The Gunn Peterson effect: a test for a black holes induced   photoionization of the intergalactic medium
  Many experimental evidences indicate the presence of a ionizing background radiation flux at large redshifts; in some previous works I suggested the possibility that this ionizing flux comes from the quantum evaporation of primordial black holes (PBHs). Here I discuss the constraints that the experimental measurements put upon the free parameters of this reionization model and I study the absorption of the ionizing background due to $\lia$ clouds: in particular, I discuss this phenomenon in presence of different absorption levels and I calculate the HI Gunn Peterson optical depth $\tau_{GP}(z)$; from a comparison with the experimental data I obtain a constraint on the intergalactic medium density parameter, namely $\Omega_{IGM}$ $<0.020$. A study of the characteristics of the absorbers is also performed; finally, the same kind of analysis is repeated for He II. 
Marina Gibilisco (Queen Mary and Westfield College, London,   Universit\'a degli Studi di Milano and Universit\'a degli Stud...