Statistical anisotropy in CMB spectral distortions

TL;DR

This paper investigates how measurements of CMB spectral y-distortion anisotropies can reveal deviations from statistical isotropy in primordial density perturbations, providing theoretical predictions for anisotropic signals.

Contribution

It calculates the expected y-distortion anisotropies for anisotropic primordial perturbations, especially the quadrupole component, and discusses cosmic variance implications.

Findings

Quadrupole anisotropy in y-distortion is proportional to the anisotropic power spectrum coefficient.

Explicit formula for the expected y-distortion anisotropy in terms of spherical harmonics and anisotropy parameters.

Discussion of cosmic variance effects on detecting anisotropic signals.

Abstract

Measurements of the cosmic microwave background (CMB) spectral $y$-distortion anisotropy offer a test for the statistical isotropy of the primordial density perturbations on $0.01\lesssim k{\rm Mpc}\lesssim 1$. We compute the 1-point ensemble averages of the $y$-distortion anisotropies which vanish for the statistically isotropic perturbations. For the quadrupole statistical anisotropy, we find $4\pi\langle y_{2m}\rangle=-6.8A_2\times 10^{-9}Y_{2m}(\mathbf d )$ with the quadruple Legendre coefficient of the anisotropic powerspectrum $A_2$ and the $\ell=2$ spherical harmonics $Y_{2m}(\mathbf d )$ for the preferred direction $\mathbf d $. Also, we discuss the cosmic variance of the $y$-distortion anisotropy in the statistically anisotropic Universe.