A Predictive Mirror Twin Higgs with Small $\mathbf{Z}_2$ Breaking
  The twin Higgs mechanism is a solution to the little hierarchy problem in
which the top partner is neutral under the Standard Model (SM) gauge group. The
simplest mirror twin Higgs (MTH) model -- where a $\mathbf{Z}_2$ symmetry
copies each SM particle -- has too many relativistic degrees of freedom to be
consistent with cosmological observations. We demonstrate that MTH models can
have an observationally viable cosmology if the twin mass spectrum leads to
twin neutrino decoupling before the SM and twin QCD phase transitions. Our
solution requires the twin photon to have a mass of $\sim 20$ MeV and
kinetically mix with the SM photon to mediate entropy transfer from the twin
sector to the SM. This twin photon can be robustly discovered or excluded by
future experiments. Additionally, the residual twin degrees of freedom present
in the early Universe in this scenario would be detectable by future
observations of the cosmic microwave background.
