Dark matter relic density in Gauss-Bonnet braneworld cosmology
  The relic density of symmetric and asymmetric dark matter in a Gauss-Bonnet
(GB) modified Randall-Sundrum (RS) type II braneworld cosmology is
investigated. The existing study of symmetric dark matter in a GB braneworld
(Okada and Okada, 2009) found that the expansion rate was reduced compared to
that in standard General Relativity (GR), thereby delaying particle freeze-out
and resulting in relic abundances which are suppressed by up to
$\mathcal{O}(10^{-2})$. This is in direct contrast to the behaviour observed in
RS braneworlds where the expansion rate is enhanced and the final relic
abundance boosted. However, this finding that relic abundances are suppressed
in a GB braneworld is based upon a highly contrived situation in which the GB
era evolves directly into a standard GR era, rather than passing through a RS
era as is the general situation. This collapse of the RS era requires equating
the mass scale $m_{\alpha}$ of the GB modification and the mass scale
$m_{\sigma}$ of the brane tension. However, if the GB contribution is to be
considered as the lowest order correction from string theory to the RS action,
we would expect $m_{\alpha} > m_{\sigma}$. We investigate the effect upon the
relic abundance of choosing more realistic values for the ratio
$\mathcal{R}_{m} \equiv m_{\alpha}/m_{\sigma}$ and find that the relic
abundance can be either enhanced or suppressed by more than two orders of
magnitude. However, suppression only occurs for a small range of parameter
choices and, overwhelmingly, the predominant situation is that of enhancement
as we recover the usual Randall-Sundrum type behaviour in the limit
$\mathcal{R}_m \gg 1$. We use the latest observational bound $\Omega_{DM}h^2 =
0.1187 \pm 0.0017$ to constrain the various model parameters and briefly
discuss the implications for direct/indirect dark matter detection experiments
as well as dark matter particle models.
