Dark energy with non-adiabatic sound speed: initial conditions and detectability

TL;DR

This paper evaluates the potential to detect dark energy perturbations with a constant sound speed using Planck CMB data and galaxy surveys, highlighting the limitations in detecting low sound speeds and discussing initial condition considerations.

Contribution

It provides a comprehensive analysis of dark energy perturbation detectability, including a full likelihood exploration and generalization of initial condition assumptions.

Findings

Detection only possible for sound speeds close to zero

Planck data alone is insufficient for detection

Attractor solutions diminish initial condition dependence

Abstract

Assuming that the universe contains a dark energy fluid with a constant linear equation of state and a constant sound speed, we study the prospects of detecting dark energy perturbations using CMB data from Planck, cross-correlated with galaxy distribution maps from a survey like LSST. We update previous estimates by carrying a full exploration of the mock data likelihood for key fiducial models. We find that it will only be possible to exclude values of the sound speed very close to zero, while Planck data alone is not powerful enough for achieving any detection, even with lensing extraction. We also discuss the issue of initial conditions for dark energy perturbations in the radiation and matter epochs, generalizing the usual adiabatic conditions to include the sound speed effect. However, for most purposes, the existence of attractor solutions renders the perturbation evolution nearly independent of these initial conditions.