Interacting Scalar Radiation and Dark Matter in Cosmology

TL;DR

This paper explores how interacting scalar radiation can influence cosmological phenomena, including effects on the CMB and large-scale structure, depending on its self-couplings and interactions with dark matter.

Contribution

It introduces a novel framework for scalar radiation self-interactions and their impact on cosmology, including a new approach to avoid scattering singularities.

Findings

Scalar radiation can behave as neutrinos or a perfect fluid depending on self-couplings.

Interactions between scalar radiation and dark matter can modify the matter power spectrum.

Debye shielding can prevent scattering singularities in the model.

Abstract

We investigate possible cosmological effects of interacting scalar radiation and dark matter. After its decoupling, scalar radiation can stream freely as neutrinos or self-interact strongly as perfect fluid, highly depending on the magnitude of its self-couplings. We obtain the general and novel structure for self-scattering rate and compare it with the expansion rate of our Universe. If its trilinear/cubic coupling is non-zero, scalar radiation can be eventually treated as perfect fluid. Possible effects on CMB are also discussed. When this scalar also mediates interaction among dark matter particles, the linear matter power spectrum for large scale structure can be modified differently from other models. We propose to use Debye shielding to avoid the singularity appearing in the scattering between scalar radiation and dark matter.