Editing Quintessence (physics)

{{Short description|Hypothetical fundamental force}}
{{About||other forms of quintessence|Quintessence (disambiguation)|theories and defunct or classical concepts named after the synonym "Aether"|Aether (disambiguation)}}
In [[physics]], '''quintessence''' is a [[hypothesis|hypothetical]] form of [[dark energy]], more precisely a [[scalar field]] minimally coupled to gravity, postulated as an explanation of the observation of an [[accelerating expansion of the universe|accelerating rate of expansion]] of the universe. The first example of this scenario was proposed by [[Bharat Ratra|Ratra]] and [[Jim Peebles|Peebles]] (1988)<ref name="RatraPeebles1988">{{cite journal|last2=Peebles |first2=L.|last1=Ratra |first1=P.
| title=Cosmological consequences of a rolling homogeneous scalar field| journal=[[Physical Review D]]
| date=1988
| volume=37
|issue=12
| pages=3406–3427
| doi=10.1103/PhysRevD.37.3406
|pmid=9958635|bibcode=1988PhRvD..37.3406R |doi-access=free
}}</ref> and [[Christof Wetterich|Wetterich]] (1988).<ref>{{Cite journal |last=Wetterich |first=C. |date=1988-06-13 |title=Cosmology and the fate of dilatation symmetry |url=https://dx.doi.org/10.1016/0550-3213%2888%2990193-9 |journal=Nuclear Physics B |language=en |volume=302 |issue=4 |pages=668–696 |doi=10.1016/0550-3213(88)90193-9 |arxiv=1711.03844 |bibcode=1988NuPhB.302..668W |s2cid=118970077 |issn=0550-3213}}</ref><ref>{{Cite journal |last=Doran |first=Michael |date=2001-10-01 |others=et al. |title=Quintessence and the Separation of Cosmic Microwave Background Peaks |url=https://iopscience.iop.org/article/10.1086/322253/fulltext/53251.text.html |journal=The Astrophysical Journal |language=en |volume=559 |issue=2 |pages=501–506 |doi= 10.1086/322253|arxiv=astro-ph/0012139 |bibcode=2001ApJ...559..501D |s2cid=119454400 |via=Iopscience}}</ref> The concept was expanded to more general types of time-varying dark energy, and the term "quintessence" was first introduced in a 1998 paper by [[Robert R. Caldwell]], Rahul Dave and [[Paul Steinhardt]].<ref name="CDS">{{cite journal |last1=Caldwell |first1=R. R. |last2=Dave |first2=R. |last3=Steinhardt |first3=P. J. |year=1998 |title=Cosmological Imprint of an Energy Component with General Equation-of-State |journal=Physical Review Letters |volume=80 |issue=8 |pages=1582–1585 |arxiv=astro-ph/9708069 |bibcode=1998PhRvL..80.1582C |doi=10.1103/PhysRevLett.80.1582 |s2cid=597168}}</ref> It has been proposed by some physicists to be a [[fifth force|fifth fundamental force]].<ref>{{cite journal |last=Carroll |first=S. M. |year=1998 |title=Quintessence and the Rest of the World: Suppressing Long-Range Interactions |journal=Physical Review Letters |volume=81 |issue=15 |pages=3067–3070 |arxiv=astro-ph/9806099 |bibcode=1998PhRvL..81.3067C |doi=10.1103/PhysRevLett.81.3067 |s2cid=14539052}}</ref><ref>{{cite web |last=Wetterich |first=C. |title=Quintessence – a fifth force from variation of the fundamental scale |url=http://www.thphys.uni-heidelberg.de/~wetterich/DEBarcelona0706.pdf |publisher=Heidelberg University}}</ref><ref>{{cite journal|url = https://cds.cern.ch/record/515241/files/0108217.pdf|title = Changing α With Time: Implications For Fifth-Force-Type Experiments And Quintessence|first1= Gia|last1= Dvali |first2=Matias|last2=  Zaldarriaga |journal = Physical Review Letters|year = 2002|volume = 88|issue = 9|pages = 091303|doi = 10.1103/PhysRevLett.88.091303|pmid = 11863992|arxiv = hep-ph/0108217|bibcode = 2002PhRvL..88i1303D|s2cid = 32730355}}</ref><ref>{{Cite journal |last1=Cicoli |first1=Michele |last2=Pedro |first2=Francisco G. |last3=Tasinato |first3=Gianmassimo |date=2012-07-23 |title=Natural Quintessence in String Theory |journal=Journal of Cosmology and Astroparticle Physics |volume=2012 |issue=7 |pages=044 |doi=10.1088/1475-7516/2012/07/044 |issn=1475-7516|arxiv=1203.6655 |bibcode=2012JCAP...07..044C |s2cid=250808223 }}</ref> Quintessence differs from the [[cosmological constant]] explanation of dark energy in that it is dynamic; that is, it changes over time, unlike the cosmological constant which, by definition, does not change. Quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Those working with this postulate believe that quintessence became repulsive about ten billion years ago, about 3.5 billion years after the [[Big Bang]].<ref>{{cite web |url = http://www.astronomytoday.com/cosmology/quintessence.html |title = Quintessence, accelerating the Universe? |first = Christopher |last = Wanjek|website = Astronomy Today}}</ref>

