Contributions to cosmic reionization from dark matter annihilation and decay

Abstract

Dark matter annihilation or decay could have a significant impact on the ionization and thermal history of the universe. In this paper, we study the potential contribution of dark matter annihilation (s-wave-or p-wave-dominated) or decay to cosmic reionization, via the production of electrons, positrons and photons. We map out the possible perturbations to the ionization and thermal histories of the universe due to dark matter processes, over a broad range of velocity-averaged annihilation cross-sections/decay lifetimes and dark matter masses. We have employed recent numerical studies of the efficiency with which annihilation/decay products induce heating and ionization in the intergalactic medium, and in this work extended them down to a redshift of 1 + z = 4 for two different reionization scenarios. We also improve on earlier studies by using the results of detailed structure formation models of dark matter haloes and subhaloes that are consistent with up-to-date N -body simulations, with estimates on the uncertainties that originate from the smallest scales. We find that for dark matter models that are consistent with experimental constraints, a contribution of more than 10% to the ionization fraction at reionization is disallowed for all annihilation scenarios. Such a contribution is possible only for decays into electron/positron pairs, for light dark matter with mass mχ 100 MeV, and a decay lifetime τχ ∼ 10 24 − 10 25 s.

General constraints on dark matter decay from the cosmic microwave background

“…Here we neglect structure formation; previous studies of the impact of DM annihilation on the CMB anisotropy spectrum have demonstrated that most of the effect arises from high redshifts, z ∼ 600, where inhomogeneities in the DM density are small [21,27]. Energy injection from DM annihilations and decays extending until late time, and the possible impact on reionization is studied in [28][29][30].…”

“…For models at the limits of the bounds we will present, the modification to the hydrogen ionization fraction x e can be as large as ∆x e ∼ 10% just prior to reionization, which is sufficient to non-trivially modify the fraction of deposited power proceeding into ionization (vs heating or excitation) for low-energy electrons [18]. However, a model with even shorter decay time / larger power injection was considered in [29], and it was shown in that case that the overall change to the ionization history from accounting for these backreaction effects was very small. Furthermore, we expect the CMB constraint from decaying DM with a long lifetime to arise mostly from higher redshifts, z ∼ 300 [21], as we will discuss in more depth in the next section.…”

Kinetic mixing, dark photons and an extra dimension. Part I

“…This remains true in the 5-D extensions of these models that we consider. These constraints essentially place bounds on the DM annihilation cross section into various electromagnetically interacting final states that can lead to re-ionization in the early universe with the sensitivity peaking near z ∼ 600 [54,55]. For example, for 50-100 MeV DM that has an s-wave annihilation into the e + e − final state (which we might expect to be the dominant mode in this mass range), the annihilation cross-section is constrained to roughly satisfy the bound < σv > (1 − 3) · 10 −29 cm 3 s −1 This is ∼ 3 orders of magnitude smaller than the canonical thermal cross section needed at freeze-out to achieve the observed relic density [56], i.e., < σv > 4.5 · 10 −26 cm 3 s −1 for (self-conjugate) DM in this mass range.…”

Prospects for indirect MeV dark matter detection with gamma rays in light of cosmic microwave background constraints

“…Given that constraints from CMB for p-wave annihilation are both weak and model-dependent, and that, moreover, the corresponding detectability of a gamma-ray signal is highly dependent on the velocity distribution in the target dark matter halo, in this work we exclusively focus on s-wave annihilators. Limits on p-wave annihilating dark matter from CMB for larger dark matter masses in standard WIMP scenarios have been presented in [39,43,44].…”

“…Equation (10) results in a suppression on the energy injection and thus will not alter the thermal history until low redshift. At the redshifts where dark matter contributes one must also consider the clumping effect due to the formation of dark matter halos [39]. In addition, to compute the z ref one must know the temperature of kinetic decoupling (T kd ), which is model dependent (see e.g.…”