Observed distortions of the cosmic microwave background spectrum are used to detect the density perturbations of the universe.

In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity.

Baryonic acoustic oscillations are imprints of sound waves on scales where radiation pressure stabilized the density perturbations against gravitational collapse in the early universe.

The left hand side is a wave operator, which is applied to the density perturbation or pressure perturbation respectively.

Supersymmetry mechanism for naturally small density perturbations and baryogenesis, Physical Review Letters (1993)

PIB models, which ascribe all cosmic density perturbations to isocurvature modes, predict results that are inconsistent with the observational data.

Often, "late-time ISW" implicitly refers to the late-time ISW effect to linear/first order in density perturbations.

Current work on this model centers on whether it can succeed in stabilizing the size of the compactified dimensions and produce the correct spectrum of primordial density perturbations.

As a consequence, the ions are accelerated out of the cavity, amplifying the density perturbation.

A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures.