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The Big Bang theory is currently (2003) the dominant theory in cosmology about the early development and current shape of the universe. According to this theory, the universe expanded rapidly starting 13.7 ± 0.2 billion years ago.

Extrapolating the history of the universe backwards using current physical models leads to a gravitational singularity, at which all distances become zero and temperatures and pressures become infinite. What this means is unclear, and most physicists believe that this result is due to our limited understanding of the laws of physics with regard to this type of situation, and in particular, the lack of a theory of quantum gravity.

The universe as we know it was initially almost uniformly filled with energy and extremely hot. As the distances in the universe rapidly grew, the temperature dropped, leading to the creation of the known forces of physics, elementary particles, and eventually hydrogen and helium atoms in a process called Big bang nucleosynthesis.

Over time, the slightly denser regions of the almost, but not quite, uniformly distributed matter were pulled together by gravity into clumps, forming gas clouds, stars, galaxies, and the other astronomical structures seen today. The details of how the process of galaxy formation occurred depends on the type of matter in the universe, and the three competing pictures of how this occurred are known as cold dark matter, hot dark matter, and baryonic matter.

It is at present unknown whether the singularity of spacetime described above is a physical reality or just a mathematical extrapolation of general relativity beyond its limits of applicability. The resolution of this question has to wait until a confirmed theory of quantum gravity is available.

In general relativity, one usually talks about spacetime and cannot cleanly separate space from time. In the Big Bang theory, this difficulty does not arise; Weyl's postulate is assumed and time can be unambiguously measured at any point as the "time since the Big Bang".

The Big Bang was not an explosion of matter moving outward to fill an empty universe. Instead, it involved the rapid growth of the universe itself. Because of this, the distance (in the sense of comoving distance) between far removed galaxies increases faster than the speed of light. This does not violate the laws of special relativity, a theory which is physically valid only as a local theory. It states, among other things, that matter and information cannot travel through space faster than the speed of light, and it is empirically invalid for global space-time concepts (because it ignores gravity).

In 1927, the Belgian priest Georges Lemaître was the first to propose that the universe began with the explosion of a "primeval atom". Earlier, in 1918, the Strasbourg astronomer Wirtz had measured a systematic redshift of certain "nebulae", and called this the K-correction, but he wasn't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.

Years later, Edwin Hubble found experimental evidence to help justify Lemaître's theory. Again using redshift measurements, Hubble determined that distant galaxies are receding in every direction at speeds (relative to the Earth) directly proportional to their distance, a fact now known as Hubble's law.

Since galaxies were receding, this suggested two possibilities. One, proposed by George Gamow, was that the universe began a finite time in the past and has been expanding ever since.
The other was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other and that the universe at one point in time would look roughly like any other point in time.
For a number of years the support for these two opposing theories was evenly divided.

In the intervening period however, all observational evidence gathered has provided overwhelming support for the Big Bang theory, and since the mid-1960s it has been regarded as the best available theory of the origin and evolution of the cosmos, and virtually all theoretical work in cosmology involves extensions and refinements to the basic big bang theory. Much of the current work in cosmology includes understanding how galaxies form within the context of the big bang, understanding what happened at the big bang, and reconciling observations with the basic theory.

Over the decades a number of weaknesses have been identified in the big bang theory, but these have thus far all been addressed by extensions and refinements such as cosmic inflation. As of 2003, there are no weaknesses in the big bang theory which are regarded as fatal by most or even a large minority of cosmologists.
However, there remain small numbers of who still support non-standard cosmologies in which the big bang is considered incorrect.

The Redshift of galaxies

By analyzing the light from distant galaxies, one notices that the shape of the light's spectrum is very similar, but the whole spectrum is shifted towards longer wavelengths for more distant galaxies. This suggests that the galaxies are moving away from us, resulting in an effect akin to the Doppler effect called redshift.

