Editing Big Bang (section)

=== Ultimate fate of the universe ===
{{Main|Ultimate fate of the universe}}
Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than 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 similar to that in which it started—a [[Big Crunch]].<ref name="kolb_c3"/>

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving [[white dwarf]]s, [[neutron star]]s, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach [[absolute zero]]—a [[Future of an expanding universe|Big Freeze]].<ref>{{cite web |url=https://map.gsfc.nasa.gov/universe/uni_fate.html |title=What is the Ultimate Fate of the Universe? |author=NASA/WMAP Science Team |date=29 June 2015 |work=Universe 101: Big Bang Theory |publisher=[[NASA]] |location=Washington, D.C. |url-status=live |archive-url=https://web.archive.org/web/20191015052245/https://map.gsfc.nasa.gov/universe/uni_fate.html |archive-date=15 October 2019 |access-date=18 December 2019}}</ref> Moreover, if protons are [[proton decay|unstable]], then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting [[Hawking radiation]]. 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.<ref name=dying>{{cite journal |last1=Adams |first1=Fred C. |author1-link=Fred Adams |last2=Laughlin |first2=Gregory |author2-link=Gregory P. Laughlin |date=April 1997 |title=A dying universe: the long-term fate and evolution of astrophysical objects |journal=[[Reviews of Modern Physics]] |volume=69 |issue=2 |pages=337–372 |arxiv=astro-ph/9701131 |bibcode=1997RvMP...69..337A |doi=10.1103/RevModPhys.69.337 |s2cid=12173790 }}.</ref>

Modern observations of accelerating 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. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, 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]].<ref>{{cite journal |last1=Caldwell |first1=Robert R. |author1-link=Robert R. Caldwell |last2=Kamionkowski |first2=Marc |author2-link=Marc Kamionkowski |last3=Weinberg |first3=Nevin N. |date=15 August 2003 |title=Phantom Energy: Dark Energy with w<−1 Causes a Cosmic Doomsday |journal=[[Physical Review Letters]] |volume=91 |issue=7 |page=071301 |arxiv=astro-ph/0302506 |bibcode=2003PhRvL..91g1301C |doi=10.1103/PhysRevLett.91.071301 |pmid=12935004|s2cid=119498512 }}</ref>