Finding Black Holes from the Big Bang: observable signatures of primordial black holes

Primordial black holes (PBHs)—black holes formed in the early universe by mechanisms other than stellar collapse—were first posited by Hawking and Carr in 1975. Fifty years later, as particle dark matter candidates continue to elude direct detection, PBHs have emerged as a compelling contender to explain all or some of the universe's missing dark matter. Unlike astrophysical black holes, PBHs are not subject to the Tolman-Oppenheimer-Volkov limit and could have formed at masses spanning over 40 orders of magnitude. While somewhat model-dependent constraints exist across much of this parameter space, the "asteroid mass range" between 10^17 and 10^22 grams remains largely unconstrained—a window where PBHs could still constitute all of the dark matter. With three decades of precision Solar System data now available, along with increasingly accurate astrophysical and cosmological measurements, we are poised to probe this mass range definitively. In this talk, I will discuss promising detection strategies with particular focus on a novel observable: orbital perturbations induced by transiting PBHs. Beyond their role as dark matter candidates, PBHs offer unique windows into early universe physics; having never been in equilibrium with the standard model bath, they preserve a snapshot of conditions at their formation, potentially revealing high-energy physics inaccessible to todays terrestrial experiments. Furthermore, through Hawking radiation, they may provide one of our few avenues for testing theories of quantum gravity. I will overview the most promising observational probes and outline the critical next steps—including improved modeling of PBH evolution and environments—needed to detect these elusive objects or exclude them entirely.