Non-baryonic models

A fundamental property of non-baryonic dark matter is that it is not coupled to radiation, at least at the epochs relevant for the origin of the primordial fluctuation spectrum. For this reason, no dissipative Silk damping is expected. However, a non-dissipative damping of fluctuations occurs in any case, due to free-streaming of dark matter particles. In fact, until such particles are relativistic they are able to freely cross the horizon within the Hubble time, thus washing out all the fluctuations below the horizon scale. This effect stops when the temperature of the Universe drops below the mass of DM particles and they become non relativistic. The size of the horizon at this epoch fixes the smallest scale of the fluctuations surviving free-streaming damping. Thus, it is clear that a crucial parameter to establish the shape of the fluctuation spectrum in a DM model is the velocity of the constituent particles.

The importance of following the evolution of the DM spectrum lies in the fact that it determines after recombination the spectrum of fluctuations of ordinary baryonic matter, so as to provide the seeds where dissipative processes occur and galaxy formation takes place. In fact, soon after recombination the Jeans mass for the baryonic component drops to a very small value. As a consequence, baryonic fluctuations starts growing again by gravitational instability, until their amplitude matches that of the non-baryonic DM perturbations.