BDM dark matter: CDM with a core profile and a free streaming scale

We present a new dark matter model BDM which is an hybrid between hot dark matter HDM and cold dark matter CDM, in which the BDM particles behave as HDM above the energy scale Ec and as CDM below this scale. Evolution of structure formation is similar to that of CDM model but BDM predicts a nonvanishing free streaming λfs scale and a inner galaxy core radius rcore, both quantities determined in terms of a single parameter Ec, which corresponds to the phase transition energy scale of the subjacent elementary particle model. For energies above Ec or for a scale factor a smaller then ac, with aac then the BDM particles acquire a large mass through a non-perturbative mechanism, as baryons do, and ρ redshifts as matter with the particles having a vanishing velocity. Typical energies are Ec=O(10-100)eV giving a λfs∝Ec-4/3≲Mpc and Mfs∝Ec-4≲109M⊙. A λfs≠0,rcore≠0 help to resolve some of the shortcomings of CDM such as over-abundance substructure in CDM halos and numerical fit to rotation curves in dwarf spheroidal and LSB galaxies. Finally, our BDM model and the phase transition scale Ec can be derived from particle physics.