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The Big Bang theory is the prevailing physical cosmology|cosmological scientific theory|model that explains the early development of the Universe . According to the Big Bang theory, the Universe was once in an extremely hot and dense state which metric expansion of space|expanded rapidly. This rapid expansion caused the Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, the Big Bang occurred approximately 13.75 billion years ago,

which is thus considered the age of the Universe .

After its initial expansion from a gravitational singularity|singularity , the Universe cooled sufficiently to allow energy to be Mass–energy equivalence|converted into various subatomic particle s, including proton s, neutron s, and electron s. While protons and neutrons big bang nucleosynthesis|combined to form the first atomic nuclei only a few minutes after the Big Bang, it would take thousands of years for electrons to recombination (cosmology)|combine with them and create electrically neutral atoms. The first element produced was hydrogen , along with traces of helium and lithium . Giant clouds of these primordial elements would coalesce through gravity to form star s and galaxy|galaxies , and the periodic table|heavier elements would be synthesized either Stellar nucleosynthesis|within stars or supernova nucleosynthesis|during supernovae .

The Big Bang is a well-tested scientific theory which is widely accepted within the scientific community because it is the most accurate and comprehensive explanation for the full range of phenomena astronomers observe. Since its conception, abundant evidence has arisen to further validate the model.
Georges Lemaître first proposed what would become the Big Bang theory in what he called his "hypothesis of the primeval atom." Over time, scientists would build on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein 's general relativity and on simplifying assumptions such as homogeneity (physics)#Translation invariance|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 galaxy|galaxies were generally proportionality (mathematics)|proportional to their redshift s—an idea originally suggested by Lemaître 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.

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 density|densities and temperature s,

and large particle accelerator s have been built to experiment on and test such conditions, resulting in significant confirmation of this model. On the other hand, these accelerators have limited capabilities to probe into such high-energy physics|high energy regimes . There is little evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theory cannot and does not provide 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 . 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 of thermal radiation from a black body , most scientists had become fairly convinced that some version of the Big Bang scenario must have occurred.

Overview

Timeline of the Big Bang

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.
This gravitational singularity|singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly no closer than the end of the Planck epoch . This singularity is sometimes called "the Big Bang", but the term can also refer to the early hot, dense phase itself, which can be considered the "birth" of our Universe. Based on measurements of the expansion using Type Ia supernova e, measurements of temperature fluctuations in the cosmic microwave background radiation|cosmic microwave background , and measurements of the correlation function of galaxies, the Universe has a calculated age of 13.75 ± 0.11& nbsp;billion years. The agreement of these three independent measurements strongly supports the Lambda-CDM model|?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 Homogeneous space|homogeneously and isotropic ally with an incredibly high energy density and huge temperature s and pressure s and was very rapidly expanding and cooling. Approximately 10^-37 seconds into the expansion, a phase transition caused a cosmic inflation , during which the Universe grew exponential growth|exponentially .
After inflation stopped, the Universe consisted of a quark–gluon plasma , as well as all other elementary particle s.
Temperatures were so high that the random motions of particles were at Special relativity|relativistic Relativistic speed|speeds , and pair production|particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number , leading to a very small excess of quark s and lepton s over antiquarks and antileptons—of the order of one part in 30& nbsp;million. This resulted in the predominance of matter over antimatter in the present Universe.Kolb and Turner (1988), chapter 6

The Universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Explicit symmetry breaking|Symmetry breaking phase transitions put the fundamental force s of physics and the parameters of elementary particles into their present form.Kolb and Turner (1988), chapter 7 After about 10^-11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10^-6 seconds, quarks and gluons combined to form baryon s such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 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 by photon s (with a minor contribution from neutrino s).

