Speaker
Description
In the standard formation picture, primordial black holes (PBHs) form in the radiation-dominated early Universe from the collapse of enhanced curvature perturbations. However, the reheating period connecting inflation to the radiation epoch is poorly constrained, as is the inflaton potential below CMB scales. Observational constraints are therefore consistent with the Universe having undergone an early matter-dominated (eMD) epoch driven by ultra-light, evaporating PBHs or oscillons -- long-lived quasi-solitonic objects formed from the fragmentation of the inflaton condensate.
In both cases, the sudden eMD-to-radiation transition strongly enhances scalar-induced gravitational waves (SIGWs) via the Poltergeist mechanism, making SIGWs a powerful probe of this epoch. In this talk we discuss the parallels between the PBH and oscillon scenarios in this context, and then focus on the latter to show how the observational bound on the effective number of relativistic species, $\Delta N_{\text{eff}}$, applied to the induced GW background sets novel constraints on the inflaton potential in regions of parameter space inaccessible to CMB measurements of the scalar power spectrum alone.