Editing Big Bang (section)

===Dark energy===
{{Main|Dark energy}}
Measurements of the redshift–[[apparent magnitude|magnitude]] relation for [[type Ia supernova]]e indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, cosmological models require that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy".<ref name="peebles" />

Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the [[Friedmann equations#Density parameter|critical density]] of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.<ref name="peebles" /> Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of [[gravitational lens]]es,<ref>{{cite journal | title=Constraining dark energy from the abundance of weak gravitational lenses  | first1=Nevin N. | last1=Weinberg | first2=Marc | last2=Kamionkowski | journal=Monthly Notices of the Royal Astronomical Society | volume=341 | issue=1 | date=May 2003 | pages=251–262 | bibcode=2003MNRAS.341..251W | arxiv=astro-ph/0210134 | doi=10.1046/j.1365-8711.2003.06421.x | doi-access=free | s2cid=1193946 }}</ref> and the other using the characteristic pattern of the large-scale structure--[[baryon acoustic oscillations]]--as a cosmic ruler.<ref>{{cite web |last1=White |first1=Martin |title=Baryon acoustic oscillations and dark energy |url=https://w.astro.berkeley.edu/~mwhite/bao/}}</ref><ref>
{{cite journal | title=Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological implications from two decades of spectroscopic surveys at the Apache Point Observatory | first1=Shadab | last1=Alam | display-authors=etal | journal=Physical Review D | volume=103 | issue=8 | date=April 2021 | page=083533 | bibcode=2021PhRvD.103h3533A | arxiv=2007.08991 | doi=10.1103/PhysRevD.103.083533}}</ref>

Negative pressure is believed to be a property of [[vacuum energy]], but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.<ref name="wmap7year" /> According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the [[far future]] as dark energy becomes even more dominant.{{citation needed|date=February 2023}}

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of [[Quintessence (physics)|quintessence]] or other modified gravity schemes.<ref>{{harvnb|Tanabashi, M.|2018|pp=[http://pdg.lbl.gov/2018/reviews/rpp2018-rev-dark-energy.pdf 406–413]|loc=chpt. 27: "Dark Energy" (Revised September 2017) by David H. Weinberg and Martin White.}}

* {{harvnb|Olive|2014|pp=[http://pdg.lbl.gov/2014/reviews/rpp2014-rev-dark-energy.pdf 361–368]|loc=chpt. 26: "Dark Energy" (November 2013) by Michael J. Mortonson, David H. Weinberg, and Martin White.}} {{bibcode|2014arXiv1401.0046M}}</ref> A [[cosmological constant problem]], sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from [[Planck units]].<ref>{{cite journal |last1=Rugh |first1=Svend E. |last2=Zinkernagel |first2=Henrik |title=The quantum vacuum and the cosmological constant problem |pages=663–705 |volume=33 |issue=4 |date=December 2002 |journal=[[Studies in History and Philosophy of Science Part B]] |arxiv=hep-th/0012253 |bibcode=2002SHPMP..33..663R |doi=10.1016/S1355-2198(02)00033-3 |s2cid=9007190 }}</ref>