Quasiadiabatic modes from viscous inhomogeneities

TL;DR

This paper investigates viscous inhomogeneities in relativistic plasmas, showing that quasiadiabatic modes are suppressed and unlikely to replace the standard adiabatic paradigm in cosmology.

Contribution

It provides a gauge-invariant and non-perturbative analysis demonstrating the suppression of quasiadiabatic modes due to viscosity effects.

Findings

Quasiadiabatic modes are suppressed compared to adiabatic solutions.

Viscous inhomogeneities cannot replace the standard adiabatic mode in accelerated cosmology.

Large tensor-to-scalar ratio excludes viscous quasiadiabatic modes as sole drivers of inflation.

Abstract

The viscous inhomogeneities of a relativistic plasma determine a further class of entropic modes whose amplitude must be sufficiently small since curvature perturbations are observed to be predominantly adiabatic and Gaussian over large scales. When the viscous coefficients only depend on the energy density of the fluid the corresponding curvature fluctuations are shown to be almost adiabatic. After addressing the problem in a gauge-invariant perturbative expansion, the same analysis is repeated at a non-perturbative level by investigating the nonlinear curvature inhomogeneities induced by the spatial variation of the viscous coefficients. It is demonstrated that the quasiadiabatic modes are suppressed in comparison with a bona fide adiabatic solution. Because of its anomalously large tensor to scalar ratio the quasiadiabatic mode cannot be a substitute for the conventional adiabatic paradigm so that, ultimately, the present findings seems to exclude the possibility of a successful accelerated dynamics solely based on relativistic viscous fluids. If the dominant adiabatic mode is not affected by the viscosity of the background a sufficiently small fraction of entropic fluctuations of viscous origin cannot be a priori ruled out.