 hori 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 hori 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 bequantum 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 hori on.


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Flatness/oldness problem

Main article:Flatness problem

The overallgeometry ofthe Universeis determined by whetherthe Omega cosmologicalparameteris less than, equalto or greaterthan 1. Shown from top to bottom are aclosed

Universe with positive curvature, a hyperbolic Universe with negative curvature and a flat

Universe with zero curvature.

The flatness problem (also known as the oldness problem) is an observational problem

associated with a Friedmann Lematre Robertson Walker metric.[57] The Universe may have

positive, negative orzero spatialcurvature depending on its total energy density. Curvature is

negative ifits density is less than the critical density, positive if greater, and zero atthecritical density, in which case space is said to beflat. The problem is that any small departure

from the critical density grows with time, and yetthe Universe today remains very close toflat.[notes 7] Given that a naturaltimescale for departure from flatness might be thePlancktime,

1043 seconds, the factthatthe Universe has reached neither aHeat Death nor a Big Crunchafter billions of years requires some explanation. Forinstance, even atthe relatively late age

of a few minutes (the time of nucleosynthesis), the Universe density must have been withinone partin 1014 ofits critical value, orit would not exist as it does today.[58]

A resolution to this problem is offered by inflationary theory. During the inflationary period,

spacetime expanded to such an extentthatitscurvature would have been smoothed out. Thus,

itis theorized thatinflation drove the Universe to a very nearly spatially flat state, with

almost exactly the critical density.

Magnetic monopoles

Main article:Magnetic monopole

The magnetic monopole objection was raised in the late 1970s.Grand unification theories

predicted topological defectsin space that would manifest as magnetic monopoles. These

objects would be produced efficiently in the hot early Universe, resulting in a density much

higherthan is consistent with observations, given thatsearches have never found any

monopoles. This problem is also resolved bycosmic inflation, which removes all point


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defects from the observable Universe in the same way thatit drives the geometry toflatness.

[57]

A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic

inflation is offered by the Weyl curvature hypothesis.[59][60]

Bar on as mmetr

Main article:Baryon asymmetry

Itis not yet understood why the Universe has morematterthan antimatter.[38] Itis generally

assumed that when the Universe was young and very hot, it was in statistical equilibrium andcontained equal numbers ofbaryons and antibaryons. However, observations suggestthatthe

Universe, including its most distant parts, is made almost entirely of matter. An unknownprocess called "baryogenesis" created the asymmetry. For baryogenesis to occur, the

Sakharov conditions must be satisfied. These require thatbaryon numberis not conserved,

thatC-symmetry and CP-symmetry are violated and thatthe Universe depart from

thermodynamic equilibrium.[61]

Allthese conditions occurin the Standard Model, butthe

effectis not strong enough to explain the present baryon asymmetry.

Globular cluster age

In the mid-1990s, observations ofglobular clusters appeared to be inconsistent with the Big

Bang. Computer simulations that matched the observations ofthestellarpopulations ofglobular clusters suggested thatthey were about 15billion years old, which conflicted with

the 13.7 billion year age ofthe Universe. This issue was generally resolved in the late 1990swhen new computer simulations, which included the effects of mass loss due tostellar winds,

indicated a much younger age for globular clusters.[62]

There still remain some questions as to

how accurately the ages ofthe clusters are measured, butitis clearthatthese objects are

some ofthe oldestin the Universe.

Dark matter

Main article:Darkmatter

Apie chartindicating the proportional composition of different energy-density components

ofthe Universe, according to the bestCDM model fits roughly 95% is in the exotic forms

of dark matter and dark energy

During the 1970s and 1980s, various observations showed thatthere is not sufficient visible

matterin the Universe to account forthe apparent strength of gravitational forces within and


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between galaxies. This led to the idea that up to 90% of the matter in the Universe is darkmatter that does not emit light or interact with normalbaryonic matter. In addition, the

assumption that the Universe is mostly normal matter led to predictions that were stronglyinconsistent with observations. In particular, the Universe today is far more lumpy and

contains far less deuterium than can be accounted for without dark matter. While dark matter

was initially controversial, it is now indicated by numerous observations: the anisotropies in

the CMB, galaxy clustervelocity dispersions, large-scale structure distributions, gravitationallensing studies, and X-ray measurements of galaxy clusters.

[63]

The evidence for dark matter comes from its gravitational influence on other matter, and no

dark matter particles have been observed in laboratories. Manyparticle physics candidates for

dark matter have been proposed, and several projects to detect them directly are underway.[64]

Dark energy

Main article $ Dark energy

Measurements of the redshiftmagnitude relation fortype Ia supernovae indicate that theexpansion 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 theUniverse consists of a component with large negative pressure, dubbed "dark energy". Dark

energy is indicated by several other lines of evidence. Measurements of the cosmicmicrowave 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 ofmass/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.[21]

Since dark energydoes 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 ofgravitational lenses, and the other using the

characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is a property ofvacuum 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.[35]

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 inthe far future as dark energy becomes even more dominant.

In the CDM, the best current model of the Big Bang, dark energy is explained by the

presence of a cosmological constant in the general theory of relativity. However, the si e ofthe constant that properly explains dark energy is surprisingly small relative to naive

estimates based on ideas about quantum gravity. Distinguishing between the cosmological

constant and other explanations of dark energy is an active area of current research.

