Quintessence (physics)

For other forms of quintessence, see Quintessence (disambiguation).

For theories and defunct or classical concepts named after the synonym "Aether", see Aether (disambiguation).

In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force. Quintessence differs from the cosmological constant explanation of dark energy in that it is a dynamic equation that changes over time, unlike the cosmological constant which always stays constant throughout history from beginning to end. It is suggested that quintessence can be either attractive or repulsive depending on the ratio of kinetic to potential energy in the universe. Specifically, it is thought that quintessence became repulsive about 10 billion years ago (the universe is approximately 14 billion years old).

Scalar field

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

${w_q=p_q/\rho_q=\frac{\frac{1}{2}\dot{Q}^2-V(Q)}{\frac{1}{2}\dot{Q}^2+V(Q)},}$.

Hence, Quintessence is dynamic, and generally has a density and equation of state that varies with time. By contrast, a Cosmological Constant is static, with a fixed energy density and w_q = −1.

Tracker behavior

Many models of quintessence have a tracker behavior, which according to Paul Steinhardt et al. (1999) 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).

Specific models

Some special cases of quintessence are phantom energy, in which w_q < −1, and k-essence (short for kinetic quintessence), which has a non-standard form of kinetic energy. If this type of energy were to exist, it would cause a big rip in the universe due to the growing energy density of dark energy which would cause the expansion of the universe to increase at a faster-than-exponential rate.

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