According to the Big Bang theory, the universe emerged from an extremely dense and hot state (singularity). Space itself has been expanding ever since, carrying galaxies with it.

In physical cosmology, the Big Bang is the scientific theory that the universe emerged from a tremendously dense and hot state about 13.7 billion years ago.

Combined with the assumption that observers anywhere in the universe would make similar observations (the Copernican principle), then this suggests that space itself is expanding.This initial state is the key premise of Big Bang theory.

Theoretical support show that a Big Bang is consistent with general relativity and with the principle that properties of the universe should be independent of position or orientation (the cosmological principle).

Modern calculations known as Big Bang nucleosynthesis (BBN), produce results that are consistent with observations of the present universe.

The Big Bang model also predicts the cosmic microwave background radiation (CMB), a background of weak microwave radiation filling the whole universe. The discovery of the CMB in 1964 led to the general acceptance amongst physicists of the Big Bang as the best theory of the origin and evolution of the cosmos.

For a while, support was split between the “steady state” and “Big Bang” theories. Eventually, the observational evidence, notably from radio source counts began to favor the latter.

The Big Bang theory depends on three assumptions:

1. The universality of physical laws     2. The cosmological principle    3. The Copernican principle

As the universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty universe; space itself expanded and caused the physical distance between two comoving points to increase. Objects that are bound together (such as atoms, people, stars, the solar system, and galaxies) do not expand with spacetime’s expansion because the forces that bind them together are strong compared with the Hubble expansion that is pulling them apart.

Observations of distant galaxies and quasars show that these objects are redshifted-the light emitted from them has been shifted to longer wavelengths.

The Hubble’s law observation has two possible explanations, one of which-that we are at the center of an explosion of galaxies-is untenable given the Copernican principle. The other explanation is that the universe is uniformly expanding everywhere as a unique property of spacetime.

A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies  appear very different from nearby galaxies . Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model.

Features, issues and problems

While very few researchers now doubt the Big Bang occurred, the scientific community was once divided between supporters of the Big Bang and those of alternative cosmological models. Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations . With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory.

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called baryogenesis created the asymmetry.

Globular cluster age

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

Dark matter

During the 1970s and 1980s, various observations  showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is not normal or baryonic matter but rather dark matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.

The detection of dark matter is sensitive only to its gravitational signature, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.

Dark energy

Measurements revealed that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed dark energy. Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. Dark energy is also required by two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic “ruler”.

Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results indicate that the universe today is 74% dark energy, 22% dark matter, and 4% regular matter. Since matter is becoming less concentrated because of the expansion of the universe, but the dark energy density remains constant.

The future according to the Big Bang theory
Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe was above the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started-a Big Crunch. Alternatively, if the density in the universe was equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero-a Big Freeze. Moreover, if the proton was unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.

Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe cools and expands. Other explanations of dark energy-so-called phantom energy theories-suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.
Philosophical and religious interpretations

The Big Bang is a scientific theory, and as such stands or falls by its agreement with observations. But as a theory which addresses, or at least seems to address, creation itself, it has always been entangled with theological and philosophical implications. Many cosmologists,  strongly preferred an eternal universe, and felt that the beginning of time implied by the Big Bang imported religious concepts into physics. This perception was enhanced by the fact that the theory’s inventor, Georges LemaՏtre, was a Roman Catholic priest. LemaՏtre himself always insisted that as a physical theory, the Big Bang has no religious implications; and yet the congruence between his scientific and religious beliefs is apparent in his famous description of the beginning of the universe as “a day without yesterday”-alluding to the creation account in Genesis. George Gamow had no compunction in describing the graphs of conditions in the Big Bang as “divine creation curves”, and sent a copy of his book The Creation of the Universe to the pope. To this day, many people’s reactions to the Big Bang theory, both positive and negative, are influenced by how well it can be harmonised with their religious and philosophical world views.

Some interpretations of the Big Bang theory go beyond science, and some purport to explain the cause of the Big Bang itself (first cause). These views have been criticized by some naturalist philosophers as being modern creation myths. Some people believe that the Big Bang theory is inconsistent with traditional views of creation such as that in Genesis, for example, while others,  believe that the Big Bang theory lends support to the idea of creation ex nihilo.

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

* A number of Christian and traditional Jewish sources have accepted the Big Bang as a possible description of the origin of the universe, interpreting it to allow for a philosophical first cause. Pope Pius XII was an enthusiastic proponent of the Big Bang even before the theory was scientifically well-established, and consequently the Roman Catholic Church has been a prominent advocate for the idea that creation ex nihilo can be interpreted as consistent with the Big Bang. This view is shared by many religious Jews in all branches .
* Some modern Islamic scholars believe that the Qur’an parallels the Big Bang in its account of creation, described as follows: “Do not the unbelievers see that the heavens and the earth were joined together as one unit of creation, before We clove them asunder?” . The claim has also been made that the Qur’an describes an expanding universe: “The heaven, We have built it with power. And verily, We are expanding it.” .Parallels with the Big Crunch and an oscillating universe have also been suggested: “On the day when We will roll up the heavens like the rolling up of the scroll for writings, as We originated the first creation, (so) We shall reproduce it; a promise (binding on Us); surely We will bring it about.” .
* Certain theistic branches of Hinduism, conceive of a creation event with similarities to the Big Bang. For example in the third book of the Bhagavata Purana , describes a primordial state which bursts forth as the Great Vishnu glances over it, transforming into the active state of the sum-total of matter (“prakriti”). Other forms of Hinduism assert a universe without beginning or end.
* Buddhism has a concept of universes that have no initial creation event, but instead go through infinitely repeated cycles of expansion, stability, destruction, and quiescence. The Big Bang may be reconciled with this view, since there are ways to conceive an eternal creation and destruction of universes within the paradigm. A number of popular Zen philosophers were intrigued, in particular, by the concept of the oscillatory universe.

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