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Big Bang

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In astrophysics, the term Big Bang is used both in a narrow sense to refer to the interval of time roughly 13.7 billion years ago when the photons observed in the microwave cosmic background radiation acquired their black-body form, and in a more general sense to refer to a hypothesized point in time when the observed expansion of the universe (Hubble's law) began.

In cosmology, the Big Bang theory is the prevailing scientific theory about the early development and shape of the universe. The central idea is that the observation that galaxies appear to be receding from each other can be combined with the theory of general relativity to extrapolate the conditions of the universe back in time. This leads to the conclusion that as one goes back in time, the universe becomes increasingly hot and dense.

There are a number of consequences to this view. One consequence is that the universe now is very different than the universe in the past or in the future. The Big Bang theory predicts that at some point, the matter in the universe was hot and dense enough to prevent light from flowing freely in space. That this period of the universe would be observable in the form of cosmic background radiation (CBR) was first predicted in the 1940s, and the discovery of such radiation in the 1960s swung most scientific opinion against the Big Bang theory's chief rival, the steady state theory.

Using current physical theories to extrapolate the Hubble expansion of the universe backwards 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 is because of 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.

There are actually many theories about the Big Bang. Some theories purport to explain the cause of the Big Bang itself, and as such have been criticized as being modern creation myths. Some people believe that the Big Bang theory lends support to traditional views of creation, for example as given in Genesis, while others believe that all Big Bang theories are inconsistent with such views. The relationship between religion and the Big Bang theory is discussed below.

Table of contents
1 Overview
2 History of the theory
3 Supporting evidence
4 Weaknesses and criticisms of the Big Bang theory
5 The future according to the Big Bang theory
6 Recent observations
7 Big Bang theory and religion
8 Origin of the term
9 See also
10 External links and references


Based on measurements of the expansion of the universe using type I supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, it is currently believed that the universe has an age of 13.7 ñ 0.2 billion years. The fact that these three separate measurements of completely different things are all consistent with each other is considered strong evidence for the model.

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 based on the properties of three types of matter known as cold dark matter, hot dark matter, and baryonic matter. These three models have been tested through computer simulations and observations of galactic correlation functions. The best measurements available (from WMAP) show that the dominant form of matter in the universe is in the form of cold dark matter. The other two types of matter make up less than 20% of the matter in the universe.

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 depends on a theory of quantum gravity, which is not currently available. Nevertheless, there has been intense theoretical work on trying to figure out what happened before the Big Bang. Some of these efforts involve the ekpyrotic universe, and there has also been interest in the anthropic principle.

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". Measurements in this system rely on so-called conformal distances and times which removes the expansion of the universe from consideration of spacetime measurements.

The Big Bang was not an explosion of matter moving outward to fill an empty universe; it is space itself that is expanding. So, bizarre as it may seem, the distance between any two fixed points in our universe is increasing. Intuitively this seems impossible: if the distance between two things increases then it seems that by definition one or both must be moving. But this is not so, as becomes clear if you consider the simplistic but logically equivalent model of a universe of constant size (whether finite or infinite), in which everything is shrinking. The people who live in this universe are shrinking too, as are all their scientific instruments. When these people measure the distance between two points that are sufficiently far apart, the distance will seem to be increasing, because the yardsticks they use to measure with are shrinking along with everything else. The fundamental assumption in this idea is that spacetime on the largest scales is unaffected by locality; objects that are bound together do not expand with spacetime's expansion because local forces keep them together. The expansion of the universe on local scales is so small that the difference of any local forces is unmeasurable by current techniques.

Because it is space itself that is expanding, and not a case of objects flying apart through space, the distance (in the sense of comoving distance) between far removed galaxies can increase faster than the speed of light without violating the laws of special relativity.

History of the theory

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.

Einstein's theory of general relativity developed during this time had the result that the universe could not remain static, a result that he himself considered wrong, and which he attempted to fix by adding a cosmological constant which did not fix the problem. Applying general relativity to cosmology was done by Alexander Friedmann whose equations describe the Friedmann-Robertson-Walker universe.

In the 1930s, Edwin Hubble found experimental evidence to help justify Lemaître's theory. Hubble had also determined that galaxies were receding back in 1913. 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, advocated and developed 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.

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 from COBE and WMAP. This data allowed astronomers to calculate many of the parameters of the Big Bang to new precision and opened up a major unexpected finding that the expansion of the universe appears to be accelerating.

