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 which has an equation of state (relating its pressure pq and density ρq) of pq = wρq, 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.
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 "No-Go" theorem indicates this situation, called the Quintom scenario, requires at least two degrees of freedom for dark energy models.
Terminology
The name comes from the classical elements of the ancient Greeks. The aether, a pure "fifth element" (quinta essentia in Latin), was thought to fill the Universe beyond Earth. This seemed fitting to modern scientists, since quintessence was the fifth known contribution to the overall mass-energy content of the Universe. (The other four in the modern interpretation, different from the Greeks' 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 self-energy.)
Further reading
- Ostriker, Jeremiah P., and Paul Steinhardt, "The Quintessential Universe", Scientific American, vol. 284, no. 1 (January 2001), pp. 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
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