# On structure formation from a small-scales-interacting dark sector

## TL;DR

This paper proposes a small-scale interacting dark sector model that affects perturbation evolution without altering background cosmology, potentially resolving discrepancies in density perturbation measurements.

## Contribution

It introduces a novel dark matter-dark energy interaction based on relative velocities, impacting small-scale structure formation and addressing observational tensions.

## Key findings

- Potential to alleviate the amplitude tension between galaxy surveys and Planck data.
- Predicts a shift in the matter power spectrum turnover independent of horizon at equality.
- Identifies a unique baryon-dark matter velocity bias due to the interaction.

## Abstract

We consider a cosmological model with an interaction between dark matter and dark energy which leaves the background cosmology unaffected and only affects the evolution of the perturbations. This is achieved by introducing a coupling given in terms of the relative velocities of dark matter and dark energy. This interaction has the distinctive feature of appearing predominantly on small scales, where peculiar velocities can become important. We confront the predictions of the model to cosmological observations and find a potential alleviation of the known tension in the amplitude of density perturbations as measured by low redshift galaxy surveys and the Planck data. The model also predicts a shift in the turnover of the matter power spectrum which does not depend on the horizon at equality (fixed by the background cosmology and, thus, unaffected by the perturbations) and is entirely due to the interaction between dark matter and dark energy. A bias in the peculiar velocity between baryons and dark matter is also shown to be a unique feature of this type of interactions in the dark sector.