In physical cosmology, the Big Bang is the scientific theory that the Universe emerged from an enormously dense and hot state about 13.7 billion years ago. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle imply that space is expanding according to the Friedmann-Lemaître model of general relativity. Extrapolated into the past, these observations show that the Universe has expanded from a primeval state, in which all the matter and energy in the Universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity.

The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the Universe (Hubble's law) began - calculated to be 13.7 billion (1.37 × 1010) years ago - and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the Universe, as well as the composition of primordial matter through nucleo-synthesis as predicted by the Alpher-Bethe-Gamow theory.

One consequence of the Big Bang is that the conditions of today's Universe are different from the conditions in the past or in the future. From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB). The CMB was discovered in the 1960s and further validated the Big Bang theory over its chief rival, the steady state theory.

History of the Big Bang

The Big Bang theory developed from observations and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way. In 1927, the Belgian Catholic priest Georges Lemaître independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the Universe began with the "explosion" of a "primeval atom" - what was later called the Big Bang.

In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. He discovered that, relative to the Earth, the galaxies are receding in every direction at speeds directly proportional to their distance from the Earth. This fact is now known as Hubble's law. Given the cosmological principle whereby the Universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the Universe was expanding.

This idea allowed for two opposing possibilities. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the Universe is roughly the same at any point in time. It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this big bang idea" during a program broadcast on March 28, 1949 by the BBC Third Programme. Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled The Nature of Things. The text of each lecture was published in The Listener a week after the broadcast, the first time that the term "big bang" appeared in print.

For a number of years support for above theories was evenly divided. However, observational evidence began to support the idea that Universe evolved from a hot dense state. Since the discovery of the cosmic microwave background radiation in 1965 it has been regarded as the best theory of origin and evolution of cosmos. Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang and reconciling observations with the basic theory.

Huge advances in Big Bang cosmology were made in the late 1990s and the early 21st century as a result of major advances in telescope technology in combination with large amounts of satellite data such as that from COBE, the Hubble Space Telescope and WMAP. Such data has allowed cosmologists to calculate many of the parameters of the Big Bang to a new level of precision and led to the unexpected discovery that the expansion of the Universe appears to be accelerating.

Based on measurements of the expansion of the Universe using Type Ia supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the Universe has a calculated age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements is considered strong evidence for the so-called ΛCDM model that describes the detailed nature of the contents of the Universe.

The early Universe was filled homogeneously and isotropically with an incredibly high energy density and concomitantly huge temperatures and pressures. It expanded and cooled, going through phase transitions analogous to the condensation of steam or freezing of water as it cools, but related to elementary particles.

A few seconds after the Planck epoch a phase transition caused the Universe to experience exponential growth during a period called cosmic inflation. After inflation stopped, the material components of Universe were in the form of a quark-gluon plasma in which the constituent particles were all moving relativistically. As the Universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called baryo-genesis, the quarks and gluons combined into baryons such as protons and neutrons, producing the observed asymmetry between matter and antimatter. Still lower temperatures led to further symmetry breaking phase transitions that put the forces of physics and elementary particles into their present form. Later, some protons and neutrons combined to form the Universe's deuterium and helium nuclei in a process called Big Bang nucleo-synthesis. As the Universe cooled, matter stopped moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 300,000 years the electrons and nuclei combined into hydrogen atoms; hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background.
 

Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the Universe. The three possible types are known as cold dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the Universe is cold dark matter. The other two types of matter make up less than 20% of the matter in the Universe.

The Universe today appears to be dominated by a mysterious form of energy known as dark energy. Approximately 70% of the total energy density of today's Universe is in this form. This component of the Universe's composition is revealed by its property of causing the expansion of the Universe to deviate from a linear velocity-distance relationship by causing spacetime to expand faster than expected at very large distances. Dark energy in its simplest formation takes the form of a cosmological constant term in Einstein's field equations of general relativity, but its composition is unknown and, more generally, the details of its equation of state and relationship with the standard model of particle physics continue to be investigated both observationally and theoretically.

