In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic, changing over time, unlike the cosmological constant which always stays constant. It is suggested[according to whom?] that quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Specifically, it is thought that quintessence became repulsive about 10 billion years ago (the universe is approximately 13.8 billion years old).
Scalar field 
Hence, Quintessence is dynamic, and generally has a density and wq parameter that varies with time. By contrast, a cosmological constant is static, with a fixed energy density and wq = −1.
Tracker behavior 
Many models of quintessence have a tracker behavior, which according to Paul Steinhardt et al. (1999) partly solves the cosmological constant problem. 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. 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, whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015-2020).
Specific models 
Some special cases of quintessence are phantom energy, in which wq < −1, 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 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.
Quintom scenario 
In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w=-1) from above to below. A proven no-go theorem[which?] indicates this situation, called the Quintom scenario, requires at least two degrees of freedom for dark energy models.
The name comes from the classical elements in ancient Greece. The aether, a pure "fifth element" (quinta essentia in Latin), was thought to fill the Universe beyond Earth. Similarly, modern quintessence would be the fifth known contribution to the overall mass-energy content of the Universe. (The other four in the modern interpretation, different from the ancient ideas, are: baryonic matter; radiation – photons and the highly relativistic neutrinos, which may be considered hot dark matter; cold dark matter; and the term due to spatial curvature – loosely, gravitational self-energy.)
- Christopher Wanjek; "Quintessence, accelerating the Universe?"; http://www.astronomytoday.com/cosmology/quintessence.html
- Zlatev, I.; Wang, L.; Steinhardt, P. (1999). "Quintessence, Cosmic Coincidence, and the Cosmological Constant". Physical Review Letters 82 (5): 896–899. arXiv:astro-ph/9807002. Bibcode:1999PhRvL..82..896Z. doi:10.1103/PhysRevLett.82.896.
- Steinhardt, P.; Wang, L.; Zlatev, I. (1999). "Cosmological tracking solutions". Physical Review D 59 (12): 123504. arXiv:astro-ph/9812313. Bibcode:1999PhRvD..59l3504S. doi:10.1103/PhysRevD.59.123504.
- Linden, Sebastian; Virey, Jean-Marc (2008). "Test of the Chevallier-Polarski-Linder parametrization for rapid dark energy equation of state transitions". Physical Review D 78 (2): 023526. arXiv:0804.0389. Bibcode:2008PhRvD..78b3526L. doi:10.1103/PhysRevD.78.023526.
- Ferramacho, L.; Blanchard, A.; Zolnierowsky, Y.; Riazuelo, A. (2010). "Constraints on dark energy evolution". A&A 514: A20. arXiv:0909.1703. Bibcode:2010A&A...514A..20F. doi:10.1051/0004-6361/200913271.
- Linder, Eric V.; Huterer, Dragan (2005). "How many cosmological parameters". Physical Review D 72 (4): 043509. arXiv:astro-ph/0505330. Bibcode:2005PhRvD..72d3509L. doi:10.1103/PhysRevD.72.043509.
- Caldwell, R. R. (2002). "A phantom menace? Cosmological consequences of a dark energy component with super-negative equation of state". Physics Letters B 545 (1-2): 23–29. arXiv:astro-ph/9908168. Bibcode:2002PhLB..545...23C. doi:10.1016/S0370-2693(02)02589-3.
- Hu, Wayne (2005). "Crossing the phantom divide: Dark energy internal degrees of freedom". Physical Review D 71 (4): 047301. arXiv:astro-ph/0410680. Bibcode:2005PhRvD..71d7301H. doi:10.1103/PhysRevD.71.047301.
Further reading 
- Ostriker JP, Steinhardt P (January 2001). "The Quintessential Universe". Scientific American 284 (1): 46–53.
- Dark Energy dominates the Universe (recently unavailable link, outreach poster from University of Bonn)
- Dark Energy dominates the Universe
- Quintessence Accelerating the Universe
- Quintessence model on arxiv.org
||This article needs attention from an expert in physics. The specific problem is: Unclear phrasing in #Tracker behavior and #Quintom scenario. (April 2009)|