“Stochastic gravitational wave backgrounds from primordial black holes”
The first detection of gravitational waves by LIGO in 2015 opened a new window of observation to the Universe and gave rise to a wide variety of research avenues. In this seminar, I will introduce gravitational waves with a focus on an intriguing signal we anticipate detecting in the near future: the stochastic gravitational wave background. I will explain how this signal is usually constrained using standard gravitational-wave detectors (LIGO, Virgo and KAGRA) and explore how astrometry can provide complementary constraints at low frequencies (sub-nHz), illustrating this approach with data from the Gaia mission. Next, I will present a class of astrophysical events that can contribute to this background: hyperbolic interactions of black holes, where I will discuss the possibility that some of them may have a primordial origin. Finally, we will see how these hyperbolic encounters can significantly modify the spins of the interacting black holes, discussing the relevance of such effects.