Gravitational wave spectra from oscillon formation after inflation
  We systematically investigate the preheating behavior of single field
inflation with an oscillon-supporting potential. We compute the properties of
the emitted gravitational waves (GWs) and the number density and
characteristics of the produced oscillons. By performing numerical simulations
for a variety of potential types, we divide the analyzed potentials in two
families, each of them containing potentials with varying large- or small-field
dependence. We find that the shape and amplitude of the emitted GW spectrum
have a universal feature, with the peak around the physical wavenumber $k/a
\sim m$ at the inflaton oscillation period, irrespective of the exact potential
shape. This can be used as a smoking-gun for deducing the existence of a
violent preheating phase and possible oscillon formation after inflation.
Despite this apparent universality, we find differences in the shape of the
emitted GW spectra between the two potential families, leading to
discriminating features between them. In particular, all potentials show the
emergence of a two-peak structure in the GW spectrum, arising at the time of
oscillon formation. However, potentials exhibiting efficient parametric
resonance tend to smear out this structure and by the end of the simulation the
GW spectrum exhibits a single broad peak. We further compute the properties of
the produced oscillons for each potential, finding differences in the number
density and size distribution of stable oscillons and transient overdensities.
We perform a linear fluctuation analysis and use Floquet charts to relate the
results of our simulations to the structure of parametric resonance. We find
that the growth rate of scalar perturbations and the associated oscillon
formation time are sensitive to the small-field potential shape while the
macroscopic physical properties of oscillons (e.g. total number) depend on the
large-field potential shape.
