19 research outputs found
Reheating the Universe in Braneworld Cosmological Models with bulk-brane energy transfer
The emergence of the cosmological composition (the reheating era) after the
inflationary period is analyzed in the framework of the braneworld models, in
which our Universe is a three-brane embedded in a five-dimensional bulk, by
assuming the possibility of the brane-bulk energy exchange. The inflaton field
is assumed to decay into normal matter only, while the dark matter is injected
into the brane from the bulk. To describe the reheating process we adopt a
phenomenological approach, by describing the decay of the inflaton field by a
friction term proportional to the energy density of the field. After the
radiation dominated epoch the model reduces to the standard four dimensional
cosmological model. The modified field equations are analyzed analytically and
numerically in both the extra-dimensions dominate reheating phase (when the
quadratic terms in energy density dominate the dynamics), and in the general
case. The evolution profiles of the matter, of the scalar field and of the
scale factor of the universe are obtained for different values of the
parameters of the model, and of the equations of state of the normal and dark
matter, respectively. The equation describing the time evolution of the ratio
of the energy density of the dark and of the normal matter is also obtained.
The ratio depends on the rate of the energy flow between the bulk and the
brane. The observational constraint of an approximately constant ratio of the
dark and of the baryonic matter requires that the dark matter must be
non-relativistic (cold). The model predicts a reheating temperature of the
order of GeV, a brane tension of the order of GeV,
and the obtained composition of the universe is consistent with the
observational data.Comment: 29 pages, 9 figures, accepted for publication in JCA
Inflation and late time acceleration in braneworld cosmological models with varying brane tension
Braneworld models with variable brane tension introduce a new
degree of freedom that allows for evolving gravitational and cosmological
constants, the latter being a natural candidate for dark energy. We consider a
thermodynamic interpretation of the varying brane tension models, by showing
that the field equations with variable can be interpreted as
describing matter creation in a cosmological framework. The particle creation
rate is determined by the variation rate of the brane tension, as well as by
the brane-bulk energy-matter transfer rate. We investigate the effect of a
variable brane tension on the cosmological evolution of the Universe, in the
framework of a particular model in which the brane tension is an exponentially
dependent function of the scale factor. The resulting cosmology shows the
presence of an initial inflationary expansion, followed by a decelerating
phase, and by a smooth transition towards a late accelerated de Sitter type
expansion. The varying brane tension is also responsible for the generation of
the matter in the Universe (reheating period). The physical constraints on the
model parameters, resulted from the observational cosmological data, are also
investigated.Comment: 25 pages, 8 figures, accepted for publication in European Physical
Journal
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
. 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 of the GB modification and the mass scale
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 . We investigate the effect upon the
relic abundance of choosing more realistic values for the ratio
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
. We use the latest observational bound 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.Comment: 18 pages, 5 figures, significant new material added and conclusions
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