quintessence (physics)

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 elements because he thought it was the essence of the celestial world. Aristotle posited it to be a pure, fine, and primigenial element. Later scholars identified this element with aether.{{clarify|text=Who? When? Sources? |date=January 2025}} 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 ancient Greek classical elements, but rather "baryons, neutrinos, dark matter, [and] 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.

Quintessence (Q) is a scalar field with an equation of state where w_q, the ratio of pressure p_q and density $\backslash rho$_q, is given by the potential energy $V(Q)$ and a kinetic term:

:$w\_q=\backslash frac\{p\_q\}\{\backslash rho\_q\}=\backslash frac\{\backslash frac\{1\}\{2\}\backslash dot\{Q\}^2-V(Q)\}\{\backslash frac\{1\}\{2\}\backslash dot\{Q\}^2+V(Q)\}$

Hence, quintessence is dynamic, and generally has a density and w_q parameter that varies with time. Specifically, w_q parameter can vary within the range [-1,1]. By contrast, a cosmological constant is static, with a fixed energy density and w_q = −1.

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.{{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

}} In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter-radiation equality, which triggers quintessence to start having characteristics similar to dark energy, eventually dominating the universe. This naturally sets the low scale of the dark energy.{{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

}} When comparing the predicted 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,{{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

}}{{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

}} whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015–2020).{{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

}}

Some special cases of quintessence are phantom energy, in which w_q < −1,{{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

}} 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{{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 }} 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 models, compared with cosmological constant models, imply a high degeneracy.{{clarify|date=August 2016}}{{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}} It has been suggested that dark energy might originate from quantum fluctuations of spacetime, and is limited by the event horizon of the universe.{{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}}

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.{{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 }}

In 2004, when scientists fitted the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary ({{mvar|w}} = –1) from above to below. A proven no-go theorem indicates this situation, called the Quintom scenario, requires at least two degrees of freedom for dark energy models involving ideal gases or scalar fields.{{cite journal |last=Hu |first=Wayne |title=Crossing the phantom divide: Dark energy internal degrees of freedom |journal=Physical Review D |year=2005 |volume=71 |issue=4 |pages=047301 |doi=10.1103/PhysRevD.71.047301 |bibcode=2005PhRvD..71d7301H |arxiv=astro-ph/0410680|s2cid=8791054 }}

In 2024, more detailed data from the Dark Energy Spectroscopic Instrument provided evidence suggesting a possible "Quintom-B" scenario, with the equation of state crossing the boundary from below to above.{{cite journal | last1=Yang | first1=Yuhang | last2=Ren | first2=Xin | last3=Wang | first3=Qingqing | last4=Lu | first4=Zhiyu | last5=Zhang | first5=Dongdong | last6=Cai | first6=Yi-Fu | last7=Saridakis | first7=Emmanuel N. | title=Quintom cosmology and modified gravity after DESI 2024 | journal=Science Bulletin | date=2024 | volume=69 | issue=17 | pages=2698–2704 | arxiv=2404.19437| doi=10.1016/j.scib.2024.07.029 }}

• Aether (classical element)

{{Reflist|25em}}

• {{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