Editing Second law of thermodynamics (section)

=== Initial conditions at the Big Bang ===
{{Further|Past hypothesis}}
As elaborated above, it is thought that the second law of thermodynamics is a result of the very low-entropy initial conditions at the [[Big Bang]]. From a statistical point of view, these were very special conditions. On the other hand, they were quite simple, as the universe - or at least the part thereof from which the [[observable universe]] developed - seems to have been extremely uniform.<ref>Carroll, S. (2017). The big picture: on the origins of life, meaning, and the universe itself. Penguin.</ref>

This may seem somewhat paradoxical, since in many physical systems uniform conditions (e.g. mixed rather than separated gases) have high entropy. The paradox is solved once realizing that gravitational systems have [[Heat capacity#Negative heat capacity|negative heat capacity]], so that when gravity is important, uniform conditions (e.g. gas of uniform density) in fact have lower entropy compared to non-uniform ones (e.g. black holes in empty space).<ref>Greene, B. (2004). The fabric of the cosmos: Space, time, and the texture of reality. Knopf.</ref> Yet another approach is that the universe had high (or even maximal) entropy given its size, but as the universe grew it rapidly came out of thermodynamic equilibrium, its entropy only slightly increased compared to the increase in maximal possible entropy, and thus it has arrived at a very low entropy when compared to the much larger possible maximum given its later size.<ref>{{cite journal | last1 = Davies | first1 = P. C. | year = 1983 | title = Inflation and time asymmetry in the universe | url = | journal = Nature | volume = 301 | issue = 5899| pages = 398–400 | doi = 10.1038/301398a0 | bibcode = 1983Natur.301..398D }}</ref>

As for the reason why initial conditions were such, one suggestion is that [[cosmological inflation]] was enough to wipe off non-smoothness, while another is that the universe was [[Hartle–Hawking state|created spontaneously]] where the mechanism of creation implies low-entropy initial conditions.<ref>[https://www.quantamagazine.org/physicists-debate-hawkings-idea-that-the-universe-had-no-beginning-20190606/ Physicists Debate Hawking's Idea That the Universe Had No Beginning. Wolchover, N. Quantmagazine, June 6, 2019. Retrieved 2020-11-28]</ref>