An Etymological Dictionary of Astronomy and Astrophysics

The energy density fluctuations in the → photon-baryon plasma
that bring about hotter and colder regions. This perturbation creates velocity distributions that are out of phase with the acoustic density mode. The fluid velocity from hot to cold regions causes blueshift of the photons, resulting in → quadrupole anisotropy.

See also: → scalar; → perturbation.