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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.

The '''Big Bang Theory''' models the Universe as expanding outwards from a very hot, dense initial state approximately 14 billion years ago. It was first proposed by [[Georges-Henri Lemaitre]] and evidence for the expansion was observed by [[Edwin Hubble]]<ref>Hubble, E. Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, pp. 168-173.</ref>. Later [[George Gamow]] predicted that the Big Bang would leave an observable microwave background radiation. This radiation was subsequently discovered by [[Bell Labs]] and found to be close to that predicted by Gamow (Gamow predicted a background radiation level equivalent to a roughly 3 K black body object, and the observed level is that of a 2.725 K body).

According to the Big Bang theory, the universe originated in an infinitely dense and physically paradoxical singularity. Space has expanded with the passage of time, objects being moved farther away from each other.

According to the Big Bang theory, the universe originated in an infinitely dense and physically paradoxical singularity. Space has expanded with the passage of time, objects being moved farther away from each other.

Observations of distant supernovae indicate that the Universe is actually undergoing accelerated expansion<ref>Riess, A. G., et al. The Astronomical Journal, Volume 116, Issue 3, pp. 1009-1038.</ref><ref>Perlmutter, S., et al. The Astrophysical Journal, Volume 517, Issue 2, pp. 565-586.</ref>. Modern galaxy surveys[https://www.sdss.org/dr5]<ref>Tegmark, M., et al. Physical Review D, vol. 74, Issue 12.</ref> and recent observations of the microwave background[http://map.gsfc.nasa.gov/]<ref>See, for example, http://arxiv.org/abs/astro-ph/0603449</ref> have corroborated these claims.

In cosmology, the Big Bang theory is the prevailing 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.

[[Young Earth Creationism|Young earth creationist]] scientists hotly contest the Big Bang Theory stating that it is scientifically unsound [http://www.apologeticspress.org/modules.php?name=Read&cat=1&itemid=22]

[http://www.creationscience.com/onlinebook/AstroPhysicalSciences16.html]  Young earth creationist use the Bible, which they believe to be a literal truth, to trace lineages to estimate the age of the Earth to be around 6,000 years.

In contrast to young earth creationists, [[Old Earth Creationists|Old Earth Creationists]] and [[Theistic Evolution|Theistic evolutionists]] agree that the Big Bang occured.[http://www.reasons.org/resources/fff/2000issue03/]. Some Old Earth Creationists and Theistic Evolutionist argue that the Big Bang is in fact mentioned in the Bible. [http://www.reasons.org/resources/fff/2000issue03/index.shtml#big_bang_the_bible_taught_it_first] However, the Bible makes no mention of the "Big Bang". While the universe has been observed to be expanding, this need not be used as evidence of a Big Bang, because the Bible states in Jeremiah that God "stretched out the heavens".

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.