Mixed dark matter with low-mass bosons
  We calculate the linear power spectrum for a range of mixed dark matter (MDM) models assuming a massive (few eV) boson, $\phi$, instead of a neutrino as the hot component. We consider both the case where the hot dark matter (HDM) particle is a boson and the cold component is some other unknown particle, and the case where there is only one dark matter particle, a boson, with the cold dark matter (CDM) component in a Bose condensate. Models resembling the latter type could arise from neutrino decays - we discuss some variants of this idea. The power spectra for MDM models with massive bosons are almost identical to neutrino MDM models for a given mass fraction of HDM if the bosons are distinct from their antiparticles ($\phi\neq\bar\phi$) and have a temperature like that of neutrinos, whereas models with $\phi=\bar\phi$ tend to overproduce small-scale structure. 
Georg B. Larsen and Jes Madsen