Sanjeev S. Seahra

Application of the Campbell–Magaard theorem to higher-dimensional physics

Stated succinctly, the original version of the Campbell–Magaard theorem says that it is always possible to locally embed any solution of four-dimensional general relativity in a five-dimensional Ricci-flat manifold. We discuss the proof of this theorem (and its variants) in n dimensions, and its application to current theories that postulate that our universe is a four-dimensional hypersurface Σ0 within a five-dimensional manifold, such as space–time–matter (STM) theory and the Randall and Sundrum (RS) braneworld scenario. In particular, we determine whether or not arbitrary spacetimes may be embedded in such theories, and demonstrate how these seemingly disparate models are interconnected. Special attention is given to the motion of test observers in five dimensions, and the circumstances under which they are confined to Σ0. For each five-dimensional scenario considered, the requirement that observers be confined to the embedded spacetime places restrictions on the 4-geometry. For example, we find that observers in the thin braneworld scenario can be localized around the brane if its total stress–energy tensor obeys the five-dimensional strong energy condition. As a concrete example of some of our technical results, we discuss a 2 symmetric embedding of the standard radiation-dominated cosmology in a five-dimensional vacuum.

Detecting extra dimensions with gravity-wave spectroscopy: the black-string brane world.

Using the black string between two branes as a model of a brane-world black hole, we compute the gravity-wave perturbations and identify the features arising from the additional polarizations of the graviton. The standard four-dimensional gravitational wave signal acquires late-time oscillations due to massive modes of the graviton. The Fourier transform of these oscillations shows a series of spikes associated with the masses of the Kaluza-Klein modes, providing in principle a spectroscopic signature of extra dimensions.

Extended Red Emission from Carbon Clusters in Interstellar Clouds

We have simulated extended red emission (ERE) spectra using a model in which this emission arises as photoluminescence from small carbon particles of mixed sp2/sp3 hybridized bonding characteristics. The emission efficiency from such particles can be highly efficient when their size is such that geminate recombination of photoexcited electron hole pairs is enhanced. The amplitude and emission profile of the ERE emission from the diffuse interstellar medium and nebular sources such as NGC 2327 and NGC 7027 can be reproduced with a range of average particle size and size distribution. These carbon particles are components of the same mixtures that yield an accurate fit to the interstellar 2175 A extinction peak and suggest that ERE emission, the 2175 A absorption, and infrared absorption at 3.4 μm may arise from the same carbonaceous components of interstellar matter.

Einstein static universes are unstable in generic f(R) models

We study Einstein static universes in the context of generic f(R) models. It is shown that Einstein static solutions exist for a wide variety of modified gravity models sourced by a barotropic perfect fluid with equation of state w = p/�, but these solutions are always unstable to either homogeneous or inhomogeneous perturbations. Our general results are in agreement with specific models investigated in that past. We also discuss how our techniques can be applied to other scenarios in f(R) gravity.

Classical confinement of test particles in higher dimensional models: Stability criteria and a new energy condition

We review the circumstances under which test particles can be localized around a spacetime section

Integrated Absorbances in the 3.4 μm CHn Band in Hydrogenated Amorphous Carbon

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Scalar perturbations in braneworld cosmology

smoothly contained within a codimension-1 embedding space M. If such a confinement is possible,

The structure of the big bang from higher-dimensional embeddings

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Cosmological perturbations in the DGP braneworld: Numeric solution

is said to be totally geodesic. Using three different methods, we derive a stability condition for trapped test particles in terms of intrinsic geometrical quantities on

Generalized uncertainty principles and quantum field theory

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