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, some cosmologists still support non-standard cosmologies in which the Big Bang does not occur.

Recent research has been refining the Big Bang by including a model for the matter within the universe to understand the process of galaxy formation. Most current models are based on the notion of cold dark matter which has supplanted other models of hot dark matter and baryonic matter. As of 2003, theories based on cold dark matter still have some conflicts with observations, namely the dwarf galaxies problem and the cuspy halo problem.

See also: Timeline of the Big Bang

Supporting evidence

In describing the evidence for the Big Bang, it is necessary to distinguish between observations which are also consistent with other theories, and observations which are not easily explained by other theories. The former category includes the observations that the universe appears to be isotropic, that galaxies appear to be receding from each other, and that the sky is dark (see Olbers' paradox). While these observations are all consistent with the Big Bang theory, each of them is also consistent with at least one other theory, such as Fred Hoyle's steady-state universe and Hannes Alfven's plasma universe.

The observations which are readily explained within the Big Bang framework but which are not so easily explained otherwise are as follows.

Cosmic background radiation

One feature of the Big Bang hypothesis was the prediction in the 1940s of the discovery of the cosmic microwave background radiation or CMBR. According to the Big Bang theory, as all the mass/energy of the universe emerged from a primordial explosion, the initial density of the universe must have been incredibly high. Since matter cools when it becomes less dense, the early universe must have been extremely hot. In fact, the temperature of the very early universe must have been so high that matter as we know it could not exist, because the subatomic particles would have been too energetic to aggregate into atoms.

However, as the temperature of the universe fell, the theory predicted that more familiar forms of matter would form from the primordial plasma. At some stage (currently reckoned to be around 500,000 years after the beginning), the temperature would fall below 3000 K. Above this temperature, electrons and protons are separate, making the universe opaque to light. Below 3000 K, atoms form, allowing light to pass freely through the gas of the universe. This is known as photon decoupling.

The Big Bang theory therefore predicts that if you look far enough into space, and hence far enough back in time, you will eventually see the location at which the universe becomes opaque to radiation. The radiation from this region will be redshifted because of the Hubble expansion. This results in the visible spectrum of the 3000 kelvin radiation from the opaque region to be redshifted to a much lower temperature. The radiation should be almost completely isotropic.

At the time they were made and for the next 20 years, the predictions of the Big Bang theory regarding CMBR were largely ignored, simply because they remained unverifiable due to inadequate technology. Initially, George Gamow calculated that the CMBR should appear as a black body radiating at 50K. He later revised the calculation and estimated the temperature of the CMBR as about 5K. This was an error being somewhat higher than the 2.73K black body later observed.

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 made their serendipitous discovery. 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 the general CMBR predictions (though it required correction of inaccurate values), and pitched the balance of opinion in favour 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 theory's predictions regarding CMBR, finding a local residual temperature of 2.726 K, determining that the CMBR was generally isotropic, and confirming the "haze" effect as distance increased. During the 1990s, CMBR data was studied further to see if small anisotropies predicted by the Big Bang theory would be observed. They were found in the late 1990s.

In early 2003 the results of the Wilkinson Microwave Anisotropy satellite (WMAP) were analysed, giving the most accurate cosmological values we have to date. This satellite also disproved several specific inflationary 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. All the abundances depend on a single parameter, the ratio of photons to baryons. The abundances predicted are about 25 percent for 4He, a 2H/H ratio of about 10-3, a 3He/H of about 10-4 and a 7Li/H abundance of about 10-9.

Measurements of primordial abundances for all four isotopes are consistent with a unique value of that parameter (see Big Bang nucleosynthesis.), and the fact that the measured abundances are in the same range as the predicted ones is considered strong evidence for the Big Bang. There is no obvious reason outside of the Big Bang that, for example, the universe should have more helium than deuterium or more deuterium than 3He. Thus far, no other theory has attempted to make the nucleosynthetic predictions that the Big Bang does.

Theories which assert that the universe has an infinite life such as the steady state theory 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 an extremely rare component of the universe suggests that the universe has a finite age.

Theories which assert that the universe has a finite life but that the Big Bang did not happen have problems with the abundance of helium-4. The observed amount of 4He is far larger than what could be created via stars or any other known process. By contrast, the abundance of 4He are very insensitive to assumptions about baryon density changing only a few percent, as the baryon density changes by several orders of magnitude. The observed value of 4He appears to be within the range calculated.

