An Etymological Dictionary of Astronomy and Astrophysics
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Assumption that the variations in the plasma parameters, due to the presence of waves, are small (to the first order) as compared to the undisturbed parameters. This makes it possible to linearize equations by dropping out second order (and higher) nonlinear terms.

→ linear; → perturbation; → theory.

The linearization of the appropriate equations governing a system by the assumption of a steady state, with departures from that steady state limited to small perturbations. Also called perturbation method.

→ method; → small; → perturbation.

1) Any departure introduced into a steady state of a system. The magnitude is often assumed to be small so that the resulting terms in the dependent variables may be neglected. The term "perturbation" is therefore sometimes used as synonymous with "small perturbation."
2) Gravitational effect of a third body that causes an alteration in the orbit of a body going around its primary.
See also: → linear perturbation theory, → method of small perturbationsn → perturbation equation, → perturbation method, → primordial curvature perturbation, → scalar perturbation, → secular perturbation, → tensor perturbation, → vector perturbation.

Verbal noun of → perturb.

Any equation governing the behavior of a → perturbation.

→ perturbation; → equation.

Approximate method of solving a difficult problem if the equations to be solved depart only slightly from those of a problem already solved.

→ perturbation; → method.

In cosmological models, the phenomenon that is supposed to seed the → cosmic microwave background anisotropies and the structure formation of the Universe.

→ primordial; → curvature; → perturbation.

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.

→ scalar; → perturbation.

A variation of planetary orbital elements which is always in the same direction as time increases.

→ secular; → perturbation.

The perturbation in the → primordial Universe plasma caused by → gravitational waves. These waves stretch and squeeze space in orthogonal directions and bring about → quadrupole anisotropy in incoming radiation temperature.

→ tensor; → perturbation.

The perturbation in the → primordial Universe plasma caused by → vorticity. These perturbations cause → Doppler shifts that result in → quadrupole anisotropy.

→ vector; → perturbation.