Power spectra in the eikonal approximation with adiabatic and non-adiabatic modes

TL;DR

This paper applies the eikonal approximation to cosmological perturbation theory to analyze how adiabatic and nonadiabatic modes influence the matter power spectrum, revealing damping effects and large-scale anisotropies.

Contribution

It demonstrates that multipoint power spectra are unaffected by adiabatic modes and uses the eikonal approximation to accurately model nonadiabatic mode damping effects.

Findings

Multipoint power spectra do not depend on large-scale adiabatic modes.

Nonadiabatic modes damp small-scale matter power spectrum.

Relative velocities induce large-scale anisotropic modulations.

Abstract

We use the so-called eikonal approximation, recently introduced in the context of cosmological perturbation theory, to compute power spectra for multi-component fluids. We demonstrate that, at any given order in standard perturbation theory, multipoint power spectra do not depend on the large-scale adiabatic modes. Moreover, we employ perturbation theories to decipher how nonadiabatic modes, such as a relative velocity between two different components, damp the small-scale matter power spectrum, a mechanism recently described in the literature. In particular, we do an explicit calculation at 1-loop order of this effect. While the 1-loop result eventually breaks down, we show how the damping effect can be fully captured by the help of the eikonal approximation. A relative velocity not only induces mode damping but also creates large-scale anisotropic modulations of the matter power spectrum amplitude. We illustrate this for the Local Group environment.