All these observations are encapsulated in the ΛCDM model of cosmology, which is a mathematical model of the Big Bang with six free parameters. Mysteries appear as one looks closer to the beginning, when particle energies were higher than can yet be studied by experiment. There is no compelling physical model for the first 10-33 seconds of the Universe, before the phase transition called for by grand unification theory. At the "first instant", Einstein's theory of gravitation predicts a gravitational singularity where densities become infinite. To resolve this paradox, a theory of quantum gravitation is needed. Understanding this period of the history of the Universe is one of the greatest unsolved problems in physics.

Theoretical underpinnings

As it stands today, the Big Bang is dependent on three assumptions:

The universality of physical laws

When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them. Tests of the universality of physical laws have found that the largest possible deviation of the fine structure constant over the age of the Universe is of order 10-5. The isotropy of the Universe that defines the Cosmological Principle has been tested to a level of 10-5 and the Universe has been measured to be homogenous on the largest scales to the 10% level. There are efforts underway to test the Copernican Principle by means of looking at the interaction of galaxy clusters with the CMB through the Sunyaev-Zel'dovich effect to a level of 1% accuracy.

The Big Bang theory uses Weyl's postulate to unambiguously measure time at any point as the "time since the Planck epoch". Measurements in this system rely on conformal coordinates in which so-called co-moving distances and conformal times remove the expansion of the Universe, parameterized by the cosmological scale factor, from consideration of space-time measurements. The co-moving distances and conformal times are defined so that objects moving with the cosmological flow are always the same co-moving distance apart and the particle horizon or observational limit of the local Universe is set by the conformal time.

As the Universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty Universe; what is expanding is space-time itself. It is this expansion that causes the physical distance between any two fixed points in our Universe to increase. Objects that are bound together (for example, by gravity) do not expand with spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the metric expansion. Moreover, the expansion of the Universe on today's local scales is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques.

Observational evidence
It is generally stated that there are three observational pillars that support the Big Bang theory of cosmology. These are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements. Additionally, the observed correlation function of large-scale structure of the cosmos fits well with standard Big Bang theory.

Hubble's law expansion

Observations of distant galaxies and quasars show that these objects are redshifted, meaning that the light emitted from them has been shifted to longer wavelengths. This is seen by taking a frequency spectrum of the objects and then matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the chemical elements interacting with the light. From this analysis, a redshift corresponding to a Doppler shift for the radiation can be measured which is explained by a recessional velocity. When the recessional velocities are plotted against the distances to the objects, a linear relationship, known as Hubble's law, is observed:

where

v is the recessional velocity of the galaxy or other distant object
D is the distance to the object and
H0 is Hubble's constant, measured to be (71 ± 4) km/s/Mpc by the WMAP probe.
The Hubble's law observation has two possible explanations. One is that we are at the center of an explosion of galaxies, a position which is untenable given the Copernican principle. The second explanation is that the Universe is uniformly expanding everywhere as a unique property of spacetime. This type of universal expansion was developed mathematically in the context of general relativity well before Hubble made his analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann-Lemaître-Robertson-Walker.

Cosmic microwave background radiation

The Big Bang theory predicted the existence of the cosmic microwave background radiation or CMB which is composed of photons emitted during baryogenesis. Because the early Universe was in thermal equilibrium, the temperature of the radiation and the plasma were equal until the plasma recombined. Before atoms formed, radiation was constantly absorbed and reemitted in a process called Compton scattering: the early Universe was opaque to light. However, cooling due to the expansion of the Universe allowed the temperature to eventually fall below 3,000 K at which point electrons and nuclei combined to form atoms and the primordial plasma turned into a neutral gas. This is known as photon decoupling. A Universe with only neutral atoms allows radiation to travel largely unimpeded.

Because the early Universe was in thermal equilibrium, the radiation from this time had a blackbody spectrum and freely streamed through space until today, becoming redshifted because of the Hubble expansion. This reduces the high temperature of the blackbody spectrum. The radiation should be observable at every point in the Universe to come from all directions of space.

In 1964, Arno Penzias and Robert Wilson, while conducting a series of diagnostic observations using a new microwave receiver owned by Bell Laboratories, discovered the cosmic background radiation. Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K —and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded the Nobel Prize for their discovery.

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and determined that the CMB was isotropic to about one part in 105. During the 1990s, CMB anisotropies were further investigated by a large number of ground-based experiments and the Universe was shown to be almost geometrically flat by measuring the typical angular size (the size on the sky) of the anisotropies.