This having been said, there are three theoretical issues with Big Bang nucleosynthesis which have some potential of undermining the theory. The first is that the baryon concentration necessary to get an exact match with the current abundances is inconsistent with a universe with mostly baryons. The second is that the Big Bang predicts that no elements heavier than lithium would have been created in the Big Bang, yet elements heavier than lithium are observed in quasars, which presumably are some of the oldest galaxies in the universe. The third problem is since big bang nucleosynthesis produces no elements heavier than lithium, then we ought to see some long lived remnant stars which have no heavy elements in them. We don't.

The standard explanation for the first are that most of the universe isn't composed of baryons. This explanation fits nicely with other evidence of dark matter such as galaxy rotation curves. The standard explanation for the second and third is that the universe underwent a period of massive star formation creating large high mass stars and that without heavy elements, forming low mass red dwarf stars is impossible. This explanation has the feature that it predicts a class of stars that, as of 2004, have not been observed. Hence, in a few years we should have either seen them, which would support the big bang scenario, or we won't, in which case there is a possibility that we will have to fundamentally alter our views of the universe.

Galactic evolution and quasar distribution

One observation that has become increasing apparent since the early 1970s is that while the universe appears to be isotropic in space (i.e. the universe in one direction looks very much like the universe in another direction) it is not uniform with respect to distance (due to the finite speed of light, greater distances represent earlier times in the past). As one looks to increasingly large distances, the universe looks very different. For example, there are no nearby quasars, but there are many quasars once you pass a given redshift, and then the quasars disappear at a still further distance. Similarly, the types and distribution of galaxies appears to change markedly over time and once one passes a given distance, the number of galaxies fall off considerably.

Weaknesses and criticisms of the Big Bang theory

Throughout its history, a number of criticisms have been offered against the Big Bang theory. Some of them are today mainly of historical interest, and have been removed either through modifications to the theory or as the result of better observations. Others issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are considered to be non-fatal as they can be addressed through relatively minor adjustments to the theory. Finally, there are proponents of non-standard cosmologies who believe that there was no Big Bang at all.

The initial condition problem

One unanswered question is how the Big Bang might have 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.

Magnetic monopole problem

The magnetic monopole problem was an objection that was raised in the late-1970s. Grand unification theories predicted point defects in space which would manifest themselves as magnetic monopoles, and the density of these monopoles was much higher than what could be accounted for. This problem is resolvable by the addition of cosmic inflation.

The horizon problem

The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are expanding at faster than the speed of light relative to each other cannot communicate. This means that there is no mechanism to ensure that they have the same temperature. In the 1970s, no anisotropies had been observed which contradicted non-inflationary theories of the Big Bang. This problem was partially resolved by cosmic inflation which reduced the horizon problem by arguing that the early universe suddenly underwent a period of massive expansion before which regions that would later not be in contact with each other could equalize their temperatures.

However, cosmic inflation predicted that the anisotropies in the Big Bang would be reduced but not eliminated. Even with inflation, there would be regions of space that could not be in thermal contact. In the early 1990s, there was some excitement and nervousness, as satellite detectors such as COBE at first failed to detect any anisotropy, and various inflationary scenarios began to be invalidated. Had another few years passed without any detections of anisotropy, the Big Bang would have been very badly hurt, but this was not the case, and the expected anisotropies were detected.

The horizon problem is still of major interest because it allows one to deduce large amounts of information from the CMB. Different expansion rates will result in different amounts of lumpiness in the CMB as a result of material falling past the horizons at different times, and this provides much data about the conditions within the universe at the time the CMB was formed.

Globular cluster age

One major issue that had the potential of challenging the Big Bang occurred in the mid-1990s. Computer simulations of globular clusters suggested that they were about 15 billion years old, which conflicted with some values of the Hubble constant suggesting that the universe was 10 billion years old. This issue was resolved in the late 1990s with other new computer simulations which included the effects of mass loss due to stellar winds indicated a much younger age for globular clusters.

Elemental abundance arguments

During the mid-1990s, measurements of the amount of primordial helium abundance suggested the possibility that the helium abundance of the first stars would have been less than 20%. If this were the case, it would have posed major problems for the Big Bang theory, as it is very difficult to get low amounts of helium from the Big Bang. This potential problem was resolved in the late-1990s by better measurements of helium abundances.

