Abstract
Decaying dark matter cosmological models have been proposed to remedy the overproduction problem at small scales in the standard cold dark matter paradigm. We consider a decaying dark matter model in which one CDM mother particle decays into two daughter particles, with arbitrary masses. A complete set of Boltzmann equations of dark matter particles is derived which is necessary to calculate the evolutions of their energy densities and their density perturbations. By comparing the expansion history of the universe in this model and the free-streaming scale of daughter particles with astronomical observational data, we give constraints on the lifetime of the mother particle, Γ−1, and the mass ratio between the daughter and the mother particles mD/mM. From the distance to the last scattering surface of the cosmic microwave background, we obtain Γ−1 > 30 Gyr in the massless limit of daughter particles and, on the other hand, we obtain mD > 0.97mM in the limit Γ−1 → 0. The free-streaming constraint tightens the bound on the mass ratio as (Γ−1/10−2Gyr)≲((1−mD1/mM)/10−2)−3/2 for Γ−1 < H−1(z = 3).