Anisotropic dark energy and CMB anomalies

TL;DR

This paper explores how anisotropic dark energy with crystalline symmetry could cause CMB anomalies, developing new methods to analyze such models and finding they are observationally indistinguishable from isotropic models.

Contribution

It introduces an adaptation of the line of sight integration method for anisotropic perturbations and applies it to crystalline symmetry models in dark energy.

Findings

Models with anisotropic dark energy are indistinguishable from isotropic models in CMB observations.

Anisotropic initial conditions from inflation produce detectable CMB anisotropies.

Dark energy anisotropies mainly affect large scales via the integrated Sachs-Wolfe effect.

Abstract

We investigate the breaking of global statistical isotropy caused by a dark energy component with an energy-momentum tensor which has point symmetry, that could represent a cubic or hexagonal crystalline lattice. In such models Gaussian, adiabatic initial conditions created during inflation can lead to anisotropies in the cosmic microwave background whose spherical harmonic coefficients are correlated, contrary to the standard assumption. We develop an adaptation of the line of sight integration method that can be applied to models where the background energy-momentum tensor is isotropic, but whose linearized perturbations are anisotropic. We then show how this can be applied to the cases of cubic and hexagonal symmetry. We compute quantities which show that such models are indistinguishable from isotropic models even in the most extreme parameter choices, in stark contrast to models with anisotropic initial conditions based on inflation. The reason for this is that the dark energy based models contribute to the CMB anistropy via the inegrated Sachs-Wolfe effect, which is only relevent when the dark energy is dominant, that is, on the very largest scales. For inflationary models, however, the anisotropy is present on all scales.