Planck Data and Ultralight Axions

TL;DR

This paper explores how photon-axion mixing in the presence of magnetic fields in underdense regions can cause observable non-thermal features in the CMB, including the cold spot, revealing potential for CMB-based axion detection.

Contribution

It demonstrates that photon-axion mixing can produce detectable non-thermal signatures in the CMB, offering a new method to probe ultralight axions.

Findings

Photon-axion mixing can induce non-thermal spots in the CMB.

The cold spot's weak distortion can be explained by this mixing.

CMB observations can be sensitive to ultralight axions.

Abstract

We examine the effects of photon-axion mixing on the CMB. We show that if there are very underdense regions between us and the last scattering surface which contain coherent magnetic fields (whose strength can be orders of magnitude weaker than the current bounds), then photon-axion mixing can induce observable deviations in the CMB spectrum. Specifically, we show that the mixing can give rise to non-thermal spots on the CMB sky. As an example we consider the well known CMB cold spot, which according to the Planck data has a weak distortion from a black body spectrum, that can be fit by our model. While this explanation of the non-thermality in the region of the cold spot is quite intriguing, photon-axion oscillation do not explain the temperature of the cold spot itself. Nevertheless we demonstrate the possible sensitivity of the CMB to ultralight axions which could be exploited by observers.