Editing Quintessence (physics) (section)

== 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>