  The concentrations of the cuspy dark matter halos predicted by simulations of
cold dark matter are related to the cosmology in which the halos form.
Observational constraints on halo concentration therefore map into constraints
on cosmological parameters. In order to explain the observed concentrations of
dark matter dominated low surface brightness galaxies, we require a cosmology
with rather little power on galaxy scales. Formally, we require s_8 G_0.6 <
0.23, where G_0.6 is a modified shape parameter appropriate to this problem.
Practically, this means either Omega_m < 0.2 or s_8 < 0.8. These limits apply
as long as we insist that the cuspy halos found in simulations must describe
the halos of low surface brightness galaxies. A low density cosmology helps
with the low observed concentrations, but it offers no explanation of the many
cases where the shape of the density profile deviates from the predicted cuspy
form. These cases must have suffered very extensive mass redistribution if the
current halo formation picture is not to fail outright. It is far from clear
whether any of the mass redistribution mechanisms which have been suggested
(e.g., feedback) are viable.
