Constraints on primordial black holes from the Galactic gamma-ray background

Abstract

The fraction of the Universe going into primordial black holes (PBHs) with initial mass M * ≈ 5 × 10 14 g , such that they are evaporating at the present epoch, is strongly constrained by observations of both the extragalactic and Galactic gamma-ray backgrounds. However, while the dominant contribution to the extragalactic background comes from the time-integrated emission of PBHs with initial mass M * , the Galactic background is dominated by the instantaneous emission of those with initial mass slightly larger than M * and current mass below M * . Also, the instantaneous emission of PBHs smaller than 0.4 M * mostly comprises secondary particles produced by the decay of directly emitted quark and gluon jets. These points were missed in the earlier analysis by Lehoucq et al. using EGRET data. For a monochromatic PBH mass function, with initial mass (1 + µ) M * and µ ≪ 1, the current mass is (3µ) 1/3 M * and the Galactic background constrains the fraction of the Universe going into PBHs as a function of µ . However, the initial mass function cannot be precisely monochromatic and even a tiny spread of mass around M * would generate a current lowmass tail of PBHs below M * . This tail would be the main contributor to the Galactic background, so we consider its form and the associated constraints for a variety of scenarios with both extended and nearly-monochromatic initial mass functions. In particular, we consider a scenario in which the PBHs form from critical collapse and have a mass function which peaks well above M * . In this case, the largest PBHs could provide the dark matter without the M * ones exceeding the gamma-ray background limits.

Primordial Black Holes as a Dark Matter Candidate Are Severely Constrained by the Galactic Center 511 keVγ-Ray Line

“…These bracket the uncertainty due to the dark matter profile and positron propagation. The other constraints shown in the paper are from the observations of Voyager 1 [7], cosmic microwave background [20], and gamma-rays [21]. The constraints from the Galactic gamma-ray background and the cosmic microwave background is relatively robust with respect to the underlying parameters.…”

“…While deriving the limit for the "3 kpc" ("1.5 kpc") constraint, we assume that low-energy positrons can travel about a kpc (100 pc) before annihilating. The upper limits from Voyager 1 (two lines denote the propagation and background uncertainties) [7], Planck [20], and gamma-ray observatories [21] are shown by red dotted lines.…”

Primordial black hole formation in inflationary α -attractor models

“…It is because of this that PBHs have become an attractive alternative for a dark matter candidate [20,21]. If they exist, they are likely to have been formed before the Big Bang Nucleosynthesis (BBN) and escape the restrictions imposed on baryonic dark matter [22]. They are also collisionless and non-relativistic -properties which are characteristic of cold dark matter (CDM).…”

Evaporating primordial black holes as varying dark energy

“…In such a case, the radiation, however, is not dark since it is composed of relativistic standard model particles and as such it is subjected to constraints. The photons and charged leptons emitted by sufficiently low mass BHs are constrained from γ-and cosmic-ray observations [15,16], while neutrino emission has been studied in [17]. The lowest possible mass for which DM can be entirely due to PBHs is the so-called asteroid window M PBH 2 × 10 16 g [18], however, the concensus on the constraints is not fully settled [19].…”