# New CMB spectral distortion constraints on decaying dark matter with full evolution of electromagnetic cascades before recombination

## TL;DR

This paper refines constraints on decaying dark matter by accurately modeling electromagnetic cascades before recombination, showing that previous limits assuming simple distortions may be overly restrictive.

## Contribution

It introduces exact calculations of spectral distortions from electromagnetic cascades, improving upon previous approximations in constraining early-universe energy injections.

## Key findings

- Spectral distortions differ significantly from simple y and μ types.
- Constraints on dark matter decay can be relaxed by up to a factor of 5.
- Accurate cascade modeling alters previous bounds on energy injection.

## Abstract

Current constraints on energy injection in the form of energetic particles before the epoch of recombination using CMB spectral distortions assume that all energy goes into $y$ and $\mu$-type distortions. We revisit these constraints with exact calculations of the spectral distortions by evolving the electromagnetic cascades. The actual spectral distortion differs in shape and amplitude from the $y$-type distortion and depends on the energy and nature of injected particles. The constraints on the energy injection processes such as dark matter decay can be relaxed by as much as a factor of 5.