A few minutes into the expansion, when the temperature was about a billion (one thousand million; 10⁹; SI prefix giga- ) kelvin and the density was about that of air, neutrons combined with protons to form the Universe's deuterium and helium atomic nucleus|nuclei in a process called Big Bang nucleosynthesis . Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density of matter came to gravity|gravitationally dominate that of the photon electromagnetic radiation|radiation . After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen ); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation .Peacock (1999), chapter 9

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, star s, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the Universe. The four possible types of matter are known as cold dark matter , warm dark matter , hot dark matter and baryonic matter . The best measurements available (from WMAP ) show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold ( warm dark matter is ruled out by early reionization
), and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrino s, then if the "physical baryon density" O_bh² is estimated at about 0.023 (this is different from the 'baryon density' O_b expressed as a fraction of the total matter/energy density, which as noted above is about 0.046), and the corresponding cold dark matter density O_ch² is about 0.11, the corresponding neutrino density O_vh² is estimated to be less than 0.0062.

Independent lines of evidence from Type Ia supernova e and the Cosmic microwave background radiation|CMB imply that the Universe today 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'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 Hubble's law|expansion of the Universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equation s of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state (cosmology)|equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the Lambda-CDM model|?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^-15 seconds or so. Apparently a new unified theory of quantum gravity|quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the Universe is currently one of the greatest unsolved problems in physics .

Underlying assumptions

The Big Bang theory depends on two major assumptions: the universality of physical law s, and the cosmological principle . The cosmological principle states that on large scales the Universe is Homogeneous space|homogeneous and isotropy|isotropic .

These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10^-5.
Also, general relativity has passed stringent tests of general relativity|tests on the scale of the Solar System and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.Detailed information of and references for tests of general relativity are given in the article tests of general relativity .

If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican principle , which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10^-5 via observations of the CMB.This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solar system through the radiation field. The Universe has been measured to be homogeneous on the largest scales at the 10% level.

FLRW metric

General relativity describes spacetime by a metric tensor|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 space|homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker metric (FLRW metric). This metric contains a scale factor , which describes how the size of the Universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates . In this coordinate system the grid expands along with the Universe, and objects that are moving only due to the expansion of the Universe remain at fixed points on the grid. While their coordinate distance ( comoving distance ) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the Universe.

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, metric expansion of space|space itself expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our Universe only on large scales—local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.

Horizons

An important feature of the Big Bang spacetime is the presence of Particle_horizon#Particle_horizon|horizons . Since the Universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon , which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our Universe. Our understanding of the Universe back to very early times Big Bang#Horizon problem|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 accelerating universe|accelerate , there is a future horizon as well.Kolb and Turner (1988), chapter 3

History

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 universe|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.

Development

The Big Bang theory developed from observations of the structure of the Universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a " spiral nebula " (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was Shapley–Curtis debate|highly controversial whether or not these nebulae were "island universes" outside our Milky Way .

Ten years later, Alexander Friedmann, a Russia n physical cosmology|cosmologist and mathematician , derived Friedmann equations|the Friedmann equations from Einstein equation|Albert Einstein's equations of general relativity , showing that the Universe might be expanding in contrast to the static Universe model advocated by Einstein at that time.
:(English translation in: ) In 1924 Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître , a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the Universe.
:(Translated in: )

In 1931 Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the Universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.

Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder , using the Hooker telescope at Mount Wilson Observatory . This allowed him to estimate distances to galaxies whose redshift s had already been measured, mostly by Slipher. In 1929 Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law .
Lemaître had already shown that this was expected, given the Cosmological Principle .

In the 1920s and 1930s almost every major cosmologist preferred an eternal Steady state theory|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 .
This perception was enhanced by the fact that the originator of the Big Bang theory, Monsignor Georges Lemaître, was a Roman Catholic priest. Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that Arguments_for_eternity#Argument_from_the_nature_of_matter|matter is eternal . A beginning in time was "repugnant" to him. Lemaître, however, thought that

If the world has begun with a single Quantum mechanics|quantum , the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the Creation myth|beginning of the world happened a little before the beginning of space and time.

