On the Density of Primordial Black Holes in the Galactic Halo

Calculations of the rate of local primordial black hole (PBH) explosions often assume that the PBHs can be highly concentrated into galaxies, thereby weakening the Page-Hawking limit on the cosmological density of PBHs. But if the PBHs are concentrated by a factor exceeding c/(H_{0 }R₀) 4 x 10 , where R_{0 }= 8.5 kpc is the scale of the Milky Way, then the steady emission from the PBHs in the halo will produce an anisotropic high-latitude diffuse gamma-ray intensity larger than the observed anisotropy. This provides a limit on the rate density of evaporating PBHs of 0.4 pc -3 yr -1, which is more than 6 orders of magnitude lower than recent experimental limits. However, the weak observed anisotropic high-latitude diffuse gamma-ray intensity is consistent with the idea that the dark matter that closes the universe is Planck mass remnants of evaporated black holes.