Editing Cosmological constant (section)

== Positive value ==
[[File:Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation.jpg|thumb|upright=2.4|Lambda-CDM, accelerated expansion of the universe. The time-line in this schematic diagram extends from the Big Bang/inflation era 13.7 Byr ago to the present cosmological time.]]
Observations announced in 1998 of distance–redshift relation for [[Type Ia supernovae]]<ref name="1998 Discovery"/> indicated that the expansion of the universe is accelerating, if one assumes the [[cosmological principle]].<ref name="Ellis 2009"/><ref name="Colin et al"/> When combined with measurements of the [[cosmic microwave background radiation]] these implied a value of {{math|Ω<sub>Λ</sub>}} ≈ 0.7,<ref>See e.g. {{harvp|Baker et al.|1999}}.</ref> a result which has been supported and refined by more recent measurements<ref>See for example Table 9 in {{harvp|The Planck Collaboration|2015a|p=27}}.</ref> (as well as previous works<ref>{{cite journal|bibcode=1992Ap&SS.191..107P|title=Inflation and compactification from Galaxy redshifts?|year=1992 |doi = 10.1007/BF00644200 |last1=Paál |first1=G. |last2=Horváth |first2=I. |last3=Lukács |first3=B. |journal=Astrophysics and Space Science |volume=191 |issue=1 |pages=107–124 |s2cid=116951785 }}</ref><ref>{{cite journal|bibcode=1994Ap&SS.222...65H|title=Once More on Quasar Periodicities|year=1994 |doi = 10.1007/BF00627083 |last1=Holba |first1=Ágnes |last2=Horváth |first2=I. |last3=Lukács |first3=B. |last4=Paál |first4=G. |journal=Astrophysics and Space Science |volume=222 |issue=1–2 |pages=65–83 |s2cid=118379051 }}</ref>). If one assumes the cosmological principle, as in the case for all models that use the [[Friedmann–Lemaître–Robertson–Walker metric]], while there are other possible causes of an [[accelerating universe]], such as quintessence, the cosmological constant is in most respects the [[Occam's razor|simplest solution]]. Thus, the Lambda-CDM model, the current standard model of cosmology which uses the FLRW metric, includes the cosmological constant, which is measured to be on the order of {{val|e=-52|u=m-2}}. It may be expressed as {{val|e=-35|u=s-2}} (multiplying by {{nowrap|{{math|''c''{{sup|2}}}} ≈ {{val|e=17|u=m<sup>2</sup>⋅s<sup>−2</sup>}}}}) or as 10<sup>−122</sup>&nbsp;''ℓ''{{sub|P}}<sup>−2</sup> <ref>{{harvp|Barrow|Shaw|2011}}.</ref> (where ''ℓ''{{sub|P}} is the Planck length). The value is based on recent measurements of vacuum energy density, {{math|''ρ''{{sub|vac}}}} = {{val|5.96|e=-27|u=kg/m3}} ≘ {{val|5.3566|e=-10|u=J/m3}} = {{val|3.35|u=GeV/m3}}.<ref>Calculated based on the Hubble constant and {{math|Ω{{sub|Λ}}}} from {{harvp|The Planck Collaboration|2015b}}.</ref> However, due to the [[Hubble tension]] and the [[CMB dipole]], recently it has been proposed that the cosmological principle is no longer true in the late universe and that the FLRW metric breaks down,<ref name="Snowmass21">{{citation |last1=Abdalla |first1=Elcio |title=Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies |date=11 Mar 2022 |journal=Journal of High Energy Astrophysics |volume=34 |page=49 |arxiv=2203.06142v1 |bibcode=2022JHEAp..34...49A |doi=10.1016/j.jheap.2022.04.002 |s2cid=247411131 |display-authors=2 |last2=Abellán |first2=Guillermo Franco |last3=Aboubrahim |first3=Amin}}.</ref><ref name="FLRW breakdown">{{cite journal |last1=Krishnan |first1=Chethan |last2=Mohayaee |first2=Roya |last3=Colgáin |first3=Eoin Ó |last4=Sheikh-Jabbari |first4=M. M. |last5=Yin |first5=Lu |title=Does Hubble Tension Signal a Breakdown in FLRW Cosmology? |journal=Classical and Quantum Gravity |date=16 September 2021 |volume=38 |issue=18 |pages=184001 |doi=10.1088/1361-6382/ac1a81 |arxiv=2105.09790 |bibcode=2021CQGra..38r4001K |s2cid=234790314 |issn=0264-9381}}</ref><ref name="Heinesen-MacPherson">{{cite journal |last1=Heinesen |first1=Asta |last2=Macpherson |first2=Hayley J. |date=15 July 2021 |title=Luminosity distance and anisotropic sky-sampling at low redshifts: A numerical relativity study |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.023525 |journal=Physical Review D |volume=104 |issue=2 |page=023525 |arxiv=2103.11918 |bibcode=2021PhRvD.104b3525M |doi=10.1103/PhysRevD.104.023525 |s2cid=232307363 |access-date=25 March 2022}}</ref> so it is possible that observations usually attributed to an accelerating universe are simply a result of the cosmological principle not applying in the late universe.<ref name="Ellis 2009"/><ref name="Colin et al"/>

As was only recently seen, by works of [[Gerard 't Hooft|'t Hooft]], [[Leonard Susskind|Susskind]] and others, a positive cosmological constant has surprising consequences, such as a finite maximum [[entropy]] of the observable universe (see ''[[Holographic principle]]'').<ref>{{harvp|Dyson|Kleban|Susskind|2002}}.</ref>