Formation of inflaton halos after inflation
  The early Universe may have passed through an extended period of
matter-dominated expansion following inflation and prior to the onset of
radiation domination. Sub-horizon density perturbations grow gravitationally
during such an epoch, collapsing into bound structures if it lasts long enough.
The strong analogy between this phase and structure formation in the
present-day universe allows the use of N-body simulations and approximate
methods for halo formation to model the fragmentation of the inflaton
condensate into inflaton halos. For a simple model we find these halos have
masses of up to $20\,\mathrm{kg}$ and radii of the order of
$10^{-20}\,\mathrm{m}$, roughly $10^{-24}$ seconds after the Big Bang. We find
that the N-body halo mass function matches predictions of the mass-Peak Patch
method and the Press-Schechter formalism within the expected range of scales. A
long matter-dominated phase would imply that reheating and thermalization
occurs in a universe with large variations in density, potentially modifying
the dynamics of this process. In addition, large overdensities can source
gravitational waves and may lead to the formation of primordial black holes.
