Quintessence (physics)
In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of the observation of an accelerating rate of expansion of the universe announced in 1998. 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, that is, it changes over time, unlike the cosmological constant which always stays constant. It is suggested 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 ten billion years ago (the universe is approximately 13.8 billion years old).^{[1]}
Contents
 Scalar field 1
 Tracker behavior 2
 Specific models 3
 Quintom scenario 4
 Terminology 5

References 6
 Further reading 6.1
 External links 7
Scalar field
Quintessence is a scalar field with an equation of state where w_{q}, the ratio of pressure p_{q} and density \rho_{q}, is given by the potential energy V(Q) and a kinetic term:
 w_q=p_q/\rho_q=\frac{\frac{1}{2}\dot{Q}^2V(Q)}{\frac{1}{2}\dot{Q}^2+V(Q)}
Hence, quintessence is dynamic, and generally has a density and w_{q} parameter that varies with time. By contrast, a cosmological constant is static, with a fixed energy density and w_{q} = −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.^{[2]} In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matterradiation 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.^{[3]} 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,^{[4]}^{[5]} whereas it has been shown that at most a twoparameter model can optimally be constrained by midterm future data (horizon 20152020).^{[6]}
Specific models
Some special cases of quintessence are phantom energy, in which w_{q} < −1,^{[7]} and kessence (short for kinetic quintessence), which has a nonstandard 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 fasterthanexponential 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 nogo theorem indicates this situation, called the Quintom scenario, requires at least two degrees of freedom for dark energy models.^{[8]}
Terminology
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 massenergy 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 selfenergy.)
References
 ^ 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".
 ^ Steinhardt, P.; Wang, L.; Zlatev, I. (1999). "Cosmological tracking solutions".
 ^ Linden, Sebastian; Virey, JeanMarc (2008). "Test of the ChevallierPolarskiLinder parametrization for rapid dark energy equation of state transitions".
 ^ Ferramacho, L.; Blanchard, A.; Zolnierowsky, Y.; Riazuelo, A. (2010). "Constraints on dark energy evolution".
 ^ Linder, Eric V.; Huterer, Dragan (2005). "How many cosmological parameters".
 ^ Caldwell, R. R. (2002). "A phantom menace? Cosmological consequences of a dark energy component with supernegative equation of state".
 ^ Hu, Wayne (2005). "Crossing the phantom divide: Dark energy internal degrees of freedom".
Further reading
 Ostriker JP, Steinhardt P (January 2001). "The Quintessential Universe". Scientific American 284 (1): 46–53.
External links
 Dark Energy dominates the Universe (recently unavailable link, outreach poster from University of Bonn)
 Dark Energy dominates the Universe
 Quintessence Accelerating the Universe
 Quintessence
 Quintessence model on arxiv.org