New constraints on the primordial black hole number density from Galactic γ-ray astronomy

Abstract

Context. Primordial black holes are unique probes of cosmology, general relativity, quantum gravity and non standard particle physics. They open a new window on the very small scales in the early Universe and also can be considered as the ultimate particle accelerator in their last (explosive) moments since they are supposed to reach, very briefly, the Planck temperature. Aims. Upper limits on the primordial black hole number density of mass M = 5 × 10 14 g, the Hawking mass (born in the big-bang terminating their life presently), is determined comparing their predicted cumulative γ-ray emission, galaxy-wise, to the one observed by the EGRET satellite, once corrected for non thermal γ-ray background emission induced by cosmic ray protons and electrons interacting with light and matter in the Milky Way. Methods. A model with free gas emissivities is used to map the Galaxy in the 100 MeV photon range, where the peak of the primordial black hole emission is expected. The best gas emissivities and additional model parameters are obtained by fitting the EGRET data and are used to derive the maximum emission of the primordial black hole of the Hawking mass, assuming that they are distributed like the dark matter in the Galactic halo.Results. The bounds we obtain, depending on the dark matter distribution, extrapolated to the whole Universe (Ω PBH (M ) = 2.4×10 −10 to 2.6 × 10 −9 ) are more stringent than the previous ones derived from extragalactic γ-ray background and antiprotons fluxes, though less model dependent and based on more robust data. Conclusions. These new limits have interesting consequences on the theory of the formation of small structures in the Universe, since they are the only constraint on very small scale density fluctuations left by inflation. Significant improvements by data gathered by the FERMI γ-ray satellite are expected in the near future. The interest of a generalisation of this work beyond the standard particle model and in extradimensions is briefly alluded.

New cosmological constraints on primordial black holes

“…PBHs with M ∼ 10 15 g. As already noted, these would be evaporating today and, since they are dynamically cold, one would expect some of them to have clustered within the Galactic halo. Besides contributing to the cosmological γ-ray background, an effect we reassess in this paper, such PBHs could contribute to the Galactic γ-ray background [106,107] and the antiprotons or positrons in cosmic rays [88,108,109]. They might also generate gamma-ray bursts [110], radio bursts [111] and the annihilation-line radiation coming from center of the Galaxy [112,113].…”

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

“…Constraints on the PBH scenario from EGRET observations of the Galactic γ-ray background were first studied by Wright [3] and Lehoucq et al [6]. Wright derived limits on the PBH clustering factor and explosion rate, while Lehoucq et al limited the PBH collapse fraction and density parameter.…”

Primordial Black Holes and Quantum Effects

“…Some time ago it was claimed that such a background had been detected by EGRET between 30 MeV and 120 GeV and that this could be attributed to PBHs [13]. A more recent analysis of EGRET data between 70 MeV and 150 GeV gives a limit Ω PBH (M * ) ≤ 2.6 × 10 −9 or β (M * ) < 1.4 × 10 −26 [14], which is a factor of 5 above the EGB constraint. However, CKSY point out that the EGB constraint on β (M) comes from the timeintegrated contribution of the M * black holes, which peaks at 120 MeV, whereas the Galactic background is dominated by PBHs which are slightly larger than this.…”