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model ,
the oscillatory Universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman )
and Fritz Zwicky 's tired light hypothesis.

After World War II , two distinct possibilities emerged. One was Fred Hoyle 's steady state model , whereby new matter would be created as the Universe seemed to expand. In this model the Universe is roughly the same at any point in time.
The other was Lemaître's Big Bang theory, advocated and developed by George Gamow , who introduced big bang nucleosynthesis (BBN)
and whose associates, Ralph Alpher and Robert Herman , predicted the cosmic microwave background radiation (CMB).
Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949.
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
secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the Universe at earlier and earlier times, and reconciling observations with the basic theory.

Significant progress in Big Bang cosmology have been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as Cosmic Background Explorer|COBE ,
the Hubble Space Telescope and WMAP .
Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the Universe appears to be accelerating.

Observational evidence

The earliest and most direct kinds of observational evidence are the Hubble's law|Hubble-type expansion seen in the redshift s of galaxies, the detailed measurements of the cosmic microwave background , the relative abundances of light elements produced by Big Bang nucleosynthesis , and today also the Large-scale structure of the cosmos|large scale distribution and apparent Galaxy formation and evolution|evolution of galaxies
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" .

Hubble's law and the expansion of space

Observations of distant galaxies and quasar s show that these objects are redshift ed—the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopy|spectroscopic pattern of emission line s or absorption line s corresponding to atom s of the chemical element s interacting with the light. These redshifts are Homogeneity (physics)|uniformly Isotropy|isotropic , distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift , the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder . When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed:
: 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 kilometers|km / second|s / Megaparsec|Mpc by the WMAP probe.

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 Metric expansion of space|uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander Friedmann in 1922 and Georges Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann–Lemaître–Robertson–Walker metric|Friedmann, Lemaître, Robertson and Walker .

The theory requires the relation v = HD to hold at all times, where D is the comoving distance , v is the recessional velocity , and v , H , and D vary as the Universe expands (hence we write H ₀ to denote the present-day Hubble "constant"). For distances much smaller than the 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.Peacock (1999), chapter 3

That Metric expansion of space|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 redshift s are extremely isotropic and Homogeneity (physics)#Translation invariance|homogenous , supporting the Cosmological principle that the Universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position.
Radiation from the Big Bang was demonstrably warmer at earlier times throughout the Universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the Universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

Cosmic microwave background radiation

In 1964 Arno Penzias and Robert Woodrow Wilson|Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of the general CMB predictions: the radiation was found to be consistent with an almost perfect black body spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725& nbsp;K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded a Nobel Prize in 1978.

The surface of last scattering corresponding to emission of the CMB occurs shortly after Recombination (cosmology)|recombination , the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly Thomson scattering|scattered from free charged particles. Peaking at around , the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.

The data point s and standard error of estimation|error bars on this graph are obscured by the theoretical curve.

In 1989 NASA launched the Cosmic Background Explorer satellite (COBE). Its findings were consistent with predictions regarding the CMB, finding a residual temperature of 2.726& nbsp;K (more recent measurements have revised this figure down slightly to 2.725& nbsp;K) and providing the first evidence for fluctuations (anisotropies) in the CMB, at a level of about one part in 10⁵. John C. 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 of ground-based and balloon experiments. In 2000–2001 several experiments, most notably BOOMERanG experiment|BOOMERanG , found the shape of the Universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies.

In early 2003 the first results of the WMAP|Wilkinson Microwave Anisotropy Probe (WMAP) were released, 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 consistent with the inflation theory in general. The Planck (spacecraft)|Planck space probe was launched in May 2009. Other ground and balloon based List of cosmic microwave background experiments|cosmic microwave background experiments are ongoing.

