Resonant high energy graviton to photon conversion at post recombination
  epoch
  Resonant conversion of high energy gravitons into photons in large scale
cosmological magnetic fields at the post recombination epoch is considered. It
is shown that the probability of the resonance photon production is much higher
than the non-resonant one. As a result an observable isotropic background of
cosmic gamma rays might be created. As shown in our previous paper, an early
population of primordial black holes (PBHs) prior to big bang nucleosynthesis
(BBN) could be an efficient source of high frequency gravitational waves. For
the primordial black hole mass about $10^8$ g the produced photons would be the
dominant component of the soft to hard Cosmic X-ray Background (CXB) and for
lower masses the spectrum is shifted down to the ultraviolet and optic.
