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 ($\phi \ne \overline{\phi }$) and have a temperature like that of neutrinos, whereas models with $\phi =\overline{\phi }$ tend to overproduce small-scale structure.