|Standard scientific model of the big bang theory.|
|Typical laymanized half-scientific (right) half-religious (left) view of the big bang.|
(add early theories)
In 1920, American polymath William Sidis postulated: 
“The second law of thermodynamics must date from some sort of great collision out of which the present universe evolved.”
In 1929, American astronomer Edwin Hubble published his findings that the degree of "Doppler shift" (specifically "redshift") observed in the light spectra from other galaxies increases in proportion to a particular galaxy's distance from earth. This relationship became known as Hubble's law, and helped establish that the model that universe is expanding. 
In 1931, Belgian physicist Georges Lemaitre suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the Universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.
In 1949, Fred Hoyle coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast. In 1967, together with Jayant Narlikar, Hoyle would go on to state: 
“In the ‘big bang’ cosmology the universe must start with a marked degree of thermodynamic disequilibrium and must eventually run down.”
In 1964, Arno Penzias and Robert Wilson, at Bell Labs, measured cosmic microwave background radiation, the first evidence of big bang theory, using the following device, a cryogenic microwave receiver, measuring a background radiation of 3K in all directions:
In cosmological thermodynamics, the big bang is often considered to have been a singularity in the form of a low entropy state.  In religious thermodynamics, the big bang is often argued to have been initiated or created by god. 
|American evolutionary paleontologist Neil Shubin's 2013 big bang to human molecule depiction. |
In 2013, American evolutionary paleontologist Neil Shubin, in his The Universe Within, published a big bang to human molecule depiction, shown adjacent, of human formation. 
1. Penrose, Rodger. (2004). The Road to Reality: A Complete Guide to the Laws of the Universe (ch. 27: The Big Bang and its Thermodynamic Legacy, pgs. 686-734). London: Jonathan Cape.
2. Edwards, Rem B. (2001). What Caused the Big Bang (pg. 283). Rodopi.
3. Inflation (cosmology) – Wikipedia.
4. Hubble, Edwin. (1929). "A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae", Proceedings of the National Academy of Sciences, 15 (3): 168–73
5. Sidis, William J. (1920). The Animate and the Inanimate, [PDF], (published in 1925, R.G. Badger).
6. (a) Hoyle, Fred and Narlikar, Jayant V. (1967). “Article”, in: The Nature of Time (editor: Thomas Gold) (pg. 25). Cornell University Press.
(b) Layzer, David. (1975). “The Arrow of Time”, Scientific American, 233:56-69.
7. Shubin, Neil. (2013). The Universe Within: Discovering the Common History of Rocks, Planets, and People. Random House.
● Williams, David A. and Hartquist, T.W. (2013). The Cosmic-Chemical Bond: Chemistry from the Big bang to Planet Formation. RSC Publishing.
● Big Bang – Wikipedia.