Background Radiation

A (now) major aspect of the Big Bang hypothesis was the prediction in the 1940s of cosmic microwave background radiation or CMBR. The theory proposed that, as all the mass/energy of the universe emerged from the primordial explosion, the initial density of the universe was incredibly high, and hence the temperature of the universe must have been extremely hot (as matter gets hotter when compressed to a higher density). The initial temperature of the universe was so high that matter (as we know it) could not exist, as the subatomic particles[?] would have been too energetic to aggregate into atoms.

However, as the universe was expanding it would also have cooled down. As the temperature of the universe fell, matter could form from the primordial plasma[?]. The theory predicted that at some stage (currently reckoned to be around 500,000 years after the beginning), this plasma would thin out sufficiently to permit photons to be set free from the attraction of the other matter, and travel through the constantly expanding reaches of space. The process that produced this blast of free energy is known as photon decoupling[?].

Based on this premise, the theory predicted that this massive blast of radiation should have left some traces in the cosmos, and would have a number of properties. Essentially it says that as the universe was extremely hot at one point, it should still be a little bit warm even today, and calculations predicted a residual temperature of about 3 Kelvin (3 degrees Celsius above absolute zero). Additionally, as the radiation was produced simultaneously, the traces of it should be uniform or isotropic. Another prediction was that as these photons are subject to the expansion of space, their wavelengths would have been "stretched" or red-shifted. A critical further prediction was that the further away one looks, the hotter the universe should appear to be (as looking further away corresponds to looking backwards in time), and at some extremely distant point the radiation in the universe should be so thick as to become opaque.

At the time they were made, the predictions of the Big Bang theory regarding CMBR were largely ignored, simply because they remained unverifiable due to inadequate technology for nearly 20 years.

However, in 1964, Arno Penzias[?] and Robert Wilson[?] conducted a series of diagnostic observations using a new microwave receiver owned by Bell Laboratories (which was designed for normal telephone communications) and accidentally discovered the cosmic background radiation originally predicted by Gamow. This observation was later confirmed by the Peebles group at Princeton University, who were themselves trying to construct a microwave antenna with a ruby maser to detect the CMBR when Penzias and Wilson "ran across" it. It was not until Penzias and Wilson consulted with the Peebles group that they understood what it was they had detected. Penzias and Wilson published their findings jointly with the Peebles group in the Astrophysical Journal.

Their discovery provided substantial confirmation of almost every aspect of the CMBR predictions, and overwhelmingly swayed the balance of opinion in favour of the Big Bang hypothesis. Penzias and Wilson were awarded the Nobel Prize for this discovery. In 1989, NASA launched the Cosmic Background Explorer satellite[?] (COBE), and the initial findings (released in 1990) were a stunning endorsement of the Big Bang theory's predictions regarding CMBR, finding a local residual temperature of 2.726 K, determining that the CMBR was indeed isotropic, and confirming the "haze" effect as distance increased.

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.
All the abundances depend on a single parameter, the ratio of photons to baryons. Measurements of primordial abundances for all four isotopes are consistent with a unique value of that parameter (see big bang nucleosynthesis.) Steady State theories fail to account for the abundance of deuterium in the cosmos, because deuterium easily undergoes nuclear fusion in stars and there are no known astrophysical processes other than the Big Bang itself that can produce it in large quantities. Hence the fact that deuterium is not a rare component of the universe suggests that the universe has a finite age.

Distribution of quasars

Quasars are predicted to only be possible in the early stages of a dynamic cosmos by the Big Bang theory, and observational evidence supports this, as quasar populations become denser the further away one looks. (more needed)

Olbers' Paradox

One piece of evidence for the Big Bang model is that it resolves Olbers' paradox of why the sky is black at night.

Weaknesses and criticisms of the Big Bang Theory

One weakness of the Big Bang theory is the obvious question of how the Big Bang occurred. The difficulty of answering this question lies with the absence of a theory of quantum gravity. As one goes back in time, the temperature and the pressures increase to the point where the physical laws governing the behavior of matter are unknown. It is hoped that as we understand these laws that we will better be able to answer the question of what happened "before" the Big Bang.

