Cosmological effects of coupled dark matter

TL;DR

This paper investigates models where a fraction of dark matter interacts with a scalar field, affecting cosmological observations, and finds that significant coupling can occur without deviating from standard cosmological models.

Contribution

It introduces a model with partial dark matter coupling to a scalar field and analyzes its cosmological effects, expanding on previous single-species coupling studies.

Findings

Coupling fraction up to half of dark matter is possible without significant deviations.

Greater coupling strength increases deviations from LCDM.

Background evolution is sensitive to the coupled dark matter fraction and coupling strength.

Abstract

Many models have been studied that contain more than one species of dark matter and some of these couple the Cold Dark Matter (CDM) to a light scalar field. In doing this we introduce additional long range forces, which in turn can significantly affect our estimates of cosmological parameters if not properly accounted for. It is, therefore, important to study these models and their resulting cosmological implications. We present a model in which a fraction of the total cold dark matter density is coupled to a scalar field. We study the background and perturbation evolution and calculate the resulting Cosmic Microwave Background anisotropy spectra. The greater the fraction of dark matter coupled to the scalar field and the stronger the coupling strength, the greater the deviation of the background evolution from LCDM. Previous work, with a single coupled dark matter species, has found an upper limit on the coupling strength of order O(0.1). We find that with a coupling of this magnitude more than half the dark matter can be coupled to a scalar field without producing any significant deviations from LCDM.