The 21 cm signal and the interplay between dark matter annihilations and astrophysical processes

Abstract

Future dedicated radio interferometers, including HERA and SKA, are very promising tools that aim to study the epoch of reionization and beyond via measurements of the 21 cm signal from neutral hydrogen. Dark matter (DM) annihilations into charged particles change the thermal history of the Universe and, as a consequence, affect the 21 cm signal. Accurately predicting the effect of DM strongly relies on the modeling of annihilations inside halos. In this work, we use up-to-date computations of the energy deposition rates by the products from DM annihilations, a proper treatment of the contribution from DM annihilations in halos, as well as values of the annihilation cross section allowed by the most recent cosmological measurements from the Planck satellite. Given current uncertainties on the description of the astrophysical processes driving the epochs of reionization, X-ray heating and Lyman-α pumping, we find that disentangling DM signatures from purely astrophysical effects, related to early-time star formation processes or late-time galaxy Xray emissions, will be a challenging task. We conclude that only annihilations of DM particles with masses of ∼ 100 MeV, could leave an unambiguous imprint on the 21 cm signal and, in particular, on the 21 cm power spectrum. This is in contrast to previous, more optimistic results in the literature, which have claimed that strong signatures might also be present even for much higher DM masses. Additional measurements of the 21 cm signal at different cosmic epochs will be crucial in order to break the strong parameter degeneracies between DM annihilations and astrophysical effects and undoubtedly single out a DM imprint for masses different from ∼ 100 MeV.

Code package for calculating modified cosmic ionization and thermal histories with dark matter and other exotic energy injections

“…III D, but instead of performing the calculation at each step, we simply provide transfer functions D high c that act on propagating photons and return the high-energy deposition into the channels c ={'ion', 'exc', 'heat'}. 6 We can then combine this with the result from electron cooling to obtain the high-energy deposition per baryon within a log-redshift step into each channel c:…”

“…The evolution equations shown in Eqs. (6) and (50) can be integrated with all reionization terms switched off if the user is primarily interested in temperatures or ionization levels well before reionization starts at z ∼ 20. In this regime, turning off helium is also a reasonable approximation.…”

“…In particular, we will focus on interactions that allow dark matter (DM) to decay or annihilate into electromagnetically interacting Standard Model particles. This case has been studied extensively in the literature: stringent constraints on the dark matter annihilation cross section and decay lifetime have been derived from the way these Standard Model products would distort the anisotropies of the cosmic microwave background (CMB) [1][2][3][4], or increase the temperature of the Inter-Galactic Medium (IGM), consequently affecting 21-cm and Lyman-α line emission [5][6][7][8].…”

“…The epoch of reionization is currently rather poorly constrained, making it important to understand the observational signatures of different scenarios, and the degree to which exotic energy injections might be separable from uncertainties in the reionization model. Previous attempts to model the effects of DM annihilation and decay into the reionization epoch have typically either assumed a fixed ionization history [9] -requiring a slow re-computation of the cooling cascade if that history is changed [5] -or made an approximation for the effect of a modified ionization fraction on the cooling of high-energy particles [6,8,[10][11][12]. Despite our emphasis on dark matter annihilation and decay, DarkHistory can be used to explore the effect of other forms of exotic particle injection.…”

Probing the small-scale matter power spectrum with large-scale 21-cm data

“…Previous works have studied the effect of non-CDM models on the 21-cm signal [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Those models typically suppress the matter power spectrum beyond some wavenumber k, delaying the onset of cosmic dawn.…”