6 research outputs found
The Cosmological Slingshot Scenario: A Stringy Early Times Universe
A cosmological model for the early time Universe is proposed. In this model,
the Universe is a wandering brane moving in a warped throat of a Calabi-Yau
space. A non-zero angular momentum induces a turning point in the brane
trajectory, and leads to a bouncing cosmology as experienced by an observer
living on the brane. The Universe undergoes a decelerated contraction followed
by an accelerating expansion and no big-bang singularity. Although the number
of e-folds of accelerated motion is low (less than 2), standard cosmological
problems are not present in our model thanks to the absence of an initial
singularity and the violation of energy conditions of mirage matter at high
energies. Density perturbations are also calculated in our model and we find a
slightly red spectral index with negligible tensorial perturbations in
compatibility with WMAP data.Comment: v5: clarifications and references added, results unchanged, version
accepted in Class. Quant. Grav. (2008), 34 pages, 5 figure
Generating the curvature perturbation with instant preheating
A new mechanism for generating the curvature perturbation at the end of
inflaton has been investigated. The dominant contribution to the primordial
curvature perturbation may be generated during the period of instant
preheating. The mechanism converts isocurvature perturbation related to a light
field into curvature perturbation, where the ``light field'' is not the
inflaton field. This mechanism is important in inflationary models where
kinetic energy is significant at the end of inflaton. We show how one can apply
this mechanism to various brane inflationary models.Comment: 17 pages, 1 figure, To appear in JCA
The Cosmological Slingshot Scenario: Myths and Facts
We generalize the Cosmological Slingshot Scenario for a Slingshot brane
moving in a Klebanov-Strassler throat. We show that the horizon and isotropy
problems of standard cosmology are avoided, while the flatness problem is
acceptably alleviated. Regarding the primordial perturbations, we identify
their vacuum state and elucidate the evolution from the quantum to the
classical regimes. Also, we calculate their exact power spectrum showing its
compatibility with current data. We discuss the bouncing solution from a four
dimensional point of view. In this framework the radial and angular motion of
the Slingshot brane are described by two scalar fields. We show that the
bouncing solution for the scale factor in String frame is mapped into a
monotonically increasing (in conformal time) solution in the Einstein frame. We
finally discuss about the regularity of the geometry in Einstein frame.Comment: 16 pages, 2 figs. Major clarifications and references added, version
accepted in Gen. Rel. Grav. (2009
Perturbative instabilities in Horava gravity
We investigate the scalar and tensor perturbations in Horava gravity, with
and without detailed balance, around a flat background. Once both types of
perturbations are taken into account, it is revealed that the theory is plagued
by ghost-like scalar instabilities in the range of parameters which would
render it power-counting renormalizable, that cannot be overcome by simple
tricks such as analytic continuation. Implementing a consistent flow between
the UV and IR limits seems thus more challenging than initially presumed,
regardless of whether the theory approaches General Relativity at low energies
or not. Even in the phenomenologically viable parameter space, the tensor
sector leads to additional potential problems, such as fine-tunings and
super-luminal propagation.Comment: 21 pages, version published at Class. Quant. Gra