Decaying cold dark matter model and small-scale power

The canonical cosmological constant-dominated cold dark matter model (Lambd a CDM) may possess too much power on small scales at z=0, manifested as cen tral overconcentration of dark matter and overabundance of dwarf galaxies. We suggest an alternative model, Lambda DCDM, where one-half of the cold da rk matter particles decay into relativistic particles by z=0. The model suc cessfully lowers the concentration of dark matter in dwarf galaxies as well as in large galaxies like our own at low redshift while simultaneously ret aining the virtues of the Lambda CDM model. The model solves the problem of overproduction of small dwarf galaxies in the Lambda CDM, not by removing them but by identifying them with failed, "dark" galaxies, where star forma tion is quenched as a result of dark matter evaporation and consequent halo expansion. A dramatic difference between the Lambda DCDM model and other p roposed variants of the LCDM model is that the small-scale power at high re dshift (z>2) in the Lambda DCDM model is enhanced compared to the LCDM mode l. A COBE- and cluster-normalized Lambda DCDM model can be constructed with the following parameters: H-o = 60 km s(-1) Mpc(-1), lambda (o) = 0.60, Om ega (0,CDM) = 0.234, Omega (0,b) = 0.044 n = 1.0, and sigma (8) = 1.06. A c lean test of this model can be made by measuring the evolution of the gas f raction in clusters. The prediction is that the gas fraction should decreas e with redshift and is smaller by 31% at z = 1 than at z=0. X-ray and Sunya ev-Zeldovich effect observations should provide such a test.