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Many experimental evidences indicate the presence of a ionizing backgroundradiation flux at large redshifts; in some previous works I suggested thepossibility that this ionizing flux comes from the quantum evaporation ofprimordial black holes (PBHs). Here I discuss the constraints that theexperimental measurements put upon the free parameters of this reionizationmodel and I study the absorption of the ionizing background due to $\lia$clouds: in particular, I discuss this phenomenon in presence of differentabsorption levels and I calculate the HI Gunn Peterson optical depth$\tau_{GP}(z)$; from a comparison with the experimental data I obtain aconstraint 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.Comment: Plain Tex, 36 pages. Accepted for Pub. in Int. Jour. of Mod. Phys.

The Gunn–Peterson Effect: A Test for a Black-Hole-Induced Photoionization of the Intergalactic Medium