Growth of perturbations between horizon crossing and matter dominance: implications for galaxy formation

The dark matter (DM) that appears to be gravitationally dominant on all astronomical scales larger than the cores of galaxies can be classified, on the basis of its characteristic free-streaming damping mass Msub(D), as hot, warm, or cold. For the case of cold DM, the shape of the DM fluctuation spectrum is determined by (a) the primordial spectrum (on scales larger than the horizon), and (b) 'stagspansion', the stagnation of growth of DM fluctuations which enter the horizon while the universe is still radiation-dominated. Stagspansion flattens the fluctuation spectrum. The authors report the results of a numerical evaluation of the fluctuation spectrum in which all relevant physical effects have been included, together with implications for galaxy formation. (Auth.)