Probing quantum gravity at low energies
  Planck stars form when a collapsing shell of matter within a black hole
reaches the Planck density, roughly equivalent to the mass being compressed
into a volumetric size near that of the proton, and rebounds outwards. These
planck stars have been considered as accounting for both fast radio bursts and
short gamma ray bursts, whilst offering a comparatively low energy perspective
onto quantum gravity. The observation of such an event would require black hole
masses much smaller than a solar mass, which could be provided by primordial
black hole dark matter models. We discuss the low energy isotropic background
emissions produced by decaying primordial black holes at all epochs and derive
constraints from the spectrum of the extragalactic background light. We find
that, in order to avoid exceeding known extragalactic background light
emissions, we must restrict the total energy emitted at low frequencies by a
planck star exploding in the present epoch to be less than $10^{13}$ erg or
restrict the primordial black hole population far below any existing limits.
This casts doubt on whether exploding planck stars could actually account for
fast radio bursts, as they are speculated to in the literature.
