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