Anisotropic Dark Energy: Dynamics of Background and Perturbations

TL;DR

This paper explores anisotropic dark energy models within Bianchi I cosmology, analyzing their background dynamics, observational signatures, and perturbations, to explain large-scale anomalies and constrain anisotropy parameters.

Contribution

It introduces a framework for anisotropic dark energy with skewness parameters, studies their effects on cosmological observations, and derives perturbation evolution equations in anisotropic backgrounds.

Findings

CMB quadrupole tightly constrains anisotropy parameters

Some anisotropic models evade CMB bounds and explain large-angle anomalies

Anisotropic dark energy influences galaxy formation and large-scale structure

Abstract

We investigate cosmologies where the accelerated expansion of the Universe is driven by a field with an anisotropic equation of state. We model such scenarios within the Bianchi I framework, introducing two skewness parameters to quantify the deviation of pressure from isotropy. We study the dynamics of the background expansion in these models. A special case of anisotropic cosmological constant is analyzed in detail. The anisotropic expansion is then confronted with the redshift and angular distribution of the supernovae type Ia. In addition, we investigate the effects on the cosmic microwave background (CMB) anisotropies for which the main signature appears to be a quadrupole contribution. We find that the two skewness parameters can be very well constrained. Tightest bounds are imposed by the CMB quadrupole, but there are anisotropic models which avoid this bound completely. Within these bounds, the anisotropy can be beneficial as a potential explanation of various anomalous cosmological observations, especially in the CMB at the largest angles. We also consider the dynamics of linear perturbations in these models. The covariant approach is used to derive the general evolution equations for cosmological perturbations taking into account imperfect sources in an anisotropic background. The implications for the galaxy formation are then studied. These results might help to make contact between the observed anomalies in CMB and large scale structure and fundamental theories exhibiting Lorentz violation.