Viscous modes, isocurvature perturbations and CMB initial conditions

TL;DR

This paper introduces viscous modes as a new class of initial conditions in cosmology, showing how viscosity fluctuations can generate curvature perturbations distinct from traditional adiabatic and isocurvature modes.

Contribution

It presents a novel analysis of viscous modes, expanding the understanding of initial conditions in the early universe beyond conventional solutions.

Findings

Viscous modes can produce curvature perturbations independently of adiabatic and isocurvature modes.

Viscous modes are explicitly characterized and compared with traditional isocurvature solutions.

Viscosity fluctuations can influence CMB initial conditions in a computable way.

Abstract

When the predecoupling plasma is thermodynamically reversible its fluctuations are classified in terms of the adiabatic and entropic modes. A different category of physical solutions, so far unexplored, arises when the inhomogeneities of the viscosity coefficients induce computable curvature perturbations. The viscous modes are explicitly illustrated and compared with the conventional isocurvature solutions.