Large-Scale Structure Tests of Warm Dark Matter

The nature of the dark matter critically affects the large-scale structure of the universe. Under the assumptions that the universe is spatially flat with zero cosmological constant and that primordial perturbations were adiabatic with a Harrison-Zeldovich spectrum, neither hot dark matter (HDM) nor cold dark matter (CDM) appears consistent with the observed large-scale structure. Warm dark matter (WDM) is an intriguing alternative from the point of view of both cosmology and particle physics. We consider a one-parameter family of WDM models. The linear power spectra for these models is calculated and compared with the corresponding spectra for CDM, HDM, and mixed dark matter (MDM), as well as the power spectrum derived from observations. Our linear analyses suggest that a model universe dominated by a particle whose mass-to-temperature ratio mx/Tx is increased by a factor of 2 as compared with the standard HDM neutrino gives a reasonable fit to the data on large (>8 h-1 Mpc) scales. N-body simulations for this particular WDM model show features of both HDM and CDM. As in HDM, the first objects to collapse are large pancake-like structures. The final matter distribution is rather smooth, and structures as small as galaxy halos are excluded. However, there appear to be virialized rich clusters evident in the CDM but not in the HDM simulations. Unfortunately, a simple comparison of the matter distribution and its statistical properties with observations indicates that WDM, like CDM, has too much power at small scales. This is particularly evident in the small-scale pairwise velocity dispersion. The cluster multiplicity function has the wrong shape, with too many rich clusters being produced, although this conclusion is based on the simple assumption that light traces mass in groups of galaxies.