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In cosmology, the Big Bang theory is the scientific theory of the early development and shape of the universe. The central idea is that the theory of general relativity can be combined with the observations on the largest scales of galaxies receding from each other to extrapolate the conditions of the universe back or forward in time. A natural consequence of the Big Bang is that in the past the universe had a higher temperature and a higher density. 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, and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and evolution of the universe.

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.

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, theorists in the 1940s were able to predict the cosmic microwave background (CMB), a form of cosmic background radiation, that was discovered in the 1960s and served as a confirmation of the Big Bang theory over its chief rival, the steady state theory.

In current physical models, the universe 13.7 billion years ago would have had the form of a gravitational singularity, at which all time and distance measurements become meaningless and temperatures and pressures become infinite. As there are no models for the regimes on this scale, in particular, the lack of a theory of quantum gravity, this period of the universe's history remains an unsolved problem in physics.

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 Carl Wilhelm 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, admitted no static solutions (that is to say, the universe had to be either expanding or shrinking), a result that he himself considered wrong, and which he attempted to fix by adding a cosmological constant. Applying general relativity to cosmology was done by Alexander Friedmann whose equations describe the Friedmann-Lemaître-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. These data allowed astronomers to calculate many of the parameters of the Big Bang to a new level of precision and opened up a major unexpected finding that the expansion of the universe appears to be accelerating.

Descriptive overview

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 measured age of 13.7 ± 0.2 billion years. The fact that these three independent measurements are consistent is considered strong evidence for the so-called concordance model that describes the detail nature of the contents of the universe.

The early universe was filled homogeneously and isotropically with a very high energy density. At times as early as 10 seconds after the Planck epoch, the material components of the universe were in the form of a quark-gluon plasma where the constituent particles were all moving relativistically. By an as yet unknown process, baryogenesis occured producing the observed asymmetry between matter and antimatter. As the universe grew in size, the temperature dropped, leading to further symmetry breaking processes that manifested themselves as the known forces of physics, elementary particles, and later allowed for the formation of the universe's hydrogen and helium atoms in a process called Big Bang nucleosynthesis. As the universe cooled, matter gradually stopped moving relativistically and came to dominate over radiation. After about 100,000 years the radiation decoupled from the atoms 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 grew into even denser regions, forming gas clouds, stars, galaxies, and the other astronomical structures seen today. The details of this process are dependent 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 in the form of 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 has the 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 takes the form of a cosmological constant term in Einstein's Field Equations of general relativity, but the details of its equation of state and relationship with the standard model of particle physics continue to be investigated both observationally and theoretically.

Theoretical underpinnings

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

When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them. The universality of physical laws has been tested to the level that the largest deviation of physical constants over the age of the universe can be is of order 10. The isotropy of the universe that defines the Cosmological Principle has been tested to a level of 10 and the universe has been measured to be homogenous on the largest scales to the 10% level. There are efforts currently underway to test the Copernican Principle by means of looking at the interaction of clusters of galaxies and the CMB through the Sunyaev-Zeldovich Effect to a level of 1% accuracy.

Once these assumptions are in place, the Big Bang theory relies on 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 comoving distances and conformal times remove the expansion of the universe from consideration of spacetime measurements. In such a coordinate system, objects moving with the cosmological flow are always the same comoving distance away and the horizon or limit of the universe is set by the conformal time.

The Big Bang is therefore not an explosion of matter moving outward to fill an empty universe; it is spacetime itself that is expanding. 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.

Supporting 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 in the universe fits well with standard Big Bang theory.

Hubble law expansion

Main article: Hubble's law

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

v = H₀ D

where v is the recessional velocity, D is the distance to the object and H₀ is the Hubble constant measured to be 71 ± 4 km/s/Mpc by the WMAP probe.

