Late Evolution of Adiabatic Fluctuations

Abstract

We classify massive stable collisionless relics of the Big Bang into three categories of dark matter: hot, with damping mass about supercluster scale; warm, with damping mass of galactic or cluster scale; and cold, with negligible damping. The first objects that form in universes dominated by hot and warm relics are pancakes. Coupled one-dimensional N-body and Eulerian hydrodynamical simulations follow the nonlinear evolution of pancakes, the separation of baryons from dark matter via shock formation and the evolution of the shocked gas by conduction as well as by cooling. Only ~10−20% of the gas cools sufficiently to fragment on sub-galactic scales in neutrino-dominated hot theories. Cooling is efficient for warm relics. In all cases, the typical fragment size is ^~10⁹−10¹⁰ M_O. Electrons in the hot gas created by the pancake shocks can upscatter photons in the microwave background radiation, causing spectral distortions. Angular differences in these distortions lead to temperature fluctuations which are on the edge of observability, and can be used as a test of the pancake scenario.