Gravitational portals in the early Universe
  We consider the production of matter and radiation during reheating after
inflation, restricting our attention solely to gravitational interactions.
Processes considered are the exchange of a graviton, $h_{\mu \nu}$, involved in
the scattering of the inflaton or particles in the newly created radiation
bath. In particular, we consider the gravitational production of dark matter
(scalar or fermionic) from the thermal bath as well as from scattering of the
inflaton condensate. We also consider the gravitational production of radiation
from inflaton scattering. In the latter case, we also derive a lower bound on
the maximal temperature of order of $10^{12}$ GeV for a typical
$\alpha-$attractor scenario from $\phi \phi \rightarrow h_{\mu \nu}
\rightarrow$ Standard Model fields (dominated by the production of Higgs
bosons). This lower gravitational bound becomes the effective maximal
temperature for reheating temperatures, $T_{\rm{RH}} \lesssim 10^9$ GeV. The
processes we consider are all minimal in the sense that they are present in any
non-minimal extension of the Standard Model theory based on Einstein gravity
and can not be neglected. We compare each of these processes to determine their
relative importance in the production of both radiation and dark matter.
