WIMP matter power spectra and small scale power generation

Dark Matter (DM) is generally assumed to be massive, cold and collisionless from the structure formation point of view. Although this translates into a scale-free powerlaw matter power spectrum, a more correct statement is that DM indeed experiences collisional damping, but on a scale which is supposed to be too small to be relevant for structure formation. The aim of this paper is to present a Cold (although “collisional”) Dark Matter candidate whose matter power spectrum is damped, discuss its consequences on structure formation and see whether it is distinguishable from standard candidates. To achieve this purpose, we first calculate the collisional damping and free-streaming scales of conventional (namely neutralinos) and non conventional DM candidates (say light particles heavier than ∼ 1 MeV but lighter than O(10) GeV). The latter are supposed to annihilate to get the correct relic density and can be considered as Cold Dark Matter (CDM) particles in the sense that they become non relativistic before their thermal decoupling epoch. Unlike neutralinos, however, the linear matter power spectrum of light DM candidates can be damped on scales of ∼ 103M⊙ (due to their interactions). Since these scales are of cosmological interest for structure formation, we then perform a series of numerical simulations to obtain the corresponding non linear matter power spectra P (k)nl at the present epoch. We show that because of small scale regeneration, they all resemble each other at low redshifts, i.e. become very similar to a typical CDM matter power spectrum on all but the smallest scales. Therefore, even if lensing measurements (at redshift below unity) were to yield a P (k)nl consistent with CDM models, this would not constitute a sufficiently robust evidence in favour of the neutralino to rule out alternative DM candidates, even if their linear matter power spectrum is damped on relatively large scales.