In early 2003 the results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general.

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, H. All the abundances depend on a single parameter, the ratio of photons to baryons. The ratios predicted (by mass, not by number) are about 0.25 for 4He/H, about 10-3 for 2H/H, about 10-4 for 3He/H and about 10-9 for 7Li/H.

The measured abundances all agree with those predicted from a single value of the baryon-to-photon ratio. The agreement is relatively poor for 7Li and 4He, the two elements for which the systematic uncertainties are least understood. This is considered strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements. 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 essentially free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.

Galactic evolution and distribution

Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early Universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies 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.

Features, issues and problems
A number of problems have arisen within the Big Bang theory throughout its history. Some of them are mainly of historical interest today, and have been avoided either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as they can be addressed through refinements of the theory.

There are a small number of proponents of non-standard cosmologies who doubt that there was a Big Bang at all. They claim that solutions to standard problems in the Big Bang theory involve ad hoc modifications and addenda to the theory. Most often attacked are the parts of standard cosmology that include dark matter, dark energy, and cosmic inflation. However, while explanations for these features remain at the frontiers of inquiry in physics, together they are suggested by independent observations of big bang nucleo-synthesis, the cosmic microwave background, large scale structure and Type Ia supernovae. The gravitational effects of these features are understood observationally and theoretically but they have not yet been successfully incorporated into the Standard Model of particle physics. Though some aspects of the theory remain inadequately explained by fundamental physics, almost all astronomers and physicists accept that the close agreement between Big Bang theory and observation have firmly established all the basic parts of the theory.

The following is a short list of Big Bang "problems" and puzzles:

Horizon problem
The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are separated by a greater distance than the speed of light multiplied by the age of the Universe cannot be in causal contact. The observed isotropy of the cosmic microwave background (CMB) is problematic in this regard, because the horizon size at that time corresponds to a size that is about 2 degrees on the sky. If the Universe has had the same expansion history since the Planck epoch, there is no mechanism to cause these 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 a time 10-35 seconds after the Planck epoch. During inflation, the Universe undergoes exponential expansion, and regions in causal contact expand so as to be beyond each other's horizons. Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the Universe. After inflation, the Universe expands according to Hubble's law, and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB.

Flatness problem
The flatness problem is an observational problem that results from considerations of the geometry associated with a Friedmann-Lemaître-Robertson-Walker metric. In general, the Universe can have three different kinds of geometries: hyperbolic geometry, Euclidean geometry, or elliptic geometry. The geometry is determined by the total energy density of the Universe (as measured by means of the stress-energy tensor): hyperbolic results from a density less than the critical density, elliptic from a density greater than the critical density, and Euclidean from exactly the critical density. The Universe is required to be within one part in 1015 of the critical density in its earliest stages. Any greater deviation would have caused either a Heat Death or a Big Crunch, and the Universe would not exist as it does today.

A possible resolution to this problem is again offered by inflationary theory. During the inflationary period, space-time expanded to such an extent that any residual curvature associated with it would have been smoothed out to a high degree of precision. Thus, it is believed that inflation drove the Universe to be very nearly spatially flat.

Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted point defects in space that would manifest as magnetic monopoles with a density much higher than was consistent with observations, given that searches have never found any monopoles. This problem is also resolvable by cosmic inflation, which removes all point defects from the observable Universe in the same way that it drives the geometry to flatness.

Baryon asymmetry
It is not yet understood why the Universe has more matter than antimatter. It is generally assumed that when the Universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the Universe, including its most distant parts, is made almost entirely of matter. An unknown process called baryogenesis created the asymmetry. For baryogenesis to occur, the Sakharov conditions, which were laid out by Andrei Sakharov, must be satisfied. They require that baryon number be not conserved, that C-symmetry and CP-symmetry be violated, and that the Universe depart from thermodynamic equilibrium. All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry. Experiments taking place at CERN near Geneva seek to trap enough anti-hydrogen to compare its spectrum with hydrogen. Any difference would be evidence of a CPT symmetry violation and therefore a Lorentz violation.

Globular cluster age
In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7-billion-year age of the Universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the Universe.

