Quintessence (physics)

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{math, energy, light}

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


Scalar field

Quintessence is a scalar field whose equation of state wq, defined as the ratio of its pressure pq and its density ρq, is given by the potential energy V(Q) and a kinetic term:

Hence, 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.[1] 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.[2] 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,[3][4] whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015-2020).[5]

Phantom energy

Some special cases of quintessence are phantom energy, in which w < −1,[6] and k-essence (short for kinetic quintessence) which has a non-standard form of kinetic energy.

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