Cosmological Signals of a Mirror Twin Higgs
  We investigate the cosmology of the minimal model of neutral naturalness, the
mirror Twin Higgs. The softly-broken mirror symmetry relating the Standard
Model to its twin counterpart leads to significant dark radiation in tension
with BBN and CMB observations. We quantify this tension and illustrate how it
can be mitigated in several simple scenarios that alter the relative energy
densities of the two sectors while respecting the softly-broken mirror
symmetry. In particular, we consider both the out-of-equilibrium decay of a new
scalar as well as reheating in a toy model of twinned inflation, Twinflation.
In both cases the dilution of energy density in the twin sector does not merely
reconcile the existence of a mirror Twin Higgs with cosmological constraints,
but predicts contributions to cosmological observables that may be probed in
current and future CMB experiments. This raises the prospect of discovering
evidence of neutral naturalness through cosmology rather than colliders.