Dark matter

During the 1970s, observations were made that - assuming that all of the matter within the universe could be seen - created problems for the Big Bang theory, as it seemed to underestimate the amount of deuterium in the universe and lead to a universe that was much more "lumpy" than observed. These problems are resolved if one assumes that most of the matter in the universe is not visible, and this assumption seems to be consistent with observations that suggest that much of the universe consists of dark matter.

The effects that dark matter has on big bang calculations generally do not depend on the detailed properties of the dark matter. The main property of dark matter which influences cosmology is whether the dark matter consists of particles that are heavy and hence are moving slowly, thereby creating cold dark matter, or whether it consists of particles are are light and hence are moving quickly, thereby creating hot dark matter, or whether the dark matter consists of ordinary matter which is baryonic matter.

Proton-antiproton imbalance[?]

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Age of universe and values of omega, Hubble Constant

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The future according to the Big Bang theory

All the matter in the universe is gravitationally attracted to other matter which is within the observable horizon (defined by the age of the universe). This should cause the expansion rate of the universe to slow down over time. Exactly how much matter exists in any given volume, relative to how large the horizon is and how fast the universe is currently expanding can lead to one of three scenarios:

The Big Crunch

If the gravitational attraction of all the matter in the observable horizon is high enough, then it could stop the expansion of the universe, and then reverse it. The universe would then contract, in about the same time as the expansion took. Eventually, all matter and energy would be compressed back into a gravitational singularity. It is impossible to ask what would happen after this, as time would stop in this singularity as well.

The Big Freeze

If the gravitational attraction of all the matter in the observable horizon is low enough, then the expansion will never stop. As the matter disperses into ever greater and greater volumes, new star formation would drop off. The average temperature of the Universe would asymptotically approach absolute zero, and the Universe would become very still and quiet. Eventually, all the protons would decay, the black holes would evaporate, and the Universe would consist of dispersed subatomic particles. The Big Freeze is also known as the heat death[?] of the universe.

Balance

If the gravitational attraction of all the matter in the observable horizon is just right, then the expansion of the universe will asymptotically approach zero. The temperature of the universe would asymptotically approach a stable value slightly above absolute zero. Entropy would increase, and the end result (with protons decaying) would be similar to the Big Freeze.

Recent observations

One extremely puzzling recent discovery comes from observations of type I supernovae which allow one to better calculate the distance to galaxies, from observations of the cosmic microwave background, from gravitational lensing, and from the use of large length scale statistics of the distributions of galaxies and quasars as standard rulers for measuring distances. It appears that the expansion of the universe is accelerating, an observation which astrophysicists are currently trying to understand (see accelerating universe). The currently favored approach is to reintroduce a non-zero cosmological constant into Einstein's equations of General Relativity, and adjust the numerical value of that constant to match the observed acceleration. This is akin to postulating a repelling "dark energy" also called quintessence.

See also the ultimate fate of the Universe.

Big Bang theory vs. religion

When the Big Bang theory was originally proposed, it was rejected by most scientists and enthusiastically embraced by the pope, because it seemed to point to a creation event. While most scientists nowadays view the Big Bang theory as the best explanation of the available evidence, and the Catholic Church still accepts it, some conservative Christians (usually Fundamentalists) oppose it because they see a contradiction between the theory and a literal interpretation of the creation stories of the Bible. Progress toward reconciliation between science and this interpretation has been made by physicist Gerald Schroeder, who claims that his calculations confirm a relativistic correspondence between the measured age of the universe and the six days of creation described in Genesis.

Origin of the term

The term "Big Bang" was coined by Fred Hoyle in a BBC radio program in the early 1950s; Hoyle did not subscribe to the theory and intended to mock the concept.