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 which has an equation of state w (relating its pressure pq and density ρq) of pq = wρq which is less than -1/3. 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 behaving as dark energy, eventually dominating the universe. This naturally sets the low scale of the dark 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.
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