575 research outputs found
Cosmological k-essence condensation
We consider a model of dark energy/matter unification based on a k-essence
type of theory similar to tachyon condensate models. Using an extension of the
general relativistic spherical model which incorporates the effects of both
pressure and the acoustic horizon we show that an initially perturbative
k-essence fluid evolves into a mixed system containing cold dark matter like
gravitational condensate in significant quantities.Comment: 8 pages, 2 figures, presented by NB at the conference Beyond 2010,
Cape Town, 1-6 February 201
Gauges in the Bulk II: Models with Bulk Scalars
Extending previous work in Randall-Sundrum type models, we construct
low-energy effective actions for braneworlds with a bulk scalar field, with
special attention to the case of BPS branes. Holding the branes at fixed
coordinate position with a general ansatz for the bulk metric, and imposing the
Einstein frame as a gauge condition, we obtain a scalar-tensor theory with only
one scalar degree of freedom related to the proper brane separation. The
formalism is applicable even when there is direct coupling of the bulk scalar
and brane matter, as in the Horava-Witten model. We further show that the usual
moduli space approximation actually describes a non-BPS three-brane system.Comment: LaTeX file of 18 page
Chaplygin Gas Cosmology - Unification of Dark Matter and Dark Energy
The models that unify dark matter and dark energy based upon the Chaplygin
gas fail owing to the suppression of structure formation by the adiabatic speed
of sound. Including string theory effects, in particular the Kalb-Ramond field,
we show how nonadiabatic perturbations allow a successful structure formation.Comment: 7 pages, presented by N. B. at IRGAC 2006, Barcelona, 11-15 July
2006, typos corrected, concluding paragraph slightly expanded, final version,
accepted in J. Phys. A, special issu
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