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Wikipedia: Big Bang
Big Bang
From Wikipedia, the free encyclopedia.

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 blackbody 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.

Some cosmologists, however, question certain underpinnings of the Big Bang theory and have developed various non-standard cosmologies.

There are in fact many theories about the Big Bang, so the phrase "Big Bang theory" can be a source of confusion, especially since the more speculative theories purport to explain the beginning of the entire Universe. Such theorizing has led some skeptics to object that many accounts of the Big Bang read like creation myths.

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.

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 the account in Genesis. The relationship between religion and the Big Bang theory is discussed below.


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 Big Bang occurred 13.7 ± 0.2 billion years ago. 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 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.

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".

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).

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 Friedman whose equations describe the Friedman-Robertson-Walker universe.

In the 1930s, 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.

Huge advances in big bang cosmology were made in the late-1990's 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, there remain small numbers of who still support non-standard cosmologies in which the big bang is considered incorrect.

Currently research has been to refine the big bang by include 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

Isotropy of observable universe

Proponents of big bang also cite isotropy of the observable universe to one part in one hundred thousand as evidence that big bang is valid[1]. They further state that what minute anisotropy does exist is consistent with big bang hypotheses which include dark matter hypotheses, which necessitates the further ad hoc inclusion of dark energy and inflationary or accelerated universal expansion to accord with known observations.

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.

Cosmic 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.

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. It is interesting to note that the predictions of the Big Bang theory regarding CMBR were inaccurate (these values were modified later from experimentation to fit the observational data).

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 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 would be observed. They were found in the late 1990s. In early 2003 the results of the Wilkenson 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. 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

Throughout its history a number of weakness and criticisms have been offered against the big bang. Some of them are today largely of historical interest, and have been removed either through modifications in the big bang or through better observations. Others are considered issues such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter are considered to be non-fatal, and to be fixable through relatively minor fixes to the theory. Finally, there are some objections which have led small numbers of people to completely reject the big bang but are not accepted by most cosmologists.

The Initial Condition Problem

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.

Magnetic Monopole problem

The magnetic monopole problem was an objection that was raised in the late-1970's. 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 resolveable 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 insure that they have the same temperature.

Globular cluster age

One major issue that had the potential of challenging the big bang occurred in the mid-1990's. 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 1990's with other new computer simulations which indicated a much younger age for globular clusters (changing some of the previously erronous assumptions).

Redshift of galaxies

Redshift is often cited as evidence verifying big bang hypotheses. Halton Arp argues that redshift does not correlate in any observable way that the standard model attributes to the phenonomena. Arp claims that there are correlations between quasars and active galaxies that demonstrate that the cosmic redshift is not due to the expansion of the universe, but is instead local to the source of radiation.

Elemental abundance arguments

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

Distribution of quasars

Some critics of big bang have suggested that recent reviews of the proper motion of many quasars has shown that extreme distances are not possible.[1] These reviews, critics contend, cast doubt on the utility of quasar light curves to verify or falsify big bang hypotheses.

Olber's paradox

The utility of Olber's paradox in verifying or falsifying big bang hypotheses is in dispute. Some critics contend that Olber's paradox is a philosophical conundrum and constitutes and unwarranted precept.

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.

Proton-antiproton imbalance

to be written

Age of universe and values of omega, Hubble Constant

to be written

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.


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 scientists and enthusiastically embraced by the Pope, because it seemed to point to a creation event. Many 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.

One way to attempt to reconcile the two ages is by arguing that the word day as used in Genesis does not correspond to the same interval of time as our 24 hours: it should be noted that Day, in the Aramaic, means "interval" or "period of time." In fact, even the duration of a solar day varies in time. A reading of Genesis 1:14 also indicates that there were no "days" as we know them until Day Four, when lights in the firmament were created to give us Day and Night. However, one problem with this interpretation is that the Bible places the creation of the seas before the creation of the stars.

One author who believes that reconciliation is possible is Gerald Schroeder: he 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 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.

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 [ed. full of technical language, but sometimes with introductions in plain English] Analysis


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