As mentioned earlier, there are also issues with the baryon density and the observation of heavy elements with quasars. These are widely considered to be less serious challenges to the Big Bang, however, they have the potential to undermine the theory if explanations advanced for them prove inadequate.

For example, the consensus is that in order to explain heavy elements in quasars, a large burst of massive star formation is needed, and as of 2004, much current research is aimed at trying to find these stars. If these population III stars are found, this will strengthen the Big Bang theory.


There also remain small numbers of astrophysicists, including Y.P. Varshni and Halton Arp, who argue that redshifts in galaxies are not strictly due to the Doppler effect, and that this invalidates the need for the Big Bang. However, these astrophysicists propose no alternative mechanism, rather they rely on their own incredulity to criticize the standard cosmological model.

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 that 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.

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. There are theories about what happens after this, but these remain uncertain as the physics of singularities remains in question. Also, the omega point theory suggests that an infinite amount of computational capacity might be available in the finite time before the Crunch.

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.


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 and religion

When the Big Bang theory was originally proposed, it was rejected by most cosmologists and enthusiastically embraced by the Pope, because it seemed to point to a creation event. A few scientists, for example astronomer Robert Jastrow, also see the Big Bang as confirmation of the account given in Genesis. 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 the age of the universe is far higher than the one calculated from a literal reading of the book of Genesis in the Bible. Many ways have been proposed to reconcile the two including denying the fundamentalist reading of Genesis or denying the correctness of the age of the universe - see Day-Age Creationism.

Similarly, some Muslims claim that a verse in the Qur'an, the holy book of Islam, can be correlated to the Big Bang. The verse in question, the 30th in its 21st chapter, states the following: "Do the disbelievers not see that the heavens and the earth were joined together, then I split them apart".

Origin of the term

The term "Big Bang" was coined in 1949 by Fred Hoyle during a BBC radio program, The Nature of Things; the text was published in 1950. Hoyle did not subscribe to the theory and intended to mock the concept. It may have been in part a joking reference to the fact that George Gamow, the leading proponent of the theory at the time, also worked on the development of the atomic bomb.

See also

Main: Timeline of the Big Bang | Dark Ages | Big bounce | Big Crunch (Heat-death of the Universe and Oscillatory Universe) | Big Rip | Big bang nucleosynthesis | Gravitational singularity | Cosmic inflation | Cosmic variance | De Sitter universe

Creation: creation myths | Creation belief | Creationism | Estimates of the date of Creation | Young Earth Creationism

Cosmology and Astrophysics: A Brief History of Time | Beyond the standard Big Bang model | Cosmological arguments | Estimates of the date of Creation | Galaxy formation and evolution | Non-standard cosmology (Creative evolution, Ekpyrotic, Plasma cosmology, Reciprocal System of Theory, and Steady state theory) | Magnitude order | Primordial black hole | Primordial helium abundance | Stellar population | Timeline of cosmology | Theoretical astrophysics | Ultimate fate of the Universe

Astronomy: History of astronomy | CMBR Timeline | Gamma-ray Large Area Space Telescope | Massive compact halo object | Red dwarf | Shape of the universe | Solar nebula | Stars | Supermassive black hole | Universe (Large-scale structure of the cosmos)

People: Hannes Alfvén | Albert Einstein | George Gamow | Fred Hoyle | Georges Lemaître | Peter Lynds | Arno Allan Penzias | Gerald Schroeder | Janez Strnad | Robert Woodrow Wilson

List of physics topics: Arrow of time | Electronuclear force | Comoving distance | Compton effect | Dark energy | Dark matter (Cold dark matter and Hot dark matter) | Hubble's law | Integrated Sachs Wolfe effect | Magnetic monopole | Observation | Olbers' paradox | Phase transition | Quantum gravity | Redshift | Theory of everything | Triple-alpha process | Weyl's postulate

Things: Ambiplasma | Antimatter | Axion | Background radiation | Cosmic light horizon | Cosmic microwave background | Fireball | Far Ultraviolet Spectroscopic Explorer (FUSE)

Atomic Chemical Elements : Beryllium | Carbon | Chemical abundance | Deuterium | Helium | Ylem

Lists: List of astronomical topics | List of famous experiments | List of time periods | Timeline of the Universe

Other: Bang | Discworld | Galactus | Horrendous Space Kablooie

External links and references


Research articles

These are generally full of technical language, but sometimes with introductions in plain English.