  We use a combination of three large N-body simulations to investigate the
dependence of dark matter halo concentrations on halo mass and redshift in the
WMAP year 5 cosmology. The median relation between concentration and mass is
adequately described by a power-law for halo masses in the range 10^11 - 10^15
Msol/h and redshifts z < 2, regardless of whether the halo density profiles are
fit using NFW or Einasto profiles. Compared with recent analyses of the
Millennium Simulation, which uses a value of sigma_8 that is higher than
allowed by WMAP5, z = 0 halo concentrations are reduced by factors ranging from
23 per cent at 10^11 Msol/h to 16 per cent at 10^14 Msol/h. The predicted
concentrations are much lower than inferred from X-ray observations of groups
and clusters.
