The Distortion of the Cosmic Microwave Background by the Milky Way

TL;DR

This paper investigates how the Milky Way's gravitational field distorts the cosmic microwave background (CMB), causing large-scale anisotropies and leakage of low-order multipoles into higher ones, but finds this effect negligible for observed anomalies.

Contribution

The study introduces a photon ray-tracing method to quantify the Milky Way's gravitational lensing effect on the CMB and demonstrates its insensitivity to Galactic potential variations.

Findings

Milky Way causes minimal photon bending (<1 arcsecond) near the Galactic Center.

Global CMB distortions affect large angular scales and multipole leakage.

The effect does not significantly contribute to CMB anomalies like power asymmetry.

Abstract

The Milky Way can act as a large-scale weak gravitational lens of the cosmic microwave background (CMB). We study this effect using a photon ray-tracing code and a Galactic mass distribution with disk, bulge and halo components. For an observer at the Sun's coordinates in the Galaxy, the bending of CMB photon paths is limited to less than one arcsecond, and only for rays that pass within a few degrees of the Galactic Center. However, the entire sky is affected, resulting in global distortions of the CMB on large angular scales. These distortions can cause the low-order multipoles of a spherical harmonic expansion of the CMB sky temperature to leak into higher-order modes. Thus the component of the CMB dipole that results from the Local Group's motion relative to the local cosmic frame of rest contributes to higher-order moments for an observer in the solar system. With our ray-tracing code we show that the phenomenon is not sensitive to the specific choice of Galactic potential. We also quantitatively rule it out as a contributor to CMB anomalies such as power asymmetry or correlated alignment of low-order multipole moments.