Cosmic microwave background radiation

Main article: Cosmic microwave background radiation

One feature of the Big Bang theory was the prediction of the cosmic microwave background radiation or CMB. As the early universe cooled off due to the expansion, the universe's 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 radiation from this region will travel unimpeded for the remainder of the lifetime of the universe, becoming redshifted because of the Hubble expansion. This results in a redshift of the uniformly distributed blackbody spectrum of the 3000 K to 3 K. It is observed 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, 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 CMB, finding a local residual temperature of 2.726 K and determining that the CMB was isotropic to an accuracy of 10. During the 1990s, CMB data was studied further to see if small anisotropies predicted by the Big Bang theory would be observed. They were found in 2000 by the Boomerang experiment.

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

Main article: Big Bang nucleosynthesis

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 He, a H/H ratio of about 10, a He/H of about 10 and a Li/H abundance of about 10.

Measurements of primordial abundances for all four isotopes are consistent with a unique value of that parameter 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 He.

Galactic evolution and quasar distribution

The details of the distribution of galaxies and quasars are both constraints and confirmations of current theory. The finite age of the universe at earlier times means that galaxy evolution is closely tied to the cosmology of the universe. The types and distribution of galaxies appears to change markedly over time, evolving by means of the Boltzmann Equation and showing structure formation that can be measured by statisics. Observations reveal a time-dependent relationship of the galaxy and quasar distributions, star formation histories, and the type and size of the largest-scale structures in the universe (superclusters). These observations all are explained very well by the Big Bang theory and serve as constraints on model parameters.

Standard problems

Historically, a number of problems have arisen within the Big Bang theory. Some of them are today mainly of historical interest, and have been avoided 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 not considered to be fatal as they seem to be addressed through relatively minor adjustments to the theory. Finally, there are a small number of proponents of non-standard cosmologies who believe that there was no Big Bang at all.

The small number of detractors of the Big Bang cite these problems and their solutions as being what they consider to be ad hoc modifications and refinements to the theory. Most often attacked are the parts of standard cosmology that include dark matter, dark energy, and cosmic inflation. These features of the Big Bang were discovered due to observational and theoretical considerations of available data and are part of the most cutting edge of inquiry in physics. Although these aspects of standard cosmology are inadequately explained in standard model physics, the regimes in which these phenomena occur are also not well constrained by laboratory experiments. While certain scientific problems remain, the models based on the Big Bang theory are better able to explain these issues than any alternative.

What follows is a short list of standard Big Bang "problems" and puzzles:

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 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 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 allow for these regions to have the same temperature.

This apparent inconsistency is resolved by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at a time 10 seconds after the Planck epoch. During inflation, the universe undergoes exponential expansion, and regions in causal contact expand past each other's horizons. Heisenberg's uncertainty principle predicts that there would be quantum thermal fluctuations during the inflationary phase, 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 by means of a Hubble Law, and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicted that the primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB. Despite its great success, cosmic inflation has yet to make contant with the standard model of particle physics and remains controversial.

Flatness

The flatness problem is an observational problem that results from considerations of the geometery associated with Friedmann-Lemaître-Robertson-Walker metric. In general, the universe can have three different kinds of geometeries: hyperbolic geometry, Euclidean geometry, or elliptic geometry. Each one of these geometeries is tied directly to the critical density of the universe, the hyperbolic corresponding to less than the critical density, elliptic corresponding to greater than the critical density, and Euclidean corresponding to exactly equal to the critical density. The universe is measured to be required to be within one part in 10 of the critical density in its earliest stages. Any deviation more than that would have caused either a Heat Death or a Big Crunch and the universe would not exist as it does today.

The resolution to this problem is again offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that any residual curvature associated with it would have been completely smoothed out to a high degree of precision. Thus, the universe is driven to be flat by inflation.

Magnetic monopoles

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 also resolvable by the addition of cosmic inflation which removes all point defects from the observable universe in the same way that the geometery is driven to flat.

Missing 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 non-baryonic 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 in 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 particles have only been detected through their gravitational signatures, and have not 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. For the universe to be flat, as is indicated by measurements of the cosmic microwave background, this would have meant that fully 70% of the energy density of the universe was left unaccounted for. Measurements of Type Ia supernovae reveal that the universe is undergoing a non-linear acceleration, violating the Hubble Law expansion of the universe. General relativity requires that this additional 70% be made up by an energy component with large negative pressure. The nature of the so-called dark energy 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.

