Partially acoustic dark matter, interacting dark radiation, and large scale structure

Abstract

The standard paradigm of collisionless cold dark matter is in tension with measurements on large scales. In particular, the best fit values of the Hubble rate H 0 and the matter density perturbation σ 8 inferred from the cosmic microwave background seem inconsistent with the results from direct measurements. We show that both problems can be solved in a framework in which dark matter consists of two distinct components, a dominant component and a subdominant component. The primary component is cold and collisionless. The secondary component is also cold, but interacts strongly with dark radiation, which itself forms a tightly coupled fluid. The growth of density perturbations in the subdominant component is inhibited by dark acoustic oscillations due to its coupling to the dark radiation, solving the σ 8 problem, while the presence of tightly coupled dark radiation ameliorates the H 0 problem. The subdominant component of dark matter and dark radiation continue to remain in thermal equilibrium until late times, inhibiting the formation of a dark disk. We present an example of a simple model that naturally realizes this scenario in which both constituents of dark matter are thermal WIMPs. Our scenario can be tested by future stage-IV experiments designed to probe the CMB and large scale structure.

Interacting dark sector and precision cosmology

“…(PAcDM ) in [14]. The plasma has a speed of sound c 2 sp analogous to that of the baryonphoton plasma, given by the expression found in Eq.…”

“…In this paper, we have studied a class of cosmological models that allow a fraction of the dark matter to interact with a locally thermal dark radiation fluid, thus generalizing the works of [10,11,14]. These interacting Dark Sector (IDS) models have three parameters in addition to the usual six of ΛCDM: the amount ρ dr of dark radiation, the fraction f of the total DM that is interacting with this radiation, and the interaction rate Γ = Γ 0 a −2 .…”

“…Were these discrepancies to be of physical origin (rather than unaccounted-for systematics), they would be a sign of the need for new physics beyond ΛCDM, and that a new cosmological model ought to take its place. Different studies on these discrepancies have appeared in the literature ( [10][11][12][13][14][15][16][17][18][19][20][21]). …”

“…Recently, [10,11,14,22] discussed an alternative model to ΛCDM which can resolve both tensions. This model invokes a dark sector (DS) composed of mutually interacting dark matter (IDM) and dark radiation (DR) 1 .…”

Make dark matter charged again

“…, or it can be a fraction of the dark matter, as was studied in Refs. [34][35][36][37]. More generally, research on self-interacting dark matter has been motivated in part by potential discrepancies in conventional dark matter scenarios, especially on small scales.…”