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The effects of ionizing radiation emitted by quasi-stellar objects on intergalactic hydrogen are studied. The hydrogen is assumed to expand with the Universe, and the amount of ionizing radiation is estimated from observations of the QSO luminosity function. The discreteness of the radiation sources is taken into account in computing the thermal history of the medium. The amount of ionizing radiation is shown to be sufficient to allow the existence of a universal medium with a temperature less than 10 K and a density several times the smeared-out density of luminous matter in galaxies. Such a medium would escape detection with presently available techniques.
Arons et al. (Sun,) studied this question.