Gravitational perturbations from oscillons and transients after
  inflation
  We study the scalar and tensor perturbations generated by the fragmentation
of the inflaton condensate into oscillons or transients after inflation, using
nonlinear classical lattice simulations. Without including the backreaction of
metric perturbations, we find that the magnitude of scalar metric perturbations
never exceeds a few $\times 10^{-3}$, whereas the maximal strength of the
gravitational wave signal today is $\mathcal{O}(10^{-9})$ for standard
post-inflationary expansion histories. We provide parameter scalings for the
$\alpha$-attractor models of inflation, which can be easily applied to other
models. We also discuss the likelihood of primordial black hole formation, as
well as conditions under which the gravitational wave signal can be at
observationally interesting frequencies and amplitudes.
  Finally, we provide an upper bound on the frequency of the peak of the
gravitational wave signal, which applies to all preheating scenarios.
