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