Testing Alternative Theories of Dark Matter with the CMB

TL;DR

This paper introduces a method to constrain modified gravity theories for dark matter using CMB temperature and polarization data, focusing on the dark matter stress history and its effects on cosmological observables.

Contribution

It develops a parametrization of dark matter stress evolution to analyze and constrain alternative gravity theories with CMB data, independent of specific gravitational models.

Findings

CMB spectra can effectively constrain many modified gravity theories.

Some theories remain indistinguishable from CDM even with combined CMB and matter power spectrum data.

The approach uses general conservation equations without assuming specific gravity models.

Abstract

We propose a method to study and constrain modified gravity theories for dark matter using CMB temperature anisotropies and polarization. We assume that the theories considered here have already passed the matter power-spectrum test of large-scale structure. With this requirement met, we show that a modified gravity theory can be specified by parametrizing the time evolution of its dark-matter density contrast, which is completely controlled by the dark matter stress history. We calculate how the stress history with a given parametrization affects the CMB observables, and a qualitative discussion of the physical effects involved is supplemented with numerical examples. It is found that, in general, alternative gravity theories can be efficiently constrained by the CMB temperature and polarization spectra. There exist, however, special cases where modified gravity cannot be distinguished from the CDM model even by using both CMB and matter power spectrum observations, nor can they be efficiently restricted by other observables in perturbed cosmologies. Our results show how the stress properties of dark matter, which determine the evolutions of both density perturbations and the gravitational potential, can be effectively investigated using just the general conservation equations and without assuming any specific theoretical gravitational theory within a wide class.