Decaying Cold Dark Matter Model and Small-Scale Power

The canonical cold dark matter (CDM) model with a cosmological constant (LCDM) apparently possesses too much small-scale ($l\sim 1-100$\kpc) power at z=0, manifested as over-concentration of dark matter in inner regions of galaxies and over-abundance of dwarf galaxies. We suggest an alternative, LCDM based model in which one half of the CDM particles have decayed into relativistic particles by z=0. This model, called LDCDM, 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 many successes of the LCDM model at high redshift. This model solves the problem of over-production of small dwarf galaxies in the LCDM by identifying them with failed, "dark" galaxies, where star-formation is quenched due to dark matter evaporation and consequently halo expansion. A COBE-and-cluster normalized LDCDM model can be constructed with the following parameters: $H_0=60$km/sec/Mpc, $\lambda_0=0.60$, $\Omega_{0,CDM}=0.234$, $\Omega_{0,b}=0.044$, $n=1.08$, and $\sigma_8=1.00$. A noted effect from CDM decay is a shift of the first Doppler peak in the Cosmic Microwave Background angular power spectrum to a larger scale, since the non-relativistic matter density parameter is higher at high redshift ($\Omega_{i,NR}=0.512$). With the adopted parameters the model produces the location of the first Doppler peak in accord with the recent BOOMERANG result. Some other features of the model are also discussed. 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.