Dark radiation constraints on light primordial black holes

TL;DR

This paper investigates how light primordial black holes, through their Hawking radiation, could have contributed to dark radiation, and how current and future cosmological observations constrain their properties.

Contribution

It provides new constraints on light primordial black holes based on their emission of gravitons and the resulting dark radiation impact on cosmological epochs.

Findings

Dark graviton emission constrains PBH abundance before BBN.

Future CMB Stage 4 observations will tighten these constraints.

Light PBHs cannot constitute a significant fraction of dark matter.

Abstract

Light black holes could have formed in the very early universe through the collapse of large primordial density fluctuations. These primordial black holes (PBHs), if light enough, would have evaporated by now because of the emission of Hawking radiation; thus they could not represent a sizable fraction of dark matter today. However, they could have left imprints in the early cosmological epochs. We will discuss the impact of massless graviton emission by (rotating) PBHs before the onset of big bang nucleosynthesis (BBN) and conclude that this contribution to dark radiation is constrained by the cosmic microwave background (CMB) (with the future CMB Stage 4) and BBN in the lighter portion of the PBH mass range, under the hypothesis that they dominated the energy density of the universe.