Theoretical possibilities and observational constraints for radiatively decaying neutrinos with a mass near 30 eV.

Abstract

Massive relic neutrinos may contribute significantly to the density of the Universe, and if appropriately concentrated, could explain the characteristics of luminous matter in galaxies. As pointed out by Sciama, the radiative decay of 30 eV neutrinos could explain several observational puzzles including the anomalous degree of ionization of interstellar matter within the Milky Way Galaxy. Although these neutrinos are impossible in the standard particle physics model we show that various nonstandard particle models with an extended scalar sector or minimal supersymmetry have sufficient freedom to accommodate this scenario. We derive observational constraints on these neutrinos in the immediate solar neighborhood, in nearby regions of low interstellar absorption, in the Galactic halo, in clusters of galaxies, and in extragalactic space. Contrary to often-expressed opinions, present observational constraints do not rule out the Sciama scenario; they only rule out specific distributions of radiatively decaying neutrinos in clusters of galaxies.