Partially Acoustic Dark Matter, Interacting Dark Radiation, and Large Scale Structure

TL;DR

This paper proposes a two-component dark matter model with interacting dark radiation that simultaneously addresses discrepancies in Hubble constant and matter fluctuation measurements, and predicts testable signatures in future experiments.

Contribution

It introduces a novel two-component dark matter framework with dark radiation interactions that resolves large-scale structure tensions and remains consistent with thermal WIMP origins.

Findings

Dark acoustic oscillations inhibit growth of subdominant dark matter perturbations.

Tightly coupled dark radiation alleviates the H_0 tension.

Model predictions can be tested by future CMB and large-scale structure experiments.

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 $\sigma_8$ 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 $\sigma_8$ 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.