Browsing by Author "Roy, Maartens"

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Cosmological perturbations from brane inflation with a Gauss-Bonnet term
(2011-07-06) Dufaux, Jean-Francois; Lidsey, James E.; Roy, Maartens
Braneworld inﬂation is a phenomenology related to string theory that describes high-energy mod- iﬁcations to general relativistic inﬂation. The observable universe is a braneworld embedded in 5-dimensional anti de Sitter spacetime. When the 5-dimensional action is Einstein-Hilbert, we have a Randall-Sundrum type braneworld. The amplitude of tensor and scalar perturbations from in- ﬂation is strongly increased relative to the standard results, although the ratio of tensor to scalar amplitudes still obeys the standard consistency relation. If a Gauss-Bonnet term is included in the action, as a high-energy correction motivated by string theory, we show that there are important changes to the Randall-Sundrum case. We give an exact analysis of the tensor perturbations. They satisfy the same wave equation and have the same spectrum as in the Randall-Sundrum case, but the Gauss-Bonnet change to the junction conditions leads to a modiﬁed amplitude of gravitational waves. The amplitude is no longer monotonically increasing with energy scale, but decreases asymp- totically after an initial rise above the standard level. Using an approximation that neglects bulk eﬀects, we show that the amplitude of scalar perturbations has a qualitatively similar behaviour to the tensor amplitude. In addition, the tensor to scalar ratio breaks the standard consistency relation.
Observational constraints on braneworld inflation: The effect of a Gauss-Bonnet term
(2011-07-06) Tsujikawa, Shinji; Sami, M.; Roy, Maartens
High-energy modiﬁcations to general relativity introduce changes to the perturbations generated during inﬂation, and the latest high-precision cosmological data can be used to place constraints on such modiﬁed inﬂation models. Recently it was shown that Randall-Sundrum type braneworld inﬂation leads to tighter constraints on quadratic and quartic potentials than in general relativity. We investigate how this changes with a Gauss-Bonnet correction term, which can be motivated by string theory. Randall-Sundrum models preserve the standard consistency relation between the tensor spectral index and the tensor-to-scalar ratio. The Gauss-Bonnet term breaks this relation, and also modiﬁes the dynamics and perturbation amplitudes at high energies. We ﬁnd that the Gauss- Bonnet term tends to soften the Randall-Sundrum constraints. The observational compatibility of the quadratic potential is strongly improved. For a broad range of energy scales, the quartic potential is rescued from marginal rejection. Steep inﬂation driven by an exponential potential is excluded in the Randall-Sundrum case, but the Gauss-Bonnet term leads to marginal compatibility for suﬃcient e-folds.
Why do naked singularities form in gravitational
(2002-03-12) Joshi, P. S.; Dadhich, Naresh; Roy, Maartens
We investigate what are the key physical features that cause the development of a naked singularity, rather than a black hole, as the end-state of spherical gravitational collapse. We show that suﬃciently strong shearing eﬀects near the singularity delay the formation of the apparent horizon. This exposes the singularity to an external observer, in contrast to a black hole, which is hidden behind an event horizon due to the early formation of an apparent horizon