Decaying warm dark matter and structure formation
  We examine the cosmology of warm dark matter (WDM), both stable and decaying,
from the point of view of structure formation. We compare the matter power
spectrum associated to WDM masses of 1.5 keV and 0.158 keV, with that expected
for the stable cold dark matter $\Lambda$CDM$\equiv$SCDM paradigm, taken as our
reference model. We scrutinize the effects associated to the warm nature of
dark matter, as well as the fact that it decays. The decaying warm dark matter
(DWDM) scenario is well-motivated, emerging in a broad class of particle
physics theories where neutrino masses arise from the spontaneous breaking of a
continuous global lepton number symmetry. The majoron arises as a
Nambu-Goldstone boson, and picks up a mass from gravitational effects, that
explicitly violate global symmetries. The majoron necessarily decays to
neutrinos, with an amplitude proportional to their tiny mass, which typically
gives it cosmologically long lifetimes. Using N-body simulations we show that
our DWDM picture leads to a viable alternative to the $\Lambda$CDM scenario,
with predictions that can differ substantially on small scales.
