Dark Radiation Sticks Together: Dark QCD and the Hubble Tension

Abstract

We introduce a model of dark matter charged under a new confining gauge group akin to the Standard Model's quantum chromodynamics (QCD). The fermionic sector contains both heavy and light quark-like fermions charged under the dark QCD. As the Universe cools, the gauge force confines and dark radiation transitions from free particles to low-mass dark pions. This form of dark radiation behaves as an imperfect fluid with nonzero viscosity. If the confinement occurs at the MeV scale, the model can alleviate the tension between early- and late-time measurements of the Hubble parameter, with a Bayesian posterior of $H_0 = 71.60\pm 0.63$ km/s/Mpc and a $3σ$ residual tension across Planck, ACT, BBN, Pantheon+, DESI DR2, and SH0ES. In the late Universe, the heavy dark matter can evade existing constraints on dark matter self-interactions while containing a rich phenomenology that may be accessible in future observations.