Abstract: This thesis investigates primordial black holes and their observational consequences. First, we examine the behavior of domain walls produced during inflation, and their subsequent collapse, leading to the formation of PBHs. The repulsive gravitational field of large spherical domain walls drives gravitational collapse with an unusual spacetime structure, which leads to the formation of a baby universe connected to the parent universe by a wormhole. In this cosmological setup we study the energy-momentum tensor of a quantum field to elucidate the behaviour of Hawking radiation in this peculiar geometry. We find that the energy density, pressure and fluxes proceed smoothly and without pathologies thoughout the entire evolution.

In the second part of the thesis, we examine the observational consequences of primordial black holes produced by generic mechanisms, such as large curvature perturbations. More precisely, we investigate the merger rate of binaries and its impact on the stochastic gravitational wave background. Using a phenomenological model, we constrain the merger rate to redshifts around z ∼ 4 based on data from the LIGO/Virgo-O3 run. We discover distinctive features in the gravitational wave signal at intermediate frequencies for steep merger rates. Our findings have implications for future experiments such as the Einstein Telescope, LISA, and PTA, covering a wide frequency range. Additionally, we highlight the importance of direct observations of the merger rate to break degeneracies in the stochastic background, and provide constraints on the abundance of primordial black holes.