The origin of the microgauss magnetic fields observed in galaxies is unknown. One scenario is that primordial magnetic fields (PMFs) generated during inflation, larger than 0.1 nanogauss on Mpc scales, were compressed to microgauss strengths in galaxies during structure formation. Thus, detecting such a PMF just after recombination would be evidence of this inflationary origin. We find that CMB-HD measurements of anisotropic birefringence would lower the upper bound on scale-invariant PMFs to 0.072 nanogauss at the 95% CL. If inflationary PMFs exist, CMB-HD would be able to detect them with 3-sigma significance or higher, providing evidence for inflation itself.

 To understand the landscape of possible dark matter models, we propose a mass-varying dark matter (MVDM) model arising from a transition.  The one free parameter of this model is the temperature at which the phase transition occurs; for a phase transition temperature between 10 MeV and 10^7 GeV, the current dark matter density is achieved for a fermion mass in the range of 1 GeV to 10^9 GeV.  In this dark matter model, the scalar becomes a sub-dominant unclustered component of dark matter that can lower the amplitude of structure formation by up to a few percent. The mass-varying fermion component can lead to discrepant measurements of the current dark matter density of about ten percent inferred from early and late-time probes assuming ?CDM.