Known unknowns of dark matter annihilation over cosmic time

Katherine J Mack

doi:10.1093/mnras/stu129

Abstract

Dark matter self-annihilation holds promise as one of the most robust mechanisms for the identification of the particle responsible for the Universe's missing mass. In this work, I examine the evolution of the dark matter annihilation power produced by smooth and collapsed structures over cosmic time, taking into account uncertainties in the structure of dark matter halos. As we search for observational signatures of annihilation, an understanding of this time evolution will help us to best direct our observational efforts, either with local measurements or investigation of the effects of annihilation on the intergalactic medium at high redshift. As I show in this work, there are several key sources of uncertainty in our ability to estimate the dark matter annihilation from collapsed structures, including: the density profile of dark matter halos; the small-scale cut-off in the dark matter halo mass function; the redshift-dependent mass-concentration relation for small halos; and the particle-velocity dependence of the dark matter annihilation process. Varying assumptions about these quantities can result in annihilation power predictions that differ by several orders of magnitude. These uncertainties must be resolved, through a combination of observation and modeling, before robust estimations of the cosmological annihilation signal can be made.

Highlights

While it is widely accepted that dark matter makes up the bulk of the mass in the Universe, the nature of the dark matter particle is still unknown
Because the Warm dark matter (WDM) cases are qualitatively distinct, I include them as the thin dashed lines that extend outside the shaded region
Fig. 7.— Mean power output per hydrogen nucleus from dark matter annihilation as a function of redshift for the “fiducial” model compared to all cold dark matter models shown in Figures 3-6 above

Summary

Introduction

While it is widely accepted that dark matter makes up the bulk of the mass in the Universe, the nature of the dark matter particle is still unknown. Dark matter identification efforts are faced with a large set of inconclusive or contradictory results in experiments for indirect (Abdo et al 2009; Adriani et al 2009; Aguilar et al 2013) and direct detection (Aalseth et al 2011, 2013; Ahmed et al 2009, 2010, 2011; Angle et al 2008, 2011; Angloher et al 2012; Armengaud et al 2012; Bernabei et al 2010; Kelso et al 2012; LUX Collaboration et al 2013; Savage et al 2009) This confusing state of affairs highlights the importance of seeking alternative probes of dark matter’s properties. Knowing this evolution can be useful in determining if there is a “sweet spot” in which to look for signs of local energy injection, where the total annihilation

Objectives

Results

Conclusion