ABSTRACT
Experiments indicate that neutrinos possess non-zero rest mass and make up at least part of the dark matter. If different neutrino species have different rest masses, then heavier species can decay into lighter ones plus a photon. The neutrino number density, moreover, is high enough that their decay photons might be observable. This chapter discusses the testing of the decaying-neutrino hypothesis in a self-consistent way by adopting a narrow range of values. Neutrinos with rest masses and decay lifetimes as specified by the decaying-neutrino scenario produce levels of ultraviolet background radiation very close to and, in several cases, above experimental upper limits on the intensity of the extragalactic background light. While there is good experimental evidence that some of the dark matter is provided by massive neutrinos, the light of the night sky tells us that these particles cannot have the rest masses and decay lifetimes attributed to them in the decaying-neutrino hypothesis.
