Editing Entropy (section)

=== Cosmology ===
Assuming that a finite universe is an isolated system, the second law of thermodynamics states that its total entropy is continually increasing. It has been speculated, since the 19th century, that the universe is fated to a [[heat death of the universe|heat death]] in which all the energy ends up as a homogeneous distribution of thermal energy so that no more work can be extracted from any source.

If the universe can be considered to have generally increasing entropy, then – as [[Roger Penrose]] has pointed out – [[gravity]] plays an important role in the increase because gravity causes dispersed matter to accumulate into stars, which collapse eventually into [[black hole]]s. [[Black hole entropy|The entropy of a black hole]] is proportional to the surface area of the black hole's [[event horizon]].<ref>{{Cite book|last=von Baeyer|first=Christian, H.|title=Information–the New Language of Science|publisher=Harvard University Press|year=2003|isbn=978-0-674-01387-2|url=https://archive.org/details/informationnewla00vonb}}</ref><ref>{{Cite journal|author=Srednicki M|title=Entropy and area|journal=Phys. Rev. Lett. |volume=71|issue=5|pages=666–669|date=August 1993|pmid=10055336|doi=10.1103/PhysRevLett.71.666 |bibcode=1993PhRvL..71..666S |arxiv=hep-th/9303048|s2cid=9329564}}</ref><ref>{{Cite journal|author=Callaway DJE|title=Surface tension, hydrophobicity, and black holes: The entropic connection|journal=Phys. Rev. E|volume=53|issue=4|pages=3738–3744|date=April 1996|pmid=9964684|doi= 10.1103/PhysRevE.53.3738 |arxiv=cond-mat/9601111|bibcode=1996PhRvE..53.3738C|s2cid=7115890|author-link=David J E Callaway}}</ref> [[Jacob Bekenstein]] and [[Stephen Hawking]] have shown that black holes have the maximum possible entropy of any object of equal size. This makes them likely end points of all entropy-increasing processes, if they are totally effective matter and energy traps.<ref>{{cite book|first1=T.K.|last1=Sarkar|first2=M.|last2=Salazar-Palma|author2-link=Magdalena Salazar Palma|first3=Eric L.|last3=Mokole|date=2008|chapter=A Look at the Concept of Channel Capacity from a Maxwellian Viewpoint|chapter-url=https://books.google.com/books?id=chfvTMRsv38C&pg=PA162 |title= Physics of Multiantenna Systems & Broadband Processing |page=162|isbn=978-0470190401|publisher=Wiley|access-date= 31 August 2019}}</ref> However, the escape of energy from black holes might be possible due to quantum activity (see [[Hawking radiation]]).

The role of entropy in cosmology remains a controversial subject since the time of [[Ludwig Boltzmann]]. Recent work has cast some doubt on the heat death hypothesis and the applicability of any simple thermodynamic model to the universe in general. Although entropy does increase in the model of an expanding universe, the maximum possible entropy rises much more rapidly, moving the universe further from the heat death with time, not closer.<ref>{{Cite book|last=Layzer|first=David|title=Cosmogenesis : The Growth of Order in the Universe|publisher=Oxford University Press|year=1990}}</ref><ref>{{Cite book|last=Chaisson|first=Eric J.|title=Cosmic Evolution: The Rise of Complexity in Nature|publisher=Harvard University Press|year=2001|isbn=978-0-674-00342-2|url=https://archive.org/details/cosmicevolutionr00chai}}</ref><ref>{{Cite book|editor-last1=Lineweaver|editor-first1=Charles H.|editor-last2=Davies|editor-first2=Paul C. W.|editor-last3=Ruse|editor-first3=Michael|title=Complexity and the Arrow of Time|publisher=Cambridge University Press|year=2013|isbn=978-1-107-02725-1}}</ref> This results in an "entropy gap" pushing the system further away from the posited heat death equilibrium.<ref>{{Cite book|last=Stenger|first=Victor J.|title=God: The Failed Hypothesis|publisher=Prometheus Books|year=2007|isbn=978-1-59102-481-1}}</ref> Other complicating factors, such as the energy density of the vacuum and macroscopic [[quantum mechanics|quantum]] effects, are difficult to reconcile with thermodynamical models, making any predictions of large-scale thermodynamics extremely difficult.<ref>{{Cite book|author=Benjamin Gal-Or|title=Cosmology, Physics and Philosophy|publisher=Springer Verlag|year=1987|isbn=978-0-387-96526-0}}</ref>

Current theories suggest the entropy gap to have been originally opened up by [[inflation (cosmology)|the early rapid exponential expansion]] of the universe.<ref name="Albrecht">{{cite encyclopedia |year=2004 |title=Cosmic Inflation and the Arrow of Time |encyclopedia=Science and Ultimate Reality: From Quantum to Cosmos |publisher=Cambridge University Press |location=Cambridge, UK |url=https://arxiv.org/ftp/astro-ph/papers/0210/0210527.pdf |access-date=28 June 2017 |last=Albrecht |first=Andreas |author-link=Andreas Albrecht (cosmologist) |editor-last=Barrow |editor-first=John D. |editor-link=John D. Barrow |arxiv=astro-ph/0210527 |bibcode=2002astro.ph.10527A |postscript=none |editor-last2=Davies |editor-first2=Paul C.W. |editor-link2=Paul Davies |editor-last3=Harper |editor-first3=Charles L.}}. In honor of John Wheeler's 90th birthday.</ref>