Decaying Cold Dark Matter Model and Small-Scale Power

The canonical cosmological constant-dominated cold dark matter model (ΛCDM) may possess too much power on small scales at z=0, manifested as central overconcentration of dark matter and overabundance of dwarf galaxies. We suggest an alternative model, ΛDCDM, where one-half of the cold dark matter particles decay into relativistic particles by z=0. The model successfully lowers the concentration of dark matter in dwarf galaxies as well as in large galaxies like our own at low redshift while simultaneously retaining the virtues of the ΛCDM model. The model solves the problem of overproduction of small dwarf galaxies in the ΛCDM, not by removing them but by identifying them with failed, ``dark'' galaxies, where star formation is quenched as a result of dark matter evaporation and consequent halo expansion. A dramatic difference between the ΛDCDM model and other proposed variants of the ΛCDM model is that the small-scale power at high redshift (z>2) in the ΛDCDM model is enhanced compared to the ΛCDM model. A COBE- and cluster-normalized ΛDCDM model can be constructed with the following parameters: H₀=60 km s^-1 Mpc^-1, λ₀=0.60, Ω_0,CDM=0.234, Ω_0,b=0.044, n=1.0, and σ₈=1.06. A clean test of this model can be made by measuring the evolution of the gas fraction in clusters. The prediction is that the gas fraction should decrease with redshift and is smaller by 31% at z=1 than at z=0. X-ray and Sunyaev-Zeldovich effect observations should provide such a test.