The $H_0$ tension alleviated through ultra-light primordial black holes: an information insight through gravitational waves

Abstract: The Hawking evaporation of ultra-light primordial black holes (PBH) dominating the early Universe before Big Bang Nucleosynthesis can potentially increase the effective number of extra neutrino species $\Delta N_\mathrm{eff}$ through the emission of dark radiation degrees of freedom alleviating in this way the $H_0$ tension problem. Interestingly, these light PBHs can form a gas of Poisson distributed compact objects which can induce a gravitational-wave (GW) background due to second order gravitational interactions. Therefore, by considering the contribution to $\Delta N_\mathrm{eff}$ due to the production of the aforementioned GW background we revisit in this work the constraints on the relevant parameters at hand, namely the PBH mass, $m_\mathrm{PBH}$, the initial PBH abundance at PBH formation time, $\Omega_\mathrm{PBH,f}$ and the number of DR radiation degrees of freedom, $g_\mathrm{DR}$ by accounting at the same time for the relevant upper bounds constraints on $\Delta N_\mathrm{eff}$ from the Planck collaboration.