Non-Abelian dark matter and dark radiation
  We propose a new class of dark matter models with unusual phenomenology. What
is ordinary about our models is that dark matter particles are WIMPs, they are
weakly coupled to the Standard Model and have weak scale masses. What is
unusual is that they come in multiplets of a new "dark" non-Abelian gauge group
with milli-weak coupling. The massless dark gluons of this dark gauge group
contribute to the energy density of the universe as a form of weakly
self-interacting dark radiation. In this paper we explore the consequences of
having i.) dark matter in multiplets ii.) self-interacting dark radiation and
iii.) dark matter which is weakly coupled to dark radiation. We find that i.)
dark matter cross sections are modified by multiplicity factors which have
significant consequences for collider searches and indirect detection, ii.)
dark gluons have thermal abundances which affect the CMB as dark radiation.
Unlike additional massless neutrino species the dark gluons are interacting and
have vanishing viscosity and iii.) the coupling of dark radiation to dark
matter represents a new mechanism for damping the large scale structure power
spectrum. A combination of additional radiation and slightly damped structure
is interesting because it can remove tensions between global $\Lambda$CDM fits
from the CMB and direct measurements of the Hubble expansion rate ($H_0$) and
large scale structure ($\sigma_8$).