A group of researchers argued in 2021 that observations of the [[Hubble tension]] may imply that only quintessence models with a nonzero [[coupling constant]] are viable.<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>

== Terminology ==
The name comes from ''quinta essentia'' (fifth element). So called in Latin starting from the Middle Ages, this was the (first) element added by [[Aristotle]] to the other four ancient [[Classical element#Classical elements in Greece|classical elements]] because he thought it was the essence of the celestial world. Aristotle posited it to be a pure, fine, and [[Fortuna|primigenial]] element which he referred to as [[Aether (classical element)|''aether'']] in his text [[On the Heavens]]. Similarly, modern quintessence would be the fifth known "dynamical, time-dependent, and spatially inhomogeneous" contribution to the overall mass–energy content of the universe.

Of course, the other four components are not the [[Classical element#Classical elements in Greece|ancient Greek classical elements]], but rather "[[Baryonic matter|baryons]], [[neutrinos]], [[dark matter]], [and] [[Electromagnetic radiation|radiation]]." Although neutrinos are sometimes considered radiation, the term "radiation" in this context is only used to refer to massless [[photons]]. Spatial curvature of the cosmos (which has not been detected) is excluded because it is non-dynamical and homogeneous; the cosmological constant would not be considered a fifth component in this sense, because it is non-dynamical, homogeneous, and time-independent.<ref name=CDS />

== Scalar field ==
Quintessence (''Q'') is a [[scalar field]] with an [[Equation of state (cosmology)|equation of state]] where ''w''<sub>''q''</sub>, the ratio of pressure ''p''<sub>''q''</sub> and density <math>\rho</math><sub>''q''</sub>, is given by the potential energy <math>V(Q)</math> and a [[kinetic term]]:
:<math>w_q=\frac{p_q}{\rho_q}=\frac{\frac{1}{2}\dot{Q}^2-V(Q)}{\frac{1}{2}\dot{Q}^2+V(Q)}</math>
Hence, quintessence is dynamic, and generally has a density and ''w''<sub>''q''</sub> parameter that varies with time. Specifically, ''w''<sub>''q''</sub> parameter can vary within the range [-1,1]. By contrast, a cosmological constant is static, with a fixed [[energy density]] and ''w''<sub>''q''</sub>&nbsp;=&nbsp;&minus;1.

== Tracker behavior ==
Many models of quintessence have a ''tracker'' behavior, which according to Ratra and Peebles (1988) and [[Paul Steinhardt]] ''et al.'' (1999) partly solves the [[cosmological constant problem]].<ref name="Zlatev">{{cite journal
| last1=Zlatev |first1=I.
|last2=Wang   |first2=L.
|last3=Steinhardt |first3=P.
| title=Quintessence, Cosmic Coincidence, and the Cosmological Constant
| journal=[[Physical Review Letters]]
| date=1999
| volume=82
|issue=5
| pages=896–899
| doi=10.1103/PhysRevLett.82.896
|bibcode=1999PhRvL..82..896Z
|arxiv = astro-ph/9807002 |s2cid=119073006
}}</ref> In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until [[Timeline of the Big Bang#Matter domination|matter-radiation equality]], which triggers quintessence to start having characteristics similar to dark energy, eventually dominating the universe. This naturally sets the low [[energy scale|scale]] of the dark energy.<ref name="Steinhardt1999">{{cite journal
| last3=Zlatev |first3=I.
|last2=Wang   |first2=L.
|last1=Steinhardt |first1=P.
| title=Cosmological tracking solutions
| journal=[[Physical Review D]]
| date=1999
| volume=59
|issue=12
| pages=123504
| doi=10.1103/PhysRevD.59.123504
|bibcode=1999PhRvD..59l3504S
|arxiv = astro-ph/9812313 |s2cid=40714104
}}</ref> When comparing the predicted [[metric expansion of space|expansion rate]] of the universe as given by the tracker solutions with cosmological data, a main feature of tracker solutions is that one needs four parameters to properly describe the behavior of their [[equation of state (cosmology)|equation of state]],<ref name="Linden2008">{{cite journal
| last1=Linden |first1=Sebastian
|last2=Virey  |first2=Jean-Marc
| title=Test of the Chevallier-Polarski-Linder parametrization for rapid dark energy equation of state transitions
| journal=[[Physical Review D]]
| date=2008
| volume=78
|issue=2
| pages=023526
| doi=10.1103/PhysRevD.78.023526
|bibcode=2008PhRvD..78b3526L
|arxiv = 0804.0389 |s2cid=118288188
}}</ref><ref name="Ferramacho2010">{{cite journal
| last1=Ferramacho |first1=L.
|last2=Blanchard  |first2=A.
|last3=Zolnierowsky | first3=Y.
|last4=Riazuelo | first4=A.
| title=Constraints on dark energy evolution
| journal=[[Astronomy & Astrophysics]]
| date=2010
| volume=514
| pages=A20
| doi=10.1051/0004-6361/200913271
|bibcode=2010A&A...514A..20F
|arxiv = 0909.1703 |s2cid=17386518
}}</ref> whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015–2020).<ref name="LindererHuterer2005">{{cite journal
| last1=Linder |first1=Eric V.
|last2=Huterer  |first2=Dragan
| title=How many cosmological parameters
| journal=[[Physical Review D]]
| date=2005
| volume=72
|issue=4
| pages=043509
| doi=10.1103/PhysRevD.72.043509
|bibcode=2005PhRvD..72d3509L
|arxiv = astro-ph/0505330 |s2cid=14722329
}}</ref>

