Spectral distortions of the CMB dipole

TL;DR

This paper explores how primordial recombination radiation and spectral distortions cause frequency-dependent anisotropies in the CMB dipole, offering a potential new method for detecting these early universe signals.

Contribution

It introduces the concept of CMB dipole distortions caused by primordial spectral distortions and discusses their detectability with future measurements.

Findings

Relative distortions reach 10^{-6} for PRR and μ, and 10^{-5} for y in 1-700 GHz range.

Temperature differences can be up to 10 nK for PRR and μ, and 1 μK for y.

Future missions like PIXIE could detect the y-distortion dipole at a few sigma.

Abstract

We consider the distortions of the cosmic microwave background (CMB) dipole anisotropy related to the primordial recombination radiation (PRR) and primordial $y$- and $\mu$-distortions. The signals arise due to our motion relative to the CMB restframe and appear as a frequency-dependent distortion of the CMB temperature dipole. To leading order, the expected relative distortion of the CMB dipole does not depend on the particular observation directions and reaches the level of $10^{-6}$ for the PRR- and $\mu$-distortions and $10^{-5}$ for the $y$-distortion in the frequency range 1 -- 700 GHz. The temperature differences arising from the dipole anisotropy of the relic CMB distortions depend on the observation directions. For mutually opposite directions, collinear to the CMB dipole axis, the temperature differences because of the PRR- and $\mu$-dipole anisotropy attain values $\Delta T\simeq 10\,$nK in the considered range. The temperature difference arising from the $y$-dipole anisotropy may reach values of up to $1\,\mu$K. The key features of the considered effect are as follow: (i) an observation of the effect does not require absolute calibration; (ii) patches of sky with minimal foreground contamination can be chosen. Future measurements of the CMB dipole distortion thus will provide an alternative method for direct detection of the PRR-, $y$-, and $\mu$-distortions. The $y$-distortion dipole may be detectable with PIXIE at a few standard deviations.