Abundance of primordial elements

Using the Big Bang model it is possible to calculate the concentration of helium-4 , helium-3 , deuterium and lithium-7 in the Universe as ratios to the amount of ordinary hydrogen. The relative abundances depend on a single parameter, the ratio of photon s to baryon s. This value can be calculated independently from the detailed structure of cosmic microwave background radiation|CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for /, about 10^-3 for /, about 10^-4 for / and about 10^-9 for /.Kolb and Turner (1988), chapter 4

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for , and off by a factor of two ; in the latter two cases there are substantial systematic error|systematic uncertainties . Nonetheless, the general consistency with abundances predicted by Big Bang nucleosynthesis is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium.
Indeed there is no obvious reason outside of the Big Bang that, for example, the young Universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than , and in constant ratios, too.

Galactic evolution and distribution

Detailed observations of the Galaxy morphological classification|morphology and Large-scale structure of the cosmos|distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy groups and clusters|galaxy clusters and supercluster s. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early Universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation , galaxy and quasar distributions and larger structures agree well with Big Bang simulations of the formation of structure in the Universe and are helping to complete details of the theory.

Primordial gas clouds

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

Other lines of evidence

The age of Universe as estimated from the Hubble expansion and the cosmic microwave background radiation|CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev–Zel'dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.

Features and problems

While the scientific community now supports the Big Bang model over other cosmological models, it was once divided between supporters of the Big Bang and those of non-standard cosmologies|alternative cosmological models . Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the observational cosmology|cosmological observations . With the overwhelming scientific consensus|consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations.
The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many observations that are independent from any cosmological model; these include the Big Bang nucleosynthesis|abundance of light elements , the cosmic microwave background , Large-scale structure of the cosmos|large scale structure , and the Hubble diagram for Type Ia supernova e.

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics . Of these features, dark matter is currently the subject to the most active laboratory investigations. Remaining issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter , are not fatal to the dark matter explanation as solutions to such problems exist which involve only further refinements of the theory. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.

On the other hand, cosmic inflation|inflation and baryogenesis remain somewhat more speculative features of current Big Bang models: they explain important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory.If inflation is true, baryogenesis must have occurred, but not vice versa. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics .

Horizon problem

The horizon problem results from the premise that information cannot travel faster than light . In a Universe of finite age this sets a limit—the particle horizon —on the separation of any two regions of space that are in causality (physics)|causal contact.Kolb and Turner (1988), chapter 8 The observed isotropy of the CMB is problematic in this regard: if the Universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause 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 horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable Universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be primordial fluctuations|quantum thermal fluctuations , which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the Universe. Inflation predicts that the primordial fluctuations are nearly Scale invariance|scale invariant and Normal distribution|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 horizon.

Flatness problem

The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann–Lemaître–Robertson–Walker metric . The Universe may have positive, negative or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density , positive if greater, and zero at the critical density, in which case space is said to be flat . The problem is that any small departure from the critical density grows with time, and yet the Universe today remains very close to flat.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 density became significant. Given that a natural timescale for departure from flatness might be the Planck time , 10^-43 seconds, the fact that the Universe has reached neither a heat death nor a Big Crunch after billions of years requires some explanation. For instance, even at the relatively late age of a few minutes
(the time of nucleosynthesis), the Universe density must have been within one part in 10¹⁴ of its critical value, or it would not exist as it does today.

A resolution to this problem is offered by inflationary theory . During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is theorized that inflation drove the Universe to a very nearly spatially flat state, with almost exactly the critical density.

Dark energy

Measurements of the redshift – apparent magnitude|magnitude relation for type Ia supernova e indicate that the expansion of the Universe has been accelerating universe|accelerating since the Universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the Universe consists of a component with large equation of state (cosmology)|negative pressure , dubbed " dark energy ". Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the Universe is very nearly spatially flat, and therefore according to general relativity the Universe must have almost exactly the critical density of mass/energy. But the density|mass density of the Universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the Universe, one using the frequency of gravitational lens es, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is a property of vacuum energy , but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence (physics)|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. The energy density in matter decreases with the expansion of the Universe, but the dark energy density remains constant (or nearly so) as the Universe expands. Therefore matter made up a larger fraction of the total energy of the Universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

In the Lambda-CDM model|?CDM , a leading model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general relativity|general theory of relativity . However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity . Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.

