CMB Spectral Distortions from Cooling Macroscopic Dark Matter

We propose a new mechanism by which dark matter (DM) can affect the early universe. The hot interior of a macroscopic DM, or macro, can behave as a heat reservoir so that energetic photons are emitted from its surface. This results in spectral distortions (SDs) of the cosmic microwave background. The SDs depend on the density and the cooling processes of the interior, and the surface composition of the Macros. We use neutron stars as a model for nuclear-density Macros and find that the spectral distortions are mass-independent for fixed density. In our work, we find that, for Macros of this type that constitute 100$\%$ of the dark matter, the $\mu$ and $y$ distortions can be above detection threshold for typical proposed next-generation experiments such as PIXIE.