Decaying Cold Dark Matter Model and Small-Scale Power

The canonical cosmological constant dominated cold dark matter model (LCDM) may possess too much power on small scales at z=0, manifested as central over-concentration of dark matter and over-abundance of dwarf galaxies. We suggest an alternative model, LDCDM, 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 LCDM model. The model solves the problem of over-production of small dwarf galaxies in the LCDM not by removing them but by identifying them with failed, "dark" galaxies, where star-formation is quenched due to dark matter evaporation and consequent halo expansion. A dramatic difference between the LDCDM model and other proposed variants of the LCDM model is that the small-scale power at high redshift (z>2) in the LDCDM model is enhanced compared to the LCDM model. A COBE-and-cluster normalized LDCDM model can be constructed with the following parameters: H0=60km/sec/Mpc, lambda0=0.60, Omega_{0,CDM}=0.234, Omega_{0,b}=0.044, n=1.0, and sigma8=1.06. A clean test of this model can be made by measuring the evolution of 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-Zel'dovich effect observations should provide such a test.