Primordial black holes (PBHs) serve as powerful probes of the early Universe, offering unique insights into primordial physics and small-scale perturbations beyond the reach of other observational tools. In this talk, I will illustrate this idea using the example of ultra-light PBHs (M_PBH<10^9 g). Such PBHs can transiently dominate the Universe's energy density and reheat it through evaporation before Big Bang Nucleosynthesis, while their clustering properties and associated isocurvature fluctuations can induce gravitational waves (GWs) via second-order effects. I will discuss how primordial non-Gaussianity influences PBH clustering and
leads to distinctive features in the GW spectrum, including a double-peaked signal whose amplitude depends on the non-Gaussian parameter \tau_NL. This signal, detectable by future GW observatories such as LISA, ET, SKA, and BBO, provides a novel avenue for probing small-scale primordial physics, including non-Gaussianity and the early Universe's microphysical processes. By combining these GW signatures with constraints from Big Bang Nucleosynthesis, we can further refine our understanding of primordial fluctuations and their impact on PBH formation and clustering.
Organized by: Catarina Cosme