Editing Cosmological constant (section)

{{Short description|Value representing energy density of space}}
[[File:CMB Timeline300 no WMAP.jpg|thumb|upright=1.8|Sketch of the [[Timeline of cosmological epochs|timeline of the Universe]] in the [[Lambda-CDM model|ΛCDM model]]. The accelerated expansion in the last third of the timeline represents the [[scale factor (cosmology)|dark-energy dominated era]].]]
{{Cosmology}}

In [[physical cosmology|cosmology]], the '''cosmological constant''' (usually denoted by the Greek capital letter [[lambda]]: {{math|Λ}}), alternatively called '''Einstein's cosmological constant''', 
is a coefficient that [[Albert Einstein]] initially added to his [[Einstein field equations| field equations]] of [[general relativity]]. He later removed it; however, much later it was revived to express the energy density of space, or [[vacuum energy]], that arises in [[quantum mechanics]]. It is closely associated with the concept of [[dark energy]].<ref name="CC Definition" />

Einstein introduced the constant in 1917<ref name=":0">{{harvp|Einstein|1917}}.</ref> to counterbalance the effect of gravity and achieve a [[static universe]], which was then assumed. Einstein's cosmological constant was abandoned after [[Edwin Hubble]] confirmed that the universe was expanding.<ref name="Rugh 2001 3"/> From the 1930s until the late 1990s, most physicists agreed with Einstein's choice of setting the cosmological constant to zero.<ref name="Λ = 0?"/> That changed with the discovery in 1998 that the [[accelerating expansion of the universe|expansion of the universe is accelerating]], implying that the cosmological constant may have a positive value.<ref name="1998 Discovery"/>

Since the 1990s, studies have shown that, assuming the [[cosmological principle]], around 68% of the mass–energy density of the universe can be attributed to dark energy.<ref name="Ellis 2009">{{cite journal|last=Ellis|first=G. F. R.|title=Dark energy and inhomogeneity|journal=Journal of Physics: Conference Series|volume=189|doi=10.1088/1742-6596/189/1/012011|year=2009|issue=1 |page=012011|bibcode=2009JPhCS.189a2011E |s2cid=250670331 |doi-access=free}}</ref><ref name="Colin et al">{{cite journal |last1=Colin |first1=Jacques |last2=Mohayaee |first2=Roya |last3=Rameez |first3=Mohamed |last4=Sarkar |first4=Subir |date=20 November 2019 |title=Evidence for anisotropy of cosmic acceleration |url=https://www.aanda.org/articles/aa/full_html/2019/11/aa36373-19/aa36373-19.html |journal=Astronomy and Astrophysics |volume=631 |pages=L13 |arxiv=1808.04597 |bibcode=2019A&A...631L..13C |doi=10.1051/0004-6361/201936373 |s2cid=208175643 |access-date=25 March 2022}}</ref><ref>{{harvp|Redd|2013}}</ref> The cosmological constant {{math|Λ}} is the simplest possible explanation for dark energy, and is used in the standard model of cosmology known as the [[Lambda-CDM model|ΛCDM model]].

According to [[quantum field theory]] (QFT), which underlies modern [[particle physics]], empty space is defined by the [[vacuum state]], which is composed of a collection of [[quantum fields]]. All these quantum fields exhibit fluctuations in their [[ground state]] (lowest energy density) arising from the [[zero-point energy]] existing everywhere in space. These zero-point fluctuations should contribute to the cosmological constant {{math|Λ}}, but actual calculations give rise to an enormous vacuum energy.<ref>{{harvp|Rugh|Zinkernagel|2001|p=1}}</ref> The discrepancy between theorized vacuum energy from quantum field theory and observed vacuum energy from cosmology is a source of major contention, with the values predicted exceeding observation by some 120&nbsp;orders of magnitude, a discrepancy that has been called "the worst theoretical prediction in the history of physics!".<ref name="CC Problem"/> This issue is called the [[cosmological constant problem]] and it is one of the greatest mysteries in science with many physicists believing that "the vacuum holds the key to a full understanding of nature".<ref name="CC Problem 3"/>