Aspects of Dark Matter Annihilation in Cosmology

TL;DR

This paper explains why current cosmic microwave background (CMB) observations already tightly constrain dark matter annihilation effects, and why future surveys are unlikely to significantly improve these bounds.

Contribution

It provides a simple physical explanation for the saturation of CMB constraints on dark matter annihilation and analyzes their impact on large-scale structure.

Findings

CMB constraints are nearly saturated by current data.

Dark matter annihilation mainly affects ionization at neutral epochs.

Future surveys will not substantially improve existing bounds.

Abstract

Cosmic microwave background (CMB) constraints on dark matter annihilation are a uniquely powerful tool in the quest to understand the nature of dark matter. Annihilation of dark matter to Standard Model particles between recombination and reionization heats baryons, ionizes neutral hydrogen, and alters the CMB visibility function. Surprisingly, CMB bounds on dark matter annihilation are not expected to improve significantly with the dramatic improvements in sensitivity expected in future cosmological surveys. In this paper, we will present a simple physical description of the origin of the CMB constraints and explain why they are nearly saturated by current observations. The essential feature is that dark matter annihilation primarily affects the ionization fraction which can only increase substantially at times when the universe was neutral. The resulting change to the CMB occurs on large angular scales and leads to a phenomenology similar to that of the optical depth to reionization. We will demonstrate this impact on the CMB both analytically and numerically. Finally, we will discuss the additional impact that changing the ionization fraction has on large scale structure.