# Dark matter kinetic decoupling with a light particle

## TL;DR

This paper explores how the nature of dark matter kinetic decoupling, especially with a light particle, can significantly influence small-scale density perturbations, challenging the usual assumptions of acoustic damping.

## Contribution

It introduces a model with sudden kinetic decoupling and analyzes its effects on cosmological perturbations both numerically and analytically.

## Key findings

- Kinetic decoupling can enhance or suppress small-scale perturbations.
- Sudden decoupling leads to different power spectrum features than standard models.
- The nature of decoupling critically impacts structure formation at small scales.

## Abstract

We argue that the acoustic damping of the matter power spectrum is not a generic feature of the kinetic decoupling of dark matter, but even the enhancement can be realized depending on the nature of the kinetic decoupling when compared to that in the standard cold dark matter model. We consider a model that exhibits a ${\it sudden}$ kinetic decoupling and investigate cosmological perturbations in the ${\it standard}$ cosmological background numerically in the model. We also give an analytic discussion in a simplified setup. Our results indicate that the nature of the kinetic decoupling could have a great impact on small scale density perturbations.