The future according to the Big Bang theory


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Main article:Ultimate fate ofthe universe

Before observations ofdark energy, cosmologists considered two scenarios forthe future of

the Universe. Ifthe mass density ofthe Universe were greaterthan the critical density, then

the Universe would reach a maximum size and then begin to collapse. It would becomedenser and hotter again, ending with a state that was similarto thatin which it starteda Big

Crunch.[46] Alternatively, ifthe density in the Universe were equalto or below the criticaldensity, the expansion would slow down, but never stop. Star formation would cease as all

the interstellar gas in each galaxy is consumed; stars would burn outleavingwhite dwarfs,neutron stars, andblack holes. Very gradually, collisions between these would resultin mass

accumulating into larger and larger black holes. The average temperature ofthe Universewould asymptotically approachabsolute zeroa Big Freeze. Moreover, ifthe proton were

unstable, then baryonic matter would disappear, leaving only radiation and black holes.

Eventually, black holes would evaporate by emittingHawking radiation. The entropy ofthe

Universe would increase to the point where no organized form of energy could be extracted

from it, a scenario known as heat death.

Modern observations ofaccelerated expansionimply that more and more ofthe currently

visible Universe will pass beyond ourevent horizon and out of contact with us. The eventualresultis not known. The CDM model ofthe Universe contains dark energyin the form of a

cosmological constant. This theory suggests that only gravitationally bound systems, such as

galaxies, would remain together, and they too would be subjecttoheat death, as the Universe

expands and cools. Other explanations of dark energyso-calledphantom energytheories

suggestthat ultimately galaxy clusters, stars, planets, atoms, nuclei and matteritself will be

torn apart by the ever-increasing expansion in a so-called Big Rip.[65]

Speculative ph sics be ond Big Bang theor

This is an artist's concept ofthe Universe expansion, where space (including hypotheticalnon-observable portions ofthe Universe) is represented at each time by the circular sections.

Note on the leftthe dramatic expansion (notto scale) occurring in the inflationary epoch, andatthe centerthe expansion acceleration. The scheme is decorated withWMAPimages on the

left and with the representation of stars atthe appropriate level of development.

While the Big Bang modelis well established in cosmology, itis likely to be refined in the

future. Little is known aboutthe earliest moments ofthe Universe's history. ThePenrose


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Hawking singularity theorems require the existence of a singularity at the beginning ofcosmic time. However, these theorems assume that general relativity is correct, but general

relativity must break down before the Universe reaches the Planck temperature, and a correcttreatment ofquantum gravity may avoid the singularity.[66]

Some proposals, each of which entails untested hypotheses, are:

y models including the HartleHawking no-boundary condition in which the whole ofspace-time is finite; the Big Bang does represent the limit of time, but without the

need for a singularity.[67]

y big bang lattice model [68] states that the universe at the moment of the big bangconsists of an infinite lattice offermions which is smeared over the fundamental

domain so it has both rotational, translational and gauge symmetry. The symmetry is

the largest symmetry possible and hence the lowest entropy of any state.

y brane cosmology models[69] in which inflation is due to the movement of branes instring theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is

the result of a collision between branes; and the cyclic model, a variant of the

ekpyrotic model in which collisions occur periodically. In the latter model, the BigBang was preceded by a Big Crunch and the Universe endlessly cycles from one

process to the other.[70][71][72]

y chaotic inflation, in which universal inflation ends locally here and there in a randomfashion, each end-point leading to a bubble universe expanding from its own big

bang.[73][74][75]

Proposals in the last two categories see the Big Bang as an event in a much larger and older

Universe, ormultiverse, and not the literal beginning.

Religious interpretations

Main article % Religious interpretations oft&

e Big Bang t&

eory

The Big Bang is a scientific theory, and as such is dependent on its agreement with

observations. But as a theory which addresses the origins of reality, it has always carriedtheological and philosophical implications, most notably, the concept of creation ex

ni'

ilo.[76][77][78][79][80]

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.[81]

This perception was enhanced by the fact that the

originator of the Big Bang theory, MonsignorGeorges Lematre, was a Roman Catholic

Christian priest.[82]

Pope Pius XII, declared at the November 22, 1951 opening meeting of the

Pontifical Academy of Sciences that the Big Bang theory accorded with the Catholic conceptof creation.

[83]Conservative ProtestantChristian denominations have also welcomed the Big

Bang theory as supporting a historical interpretation of the doctrine ofcreation.[84]

Since the acceptance of the Big Bang as the dominant physical cosmological paradigm, there

have been a variety of reactions by religious groups as to its implications for their respective

religious cosmologies. Some accept the scientific evidence at face value, while others seek to


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reconcile the Big Bang with their religious tenets, and others completely reject or ignore theevidence for the Big Bang theory.[85]

Notes

1. ^ Lematre termed the event the "big noise". Astrophysicist Fred Hoyle, who disliked theidea, designated the creation event "Big Bang", which he considered to be an ugly name. SeeOp. cit. Ferris 1988, pp. 211, 436, citing The Los Angeles Ti( es. January 12, 1933.

2. ^ It is commonly reported that Hoyle intended this to be pejorative. However, Hoyle laterdenied that, saying that it was just a striking image meant to emphasi) e the differencebetween the two theories for radio listeners. See chapter 9 ofThe Alche0 y ofthe

1

eavens by

Ken Croswell, Anchor Books, 1995.

3. ^ There is no consensus about how long the Big Bang phase lasted. For some writers thisdenotes only the initial singularity, for others the whole history of the Universe. Usually, atleast the first few minutes (during which helium is synthesi2 ed) are said to occur "during theBig Bang".

4. ^ Detailed information of and references for tests of general relativity are given at Tests ofgeneral relativity.

5. ^

This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solarsystem through the radiation field.6. ^ If inflation is true, baryogenesis must have occurred, but not vice versa.7. ^ Strictly, dark energy in the form of a cosmological constant drives the Universe towards a

flat state; however, our Universe remained close to flat for several billion years, before the

dark energy

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