Black holes as “time capsules”: A cosmological graviton background and the Hubble tension

Abstract

Minuscule primordial black holes (BHs) before the end and after inflation can serve as “time capsules” bringing back energy from the past to a later epoch when they evaporate. As these BHs behave like matter, while the rest of the universe's content behaves like radiation, the mass fraction of these BHs, which is tiny at formation, becomes significant later. If sufficiently small, these BHs will evaporate while the Universe is still radiation dominated. We revisit this process and point out that gravitons produced during evaporation behave as “dark radiation.” If the initial BHs are uniformly distributed so will the gravitons and in this case, they will be free of Silk damping and avoid current limits on “dark radiation” scenarios. Seeds for such BHs can arise during the last phases of inflation. We show here that with suitable parameters, this background graviton field can resolve the Hubble tension. We present current observational constraints on this scenario and suggest upcoming observational tests to prove or refute it. Finally, we also elaborate on the graviton background produced by particle annihilation during the Planck era or shortly after inflation.