# Quintessence (physics)

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Quintessence (physics)

__NOTOC__In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe.

calar field

Quintessence is a scalar field which has an equation of state (relating its pressure "p""q" and density $ho$"q") of "p""q" = "w"$ho$"q", where "w" is equal to the equation of state of the energy component dominating the universe (i.e. equal to 1/3 during radiation domination and 0 during matter domination) until "w" undergoes a transition to less than -1/3 which initiates the accelerated expansion of the universe. Quintessence is dynamic, and generally has a density and equation of state that varies through time and space. By contrast, a cosmological constant is static, with a fixed energy density and "w" = −1.

Tracker behavior

Many models of quintessence have a "tracker" behavior, which 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.

Phantom energy

Some special cases of quintessence are phantom energy, in which w < −1, and k-essence (short for kinetic quintessence) which has a non-standard form of kinetic energy.

Terminology

The name comes from the Classical elements of the ancient Greeks, where a pure "fifth element," (known as "quinta essentia" in Latin) the aether, was thought to fill the Universe beyond Earth.

* [http://www.astro.uni-bonn.de/~webiaef/outreach/posters/darkenergy/ Dark Energy dominates the Universe] (outreach poster from University of Bonn)
* [http://www.astro.uni-bonn.de/~kilbinge/de/index.shtml/ Dark Energy dominates the Universe]
* [http://www.astronomytoday.com/cosmology/quintessence.html/ Quintessence Accelerating the Universe]
* [http://physicsworld.com/cws/article/print/402 Quintessence]

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