== Specific models ==
Some special cases of quintessence are [[phantom dark energy]], in which ''w''<sub>''q''</sub>&nbsp;<&nbsp;&minus;1,<ref name="Caldwell2002">{{cite journal
| last1=Caldwell |first1=R. R.
| title=A phantom menace? Cosmological consequences of a dark energy component with super-negative equation of state
| journal=[[Physics Letters B]]
| date=2002
| volume=545
|issue=1–2
| pages=23–29
| doi=10.1016/S0370-2693(02)02589-3
|bibcode=2002PhLB..545...23C
|arxiv = astro-ph/9908168 |s2cid=9820570
}}</ref> and k-essence (short for kinetic quintessence), which has a non-standard form of [[kinetic energy]]. If this type of energy were to exist, it would cause a [[big rip]]<ref name="Antoniou2016">{{cite journal
| last1=Antoniou |first1=Ioannis |last2=Perivolaropoulos |first2=Leandros
| title= Geodesics of McVittie Spacetime with a Phantom Cosmological Background
| journal=[[Phys. Rev. D]]
| volume= 93
| issue= 12
| pages= 123520
| year= 2016
| doi= 10.1103/PhysRevD.93.123520
| arxiv =1603.02569  | bibcode= 2016PhRvD..93l3520A
|s2cid=18017360 }}</ref> in the universe due to the growing energy density of dark energy, which would cause the expansion of the universe to increase at a faster-than-exponential rate.

===Holographic dark energy===
Holographic dark energy models, compared with cosmological constant models, imply a high [[degeneracy (mathematics)|degeneracy]].{{clarify|date=August 2016}}<ref>{{Cite journal|arxiv=1502.01156 |year=2015|title=Holographic Dark Energy with Cosmological Constant|journal=Journal of Cosmology and Astroparticle Physics|volume=2015|issue=8|pages=012|last1=Hu|first1=Yazhou|last2=Li|first2=Miao|last3=Li|first3=Nan|last4=Zhang|first4=Zhenhui|doi=10.1088/1475-7516/2015/08/012|bibcode=2015JCAP...08..012H|s2cid=118732915}}</ref> It has been suggested that dark energy might originate from [[quantum fluctuations]] of [[spacetime]], and is limited by the event horizon of the universe.<ref>{{cite journal |author=Gao |first=Shan |year=2013 |title=Explaining Holographic Dark Energy |url=http://philsci-archive.pitt.edu/10036/ |journal=Galaxies |volume=1 |issue=3 |pages=180–191 |bibcode=2013Galax...1..180G |doi=10.3390/galaxies1030180 |doi-access=free}}</ref>

Studies with quintessence dark energy found that it dominates gravitational collapse in a spacetime simulation, based on the holographic thermalization. These results show that the smaller the state parameter of quintessence is, the harder it is for the plasma to thermalize.<ref>{{cite journal|doi=10.1103/PhysRevD.91.046005|title=Holographic thermalization and gravitational collapse in the spacetime dominated by quintessence dark energy |journal=Physical Review D |volume=91 |issue=4 |pages=046005 |year=2015|arxiv = 1408.6632 |bibcode = 2015PhRvD..91d6005Z |last1=Zeng |first1=Xiao-Xiong |last2=Chen |first2=De-You |last3=Li |first3=Li-Fang |s2cid=119107827 }}</ref>

==See also==
*[[Aether (classical element)]]
*[[Phantom energy|Phantom dark energy]]
*[[Quintom scenario|Quintom]]

== References ==
{{Reflist|25em}}

==Further reading==
* {{cite journal |last=Christof |first=Wetterich |date=1987-09-24 |title=Cosmology and the fate of dilatation symmetry |journal=Nuclear Physics B |volume=302 |issue=4 |pages=668–696 |doi=10.1016/0550-3213(88)90193-9 |arxiv=1711.03844|bibcode=1988NuPhB.302..668W |s2cid=118970077 }}
* {{Cite journal |author=Ostriker |first1=J. P. |last2=Steinhardt |first2=P. |title=The Quintessential Universe |journal=Scientific American |volume=284 |number=1 |date=January 2001 |pages=46–53 |doi=10.1038/scientificamerican0101-46 |pmid=11132422 |bibcode=2001SciAm.284a..46O}}
* {{cite book |author=Krauss |first=Lawrence M. |author-link=Lawrence M. Krauss |title=Quintessence: The Search for Missing Mass in the Universe |publisher=[[Basic Books]] |year=2000 |isbn=978-0465037414 |title-link=Quintessence: The Search for Missing Mass in the Universe}}

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[[Category:Dark energy]]