An Etymological Dictionary of Astronomy and Astrophysics

In cosmology, a → dimensionless parameter introduced by the → equation of state
representing the ratio of the pressure to the energy density of a fluid, such as the
→ dark energy: w = p/ρ.
The → deceleration or → acceleration of an → expanding Universe depends on this parameter (→ accelerating Universe). A number of numerical values of this parameter are as follows: for the → cosmological constant: w = -1, for
→ non-relativistic matter (present-day → baryons): w = 0, and
for → relativistic matter (photons, neutrinos): w = +1/3. Together with Ω(dark energy) and Ω(matter), w provides a three-parameter description of the dark energy. The simplest parametrization of the dark energy is w = constant, although w might depend on → redshift.

See also: → equation; → state; → parameter.