Universe

=== Dark energy ===
{{Main|Dark energy}}
An explanation for why the expansion of the universe is accelerating remains elusive. It is often attributed to "dark energy", an unknown form of energy that is hypothesized to permeate space.<ref name="peebles(a)">{{cite journal|author1=Peebles, P.J.E. |author2=Ratra, Bharat |name-list-style=amp |title=The cosmological constant and dark energy|year=2003|journal=Reviews of Modern Physics|arxiv=astro-ph/0207347|volume=75|issue=2|pages=559–606|doi=10.1103/RevModPhys.75.559|bibcode=2003RvMP...75..559P|s2cid=118961123 }}</ref> On a [[mass–energy equivalence]] basis, the density of dark energy (~ 7 × 10<sup>−30</sup> g/cm<sup>3</sup>) is much less than the density of ordinary matter or dark matter within galaxies. However, in the present dark-energy era, it dominates the mass–energy of the universe because it is uniform across space.<ref>{{cite journal|title=Why the cosmological constant is small and positive |first1=Paul J. |last1=Steinhardt |first2=Neil|last2=Turok|journal=Science|volume=312|issue=5777|pages=1180–1183 |doi=10.1126/science.1126231 |arxiv=astro-ph/0605173 |year=2006 |bibcode=2006Sci...312.1180S |pmid=16675662|s2cid=14178620 }}</ref><ref>{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/astro/dareng.html |title=Dark Energy |work=Hyperphysics |access-date=January 4, 2014 |archive-url=https://web.archive.org/web/20130527105518/http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/dareng.html |archive-date=May 27, 2013 |url-status=dead }}</ref>

Two proposed forms for dark energy are the [[cosmological constant]], a ''constant'' energy density filling space homogeneously,<ref name="carroll">{{cite journal|author=Carroll, Sean |year=2001 |url=http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html |title=The cosmological constant |journal=Living Reviews in Relativity |volume=4 |issue=1 |page=1 |access-date=September 28, 2006 |doi=10.12942/lrr-2001-1 |pmid=28179856 |pmc=5256042 |url-status=dead |archive-url=https://web.archive.org/web/20061013042057/http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html |archive-date=October 13, 2006 |arxiv=astro-ph/0004075 |bibcode=2001LRR.....4....1C |author-link=Sean M. Carroll }}</ref> and [[scalar field]]s such as [[quintessence (physics)|quintessence]] or [[moduli (physics)|moduli]], ''dynamic'' quantities whose energy density can vary in time and space while still permeating then enough to cause the observed rate of expansion. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to [[vacuum energy]]. Scalar fields having only a slight amount of spatial inhomogeneity would be difficult to distinguish from a cosmological constant.