Dark matter
During the 1970s and 1980s various observations (notably of galactic rotation curves) showed that there was 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 not normal or baryonic matter but rather dark matter. In addition, assuming that the Universe was mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the Universe is far less lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now a widely accepted part of standard cosmology due to observations of the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and x-ray measurements from galaxy clusters. Dark matter has only been detected through its gravitational signature; no particles that might make it up have yet been observed in laboratories. However, there are many particle physics candidates for dark matter, and several projects to detect them are underway.

Dark energy
In the 1990s, detailed measurements of the mass density of the Universe revealed a value that was 30% that of the critical density. Since the Universe is very nearly spatially flat, as is indicated by measurements of the cosmic microwave background, about 70% of the energy density of the Universe was left unaccounted for. This mystery now appears to be connected to another one: Independent measurements of Type Ia supernovae have revealed that the expansion of the Universe is undergoing a non-linear acceleration rather than following strictly Hubble's law. To explain this acceleration, general relativity requires that much of the Universe consist of an energy component with large negative pressure. This dark energy is now thought to make up the missing 70%. Its nature remains one of the great mysteries of the Big Bang. Possible candidates include a scalar cosmological constant and quintessence. Observations to help understand this are ongoing.

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 is above 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 that was similar to that in which it started - a Big Crunch. Alternatively, if the density in the Universe is equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as the Universe grows less dense. The average temperature of the Universe would asymptotically approach absolute zero. Black holes would evaporate. 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. Moreover, if proton decay exists, then hydrogen, the predominant form of baryonic matter in the Universe today, would disappear, leaving only radiation.

Modern observations of accelerated 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 ΛCDM model of the Universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the Universe cools and expands. Other explanations of dark energy - so-called phantom energy theories - suggest that ultimately galaxy clusters and eventually galaxies themselves will be torn apart by the ever-increasing expansion in a so-called Big Rip.

Speculative physics beyond the Big Bang
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 Universe, when inflation is hypothesized to have occurred. There may also be parts of the Universe well beyond what can be observed in principle. In the case of inflation this is required: exponential expansion has pushed large regions of space beyond our observable horizon. It may be possible to deduce what happened when we better understand physics at very high energy scales. Speculations about this often involve theories of quantum gravitation. Some proposals are:

chaotic inflation
Universe in which the early Universe's hot, dense state resulted from the Big Crunch of a universe similar to ours. The Universe could have gone through an infinite number of big bangs and big crunches. The cyclic extension of the ekpyrotic model is a modern version of such a scenario. (The chief outstanding problem is that entropy would apparently be carried over to each new cycle, resulting in a condition of heat death in the remote past).

Philosophical and religious interpretations
There are a number of interpretations of the Big Bang theory that are extra-scientific. Some of these ideas purport to explain the cause of the Big Bang itself (first cause), although science cannot possibly show a first cause, so they have been criticized by some naturalist philosophers as being modern creation myths. Some people believe that the Big Bang theory lends support to traditional views of creation as given in Genesis, for example, while others believe that the Big Bang theory is inconsistent with such views.

The Big Bang, as a scientific theory, is not based on any religion. While some religious interpretations conflict with the Big Bang story of the Universe, there are many other interpretations that do not. The following is a list of various religious interpretations of the Big Bang theory:

Certain theistic branches of Hinduism, such as in Vaishnavism, conceive of a theory of creation with similarities to the theory of the Big Bang. The Hindu mythos, narrated for example in the third book of the Bhagavata Purana, describes a primordial state which bursts forth as the Great Vishnu glances over it, transforming into the active state of the sum-total of matter ("prakriti"). Other forms of Hinduism assert a Universe without beginning or end. Buddhism has a concept of a Universe that has no creation event. The Big Bang, however, is not seen to be in conflict with this since there are ways to conceive an eternal universe within the paradigm. A number of popular Zen philosophers were intrigued, in particular, by the concept of the oscillating Universe.

A number of Christian churches have accepted the Big Bang as a possible description of the origin of the Universe, interpreting it to allow for a philosophical first cause. Some students of Kabbalah, deism and other non-anthropomorphic faiths concord with the Big Bang theory, connecting it with the theory of "divine retraction" (tzimtzum) as explained by the Jewish scholar Moses Maimonides. Some modern Islamic scholars believe that the Qur'an parallels the Big Bang in its account of creation, described as follows: "Do not the unbelievers see that the heavens and earth were joined together as one unit of creation, before We clove them asunder?"