Dark matter

During the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the Universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the Universe is dark matter that does not emit light or interact with normal baryon ic matter. In addition, the assumption that the Universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the Universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy groups and clusters|galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray astronomy|X-ray measurements of galaxy clusters.

The evidence for dark matter comes from its gravitational influence on other matter, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unification theory|Grand unification theories predicted topological defect s in space that would manifest as magnetic monopole s. These objects would be produced efficiently in the hot early Universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by cosmic inflation , which removes all point defects from the observable Universe in the same way that it drives the geometry to flatness.Kolb and Turner, chapter 8

A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature hypothesis .

Baryon asymmetry

It is not yet understood why the Universe has more matter than antimatter .Kolb and Turner, chapter 6 It is generally assumed that when the Universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryon s and antibaryons. However, observations suggest that the Universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the Universe depart from thermodynamic equilibrium .

:(Translated in Journal of Experimental and Theoretical Physics Letters 5 , 24 (1967).)
All these conditions occur in the Standard Model , but the effect is not strong enough to explain the present baryon asymmetry.

Globular cluster age

In the mid-1990s observations of globular cluster s appeared to be inconsistent with the Big Bang theory. Computer simulations that matched the observations of the star|stellar populations of globular clusters suggested that they were about 15& nbsp;billion years old, which conflicted with the 13.7& nbsp;billion year age of the Universe. This issue was partially resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar wind s, indicated a much younger age for globular clusters.
There remain some questions as to how accurately the ages of the clusters are measured, but it is clear that observations of globular clusters no longer appear inconsistent with the Big Bang theory.

The future according to the Big Bang theory

Before observations of dark energy , cosmologists considered two scenarios for the future of the Universe. If the mass density of the Universe were greater than the critical density , then the Universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch .Kolb and Turner, 1988, chapter 3 Alternatively, if the density in the Universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out leaving white dwarf s, neutron star s, and black hole s. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the Universe would asymptotically approach absolute zero —a Big Freeze . Moreover, if the proton were proton decay|unstable , then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation . The entropy of the Universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as Heat death of the universe|heat death .

Modern observations of accelerating universe|accelerating expansion imply that more and more of the currently visible Universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The Lambda-CDM model|?CDM model of the Universe contains dark energy in the form of a cosmological constant . This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the Universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy groups and clusters|galaxy clusters , stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip .

Speculative physics beyond Big Bang theory

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the Universe's history. The equations of classical general relativity indicate a gravitational singularity|singularity at the origin of cosmic time, although this conclusion depends on several assumptions. Moreover, general relativity must break down before the Universe reaches the Planck temperature , and a correct treatment of quantum gravity may avoid the would-be singularity.

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

models including the Hartle–Hawking state|Hartle–Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.

Big Bang lattice model

states that the Universe at the moment of the Big Bang consists of an infinite lattice of fermions 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.

brane cosmology models

in which inflation is due to the movement of branes in string theory ; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model , a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the Universe endlessly cycles from one process to the other.

eternal inflation , in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe expanding from its own big bang.

Proposals in the last two categories see the Big Bang as an event in a much larger and older Universe, or multiverse , and not the literal beginning.

Religious and philosophical implications

As a theory relevant to the origins of the universe, the Big Bang has significant bearing on religion and philosophy. As a result, it has become one of the liveliest areas in the discourse between Relationship between religion and science|science and religion .
Some believe the Big Bang involved a creator,

while others argue that Big Bang cosmology makes the notion of a creator superfluous.

Notes

External links

Category:Physical cosmology
Category:Astrophysics theories
Category:Big Bang|*Main
Category:Universe

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