Primordial black hole ringdown: The irreducible stochastic gravitational wave background

Independently from the formation mechanism of primordial black holes in the early Universe, their generation is accompanied by a ringdown phase during which they relax to a stationary configuration and gravitational waves under the form of quasinormal modes are emitted. Such gravitational waves generate an irreducible and unavoidable stochastic background which is testable by current and future experiments. In particular, for primordial black holes with masses exceeding ${10}^{14}{M}_{\odot }$, the associated stochastic background lies within the frequency range accessible to current and upcoming cosmic microwave background experiments, thereby providing a direct observational way to probe the existence of such extremely massive objects.