Globular cluster age

A certain set of observations were made in the mid-1990s invovling the ages of globular clusters that were found to be inconsistent with the Big Bang. Computer simulations of 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 with other 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.

The future according to the Big Bang theory

Currently, observations of dark energy constrain the immediate future of the universe to expand forever. Before the component densities of the universe were as well-measured as today, there had been offered scenarios where the a universe with a mass density above the critical density would reach a maximum size and then begin to collapse in a Big Crunch. In this scenario, the universe would become denser and hotter again, ending with a state that was similar to that in which it started.

Alternatively, if the mass density in the universe was equal or below the critical density and there was no dark energy, the expansion would slow down, but never stop. New star formation would drop off as the universe grew less dense. The average temperature of the Universe would asymptotically approach absolute zero, and eventually, all the protons would decay, the black holes would evaporate, and the Universe would consist of dispersed radiation. This scenario is also known as heat death.

However, universe is not decelerating but rather accelerating in expansion. This will cause more and more of the universe that is presently visible to become unobservable as objects pass our future horizon. Ultimately, galaxy clusters and eventually galaxies themselves will be torn apart by the ever increasing expansion in a so-called Big Rip. The only remaining visible objects will be ancient gamma-ray bursters which will have their light redshifted to extremely long wavelengths.

Beyond this, the laws of physics have little to offer. Scenarios have been put forth where new "multiverses" are born out of the unimaginably large expanse of dark energy. Indeed, the eternal inflation theory put forth by some seems to demand this be the case.

See also the Ultimate fate of the universe.

Speculative physics beyond the Big Bang

There remains the possibility that a more accurate approximation or generalization than the Big Bang will be developed in the future. It might be the case that there are parts of the universe well-beyond what can currently be observed. This is required to be true for the case of cosmic inflation which holds that the initial conditions before the inflationary epoch are, on the whole, entirely erased from influencing the observable universe and that domains that are completely separate from that which can be observed may, in the materialistic sense, exist. It may be possible to deduce what happened before inflation through observational tests yet to be discovered. Extrapolations and speculations about this tend to involve theories quantum gravity.

Some proposed ideas are:

chaotic inflation
brane cosmology models (including ekpyrotic and cyclic scenarios in which the Big Bang is the result of a collision between branes)
oscillatory universe which holds that the has an eternal and repetitive quality of repeating big bangs and big crunches
models including the Hartle-Hawking boundary condition in which the whole of space-time is finite

Some of these scenarios are either qualitatively or quantitatively compatible with one another. Each involves a certain amount of untested physics and/or heuristic hypotheses.

Philosophical and religious interpretations

Philosophically, there are a number of interpretations of the Big Bang theory that are entirely speculative or extra-scientific. Some of these ideas purport to explain the cause of the Big Bang itself (first cause), and 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, for example as given in Genesis, while others believe that all Big Bang theories are inconsistent with such views.

The Big Bang as a scientific theory is not associated with any religion. While certain fundamentalist interpretations of religions conflict with the history of the universe as put forth by the Big Bang, there are also more liberal interpretations which do not.

The following is a list of various religious interpretations of the Big Bang theory:

A number of Christian apologists and the Roman Catholic Church in particular have accepted the Big Bang as a description of the origin of the universe, interpreting it to allow for a philosophical first cause.
Students of Kabbalah, deism and other non-anthropomorphic faiths concord with the Big Bang theory, notably the theory of "divine retraction" (Tzimtzum), as explained by 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: "the heavens and the earth were joined together as one unit, before We clove them asunder" (21:30). The Qur'an also appears to describe an expanding universe: "The heavens, We have built them with power. And verily, We are expanding it" (51:47).
Certain theistic branches of Hinduism, such as the Vaishnava-traditions, 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 (primarily, chapters 10 and 26), 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").
Buddhism has a concept of a universe which has no creation event per se. The Big Bang, however, is not seen to be in conflict with this since there are ways to get an eternal universe within the paradigm. A number of popular Zen philsophers were intrigued, in particular, by the concept of the oscillating universe.

External links and references

Research articles

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

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