# Shedding light on the small-scale crisis with CMB spectral distortions

## TL;DR

This paper proposes using measurements of the cosmic microwave background's spectral distortions, specifically the $$ distortion, to distinguish between early-universe and late-time suppression mechanisms responsible for the small-scale crisis.

## Contribution

It demonstrates that $$ distortion measurements can differentiate primordial from late-time matter power suppression mechanisms affecting the small-scale crisis.

## Key findings

- $$ distortion is suppressed if suppression is primordial.
- Standard $$ prediction remains unchanged for late-time suppression.
- $$ measurements can identify the origin of small-scale power suppression.

## Abstract

The small-scale crisis, discrepancies between observations and N-body simulations, may imply suppressed matter fluctuations on subgalactic distance scales. Such a suppression could be caused by some early-universe mechanism (e.g., broken scale invariance during inflation), leading to a modification of the primordial power spectrum at the onset of the radiation-domination era. Alternatively, it may be due to nontrivial dark-matter properties (e.g., new dark-matter interactions or warm dark matter) that affect the matter power spectrum at late times, during radiation domination, after the perturbations re-enter the horizon. We show that early- and late-time suppression mechanisms can be distinguished by measurement of the $\mu$ distortion to the frequency spectrum of the cosmic microwave background. This is because the $\mu$ distortion is suppressed, if the power suppression is primordial, relative to the value expected from the dissipation of standard nearly scale-invariant fluctuations. We emphasize that the standard prediction of the $\mu$ distortion remains unchanged in late-time scenarios even if the dark-matter effects occur before or during the era (redshifts $5\times 10^4 \lesssim z \lesssim 2\times 10^6$) at which $\mu$ distortions are generated.