Speaker
Description
Hawking radiation (HR) causes ultralight primordial black holes (PBHs) ($≤ 10^9g$) to
entirely evaporate and produce all of the particles in a given theory, regardless of their
other interactions. Successful baryogenesis via leptogenesis predicts the mass scale of RH
neutrinos as well as black holes if right-handed (RH) neutrinos are also created from PBH
evaporation. We show that a network of cosmic strings naturally give rise to a strong
stochastic gravitational-wave (GW) signal at the sensitivity level of pulsar timing arrays
(PTA) and LIGO5, given that the lepton number violation (generation of RH neutrino
masses) in the theory is a result of a gauged U(1) breaking followed by the formation of
PBHs. A break in the GW spectra occurs around MHz frequency due to a short period
of black hole dominance in the early universe, for which baryon asymmetry is independent
of initial PBH density. As a result, GW detectors with higher frequencies are required to
observe the break together with the regular GW signal caused by graviton emission via
HR. The NANOGrav PTA’s recent discovery of a stochastic common spectrum process
(interpreted as GWs) across a large number of pulsars conflicts with PBH baryogenesis for
large cosmic string loops ($α \simeq 0.1$).
