Black Holes as "Time Capsules": A Cosmological Graviton Background and the Hubble Tension

TL;DR

This paper proposes that primordial black holes and their graviton emissions could act as dark radiation, potentially resolving the Hubble tension, and discusses observational constraints and future tests for this scenario.

Contribution

It introduces a novel black hole graviton background model as a solution to the Hubble tension, linking early universe black holes to dark radiation.

Findings

Black hole evaporation produces gravitons as dark radiation.

The graviton background can be tuned to resolve the Hubble tension.

Observational constraints limit the parameter space of the model.

Abstract

Minuscule primordial black holes 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 black holes behave like matter, while the rest of the Universe content behaves like radiation, the mass fraction of these black holes, that is tiny at formation, becomes significant later. If sufficiently small, these black holes will evaporate while the Universe is still radiation dominated. We revisit this process and point out that gravitons produced during the evaporation behave as ``dark radiation". If the initial black holes are uniformly distributed so will be the gravitons and in this case they will be free of Silk damping and avoid current limits on "dark radiation" scenarios. Seeds for such black holes 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.