Spectral shifts between massive and massless particles from cosmic sources

Abstract

Stable neutral particles with non-zero mass, e.g. massive neutrinos, axions etc., may be produced in the vicinity of some powerful sources that are cosmological distance indicators, e.g. active galactic nuclei. Their spectral features will be altered due to the expansion of the Universe and possible beaming mechanisms. We use the geodesic equation of the Robertson-Walker metric and the Liouville theorem to describe the spectral shifts relative to massless particles, e.g. photons, due to these two effects. If these neutral particles and high energy photons are produced in a similar fashion, some vital information (e.g. distance to the source, mass of the neutral particles, velocity of the emitting source etc.) can be obtained by comparing the spectral shift between the massive particles and massless photons.