The First Billion Years of a Warm Dark Matter Universe
  We present results of cosmological N-body hydrodynamic chemistry simulations
of primordial structure growth and evolution in a scenario with warm dark
matter (WDM) having a mass of 3 keV (thermal relic) and compare with a model
consisting of standard cold dark matter (CDM). We focus on the high-redshift
universe ($z>6$), where the structure formation process should better reflect
the primordial (linear) differences in terms of matter power spectrum. We find
that early epochs can be exceptional probes of the dark-matter nature.
Non-linear WDM power spectra and mass functions are up to 2 dex lower than in
CDM and show spreads of factor of a few persisting in the whole first Gyr.
Runaway molecular cooling in WDM haloes results severely inhibited because of
the damping of power at large $k$ modes and hence cosmic (population III and
II-I) star formation rate (SFR) is usually suppressed with respect to CDM
predictions. Luminous objects formed in a WDM background are very rare at
$z>10$, due to the sparser and retarded evolution of early WDM mini-haloes
during the dark ages and their lack can be fitted with a simple analytical
formula depending only on magnitude and redshift. Future high-$z$ observations
of faint galaxies have the potential to discriminate between CDM and WDM
scenarios by means of cosmic stellar mass density (SMD) and specific SFR, as
well. When compared to the effects of alternative cosmologies (e.g.
non-Gaussian or dark-energy models) or of high-order corrections at large $z$
(e.g. primordial streaming motions or changes in the pristine IMF) the ones
caused by WDM are definitely more dramatic.
