The recombination of neutral hydrogen in the early universe is reinvestigated taking into account light neutral fermions, stable or radiatively unstable. When these fermions are stable, their main effect is to increase the expansion rate of the universe, and to increase the fossilized ionization x e of matter. Big bang nucleo-synthesis provides density constraints on the baryonic components and if we assume that the universe is closed with light neutral fermions, we can set limits upon x e:4×10 −4< x e<2×10 − instead of the previous baryon-dominated universe result: 3×10 −5< x e<3×10 −4. If the light neutral fermions decay radiatively, the emitted photon is the UV-range and reionizes the neutral matter. We point out that matter can be completely reionized at a redshift Z∼100 for radiative lifetimes in the range 10 20–10 24 s. Supersymmetry provides us with such a light “ino”. The reaction photino→photon+gravitino exhibits the good relation between the photino lifetime and the ionizing photon energy.
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