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 E c and as CDM below this scale. Evolution of structure formation is similar to that of CDM model but BDM predicts a nonvanishing free streaming λ fs scale and a inner galaxy core radius r core , both quantities determined in terms of a single parameter E c , which corresponds to the phase transition energy scale of the subjacent elementary particle model. For energies above E c or for a scale factor a smaller then a c , with a < a c < a eq , the particles are massless and ρ redshifts as radiation. However, once the energy becomes E ⩽ E c or a > a c 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 E c = O ( 10 – 100 ) eV giving a λ fs ∝ E c - 4 / 3 ≲ Mpc and M fs ∝ E c - 4 ≲ 10 9 M ⊙ . A λ fs ≠ 0 , r core ≠ 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 E c can be derived from particle physics.