Constraints on the anisotropy of dark energy

TL;DR

This paper constrains the anisotropy of dark energy by analyzing its effects on the cosmic microwave background quadrupole, setting tight limits on anisotropic stress and discussing implications for cosmological models.

Contribution

It provides the first stringent limits on dark energy anisotropy using CMB quadrupole data, improving previous bounds from supernova observations.

Findings

Limit of | w| < 2.1 10^{-4} for dark energy anisotropy.

Stronger limits are possible for specific cosmological parameters.

No evidence found for anisotropy improving the low quadrupole anomaly.

Abstract

If the equation of state of dark energy is anisotropic there will be additional quadrupole anisotropy in the cosmic microwave background induced by the time dependent anisotropic stress quantified in terms of $\Delta w$. Assuming that the entire amplitude of the observed quadrupole is due to this anisotropy, we conservatively impose a limit of $|\Delta w| < 2.1\times 10^{-4}$ for any value of $w\ge -1$ assuming that $\Omega_{\rm m}<0.5$. This is considerably tighter than that which comes from SNe. Stronger limits, upto a factor of 10, are possible for specific values of $\Omega_{\rm m}$ and $w$. Since we assume this component is uncorrelated with the stochastic component from inflation, we find that both the expectation value and the sample variance are increased. There no improvement in the likelihood of an anomalously low quadrupole as suggested by previous work on an elliptical universe.