New cosmological constraints on primordial black holes

TL;DR

<3-5 sentence high-level summary> This paper provides a comprehensive reassessment of primordial black hole constraints across a broad mass range, with a focus on 10^9–10^17 g where big bang nucleosynthesis and the extragalactic gamma-ray background impose the strongest limits. It advances the field by incorporating quark/gluon emission and QCD fragmentation in PBH spectra, updating observational inputs, and deriving a unified β'(M) diagram that consolidates BBN, EGB, and secondary constraints. The work also reviews constraints on nonevaporating PBHs and Planck-mass relics, and discusses potential 21 cm and CMB-based probes, highlighting PBHs as a sensitive test of early-Universe physics and high-energy processes. Overall, the results show that PBHs must constitute a tiny fraction of the Universe over most mass ranges, with a few narrow windows where degeneracies or model dependencies permit larger abundances.</paper_summary>

Abstract

We update the constraints on the fraction of the Universe going into primordial black holes in the mass range 10^9--10^17 g associated with the effects of their evaporations on big bang nucleosynthesis and the extragalactic photon background. We include for the first time all the effects of quark and gluon emission by black holes on these constraints and account for the latest observational developments. We then discuss the other constraints in this mass range and show that these are weaker than the nucleosynthesis and photon background limits, apart from a small range 10^13--10^14 g, where the damping of cosmic microwave background anisotropies dominates. Finally we review the gravitational and astrophysical effects of nonevaporating primordial black holes, updating constraints over the broader mass range 1--10^50 g.

Figures (10)

• Figure 1: Instantaneous emission rate of photons for four typical black hole temperatures. For each temperature, the curve with the peak to the right (left) represents the primary (secondary) component and the thick curve denotes their sum.
• Figure 2: Ratios of the time-integrated secondary to primary peak energies (solid red) and fluxes (dashed green).
• Figure 3: Upper bounds on $\beta'(M)$ from BBN, with the broken lines giving the earlier limits in Ref. Kohri:1999ex.
• Figure 4: Fluxes corresponding to the upper limit on the PBH abundance for various values of $M$ . All PBHs produce primary photons but $M \lesssim M_*$ ones also produce secondary photons and this gives a stronger constraint on $\beta$ .
• Figure 5: Upper bounds on $\beta'(M)$ from the extragalactic photon background; these are conservative in the sense that no other contributors to the background have been subtracted.