Cannibal domination and the matter power spectrum
  Decoupled hidden sectors can easily and generically result in a period of
cannibal domination, during which the dominant component of the Universe has an
equation of state intermediate between radiation and matter due to self-heating
by number-changing interactions. We present for the first time the consequences
of a cannibal-dominated era prior to big bang nucleosynthesis for structure
formation on small scales. We find that an early cannibal-dominated era
imprints a characteristic peak on the dark matter power spectrum, with scale
and amplitude directly determined by the mass, lifetime, and number-changing
interaction strength of the cannibal field. This enhancement to the small-scale
matter power spectrum will generate early-forming dark matter microhalos, and
we provide a detailed and transparent map between the properties of the
cannibal species and the characteristic mass and formation time of these
structures. These relations demonstrate how the internal workings of a hidden
sector leave a potentially observable imprint on the matter power spectrum even
if dark matter has no direct couplings to the Standard Model.
