arXiv subjects

S. Kalyana Rama

Publications and source records attributed to S. Kalyana Rama.

At least 37 records · Page 2Linked to original sources

arXiv2009

Particle Motion with Hořava -- Lifshitz type Dispersion Relations

Using super Hamiltonian formalism, we study the motion of particles whose dispersion relations are modified to incorporate Hořava -- Lifshitz type anisotropic scaling symmetry. We find the following as consequences of this modified dispersion relation: (i) The speed of a charged particle under a constant electric field grows without bound and diverges. (ii) The speed of a particle falling towards the horizon also grows without bound and diverges as the particle approaches the horizon. (iii) This particle reaches the horizon in a finite coordinate time, in contrast to the standard case where it requires infinite time.

hep-th↗

arXiv2009

Anisotropic Cosmology and (Super)Stiff Matter in Hořava's Gravity Theory

We study anisotropic cosmology in Hořava's gravity theory and obtain Kasner type solutions, valid for any number d of spatial dimensions. The corresponding exponents satisfy two relations, one involving the marginal coupling λ. Also, Hořava's (super)renormalisable theory predicts (super)stiff matter whose equation of state is p = w ρwith w \ge 1. We discuss briefly the implications of these results for the nature of cosmological collapse.

hep-th↗

arXiv2009

Stabilisation of Seven Directions in an Early Universe M -- theory Model

Our model consists of intersecting 22'55' branes in M theory distributed uniformly in the common transverse space. Equations of state follow from U duality symmetries. In this model, three spatial directions expand, and seven directions stabilise to constant sizes. From string theory perspective, the dilaton is hence stabilised. The constant sizes depend on certain imbalance among initial values. One naturally obtains M_{11} \simeq M_s \simeq M_4 and g_s \simeq 1 within a few orders of magnitude. Smaller numbers, for example M_s \simeq 10^{- 16} M_4, are also possible but require fine tuning.

hep-th↗

arXiv2007

A Description of Rotating Multicharged Black Holes in terms of Branes and Antibranes

We describe rotating multicharge black holes as stacks of intersecting branes and antibranes together with massless gases on them. Assuming the energies of the gases to be equal, we find that their angular momentum parameters, corresponding to black hole rotations, are also equal. The entropy S of this model is given by S = X S_{sg} where S_{sg} is the supergravity entropy. One can obtain X = 1 under an assumption which violates conservation of energy. We show that X = 1 can also be obtained if one assumes that there is only one single gas, which is some sort of superposition of the gases mentioned above, and that the brane tensions are reduced by a factor of four. In this interpretation, energy is conserved and the unusual assumption that energies, not temperatures, of the gases are equal becomes superfluous.

hep-th↗

arXiv2007

Consequences of U dualities for Intersecting Branes in the Universe

We consider N -- charge, intersecting brane antibrane configurations in M theory which are smeared uniformly in the common transverse space and may describe our universe. We study the consequences of U dualities and find that they imply relations among the scale factors. We find using Einstein's equations that U dualities also imply a relation among the density ρand the pressure p_i for the single charge case. We present an ansatz for ρand p_i for the N -- charge case which yields all the U duality relations among the scale factors. We then study configurations with identical charges, and also with net charges vanishing. We find among other things that, independent of the details of the brane antibrane dynamics, such four charge configurations lead asymptotically to an effective (3 + 1) -- dimensional expanding universe.

hep-th↗

arXiv2007

Entropy of Anisotropic Universe and Fractional Branes

We obtain the entropy of a homogeneous anisotropic universe applicable, by assumption, to the fractional branes in the universe in the model of Chowdhury and Mathur. The entropy for the 3 or 4 charge fractional branes thus obtained is not of the expected form E^{3/2} or E^2. One way the expected form is realised is if p \to ρfor the transverse directions and if the compact directions remain constant in size. These conditions are likely to be enforced by brane decay and annihilation, and by the S, T, U dualities. T duality is also likely to exclude high entropic cases, found in the examples, which arise due to the compact space contracting to zero size. Then the 4 charge fractional branes may indeed provide a detailed realisation of the maximum entropic principle we proposed recently to determine the number (3 + 1) of large spacetime dimensions.

hep-th↗

arXiv2006

A Principle to Determine the Number (3 + 1) of Large Spacetime Dimensions

We assume that our universe originated from highly excited and interacting strings with coupling constant g_s = {\cal O} (1). Fluctuations of spacetime geometry are large in such strings and the physics dictating the emergence of a final spacetime configuration is not known. We propose that, nevertheless, it is determined by an entropic principle that the final spacetime configuration must have maximum entropy for a given amount of energy. This principle implies, under some assumptions, that the spacetime configuration that emerges finally is a (3 + 1) -- dimensional FRW universe filled with w = 1 perfect fluid and with 6 -- dimensional compact space of size l_s; in particular, the number of large spacetime dimensions is d = 3 + 1. Such an universe may evolve subsequently into our universe, perhaps as in Banks -- Fischler scenario.

hep-th↗

arXiv2006

A Stringy Correspondence Principle in Cosmology

We study a d-dimensional FRW universe, containing a perfect fluid with p = w ρand \frac{1} {d - 1} \le w \le 1, and find a correspondence principle similar to that of Horowitz and Polchinski in the black hole case. This principle follows quite generally from thermodynamics and the conservation of energy momentum tensor, and can be stated along similar lines as in the black hole case: ``When the temperature T of the universe becomes of order string scale the universe state becomes a highly excited string state. At the transition, the entropies and energies of the universe and strings differ by factors of {\cal O}(1).'' Such a matching is absent for w \ne 1 if the transition is assumed to be when the curvature or the horizon length is of order string scale.

hep-th↗

arXiv2005

Branes in a Time Dependent Universe

Long ago, McVittie had found a class of solutions which can be thought of as Schwarzschild black holes in an FRW universe. In recent years they have been studied extensively and generalised to charged and uncharged black holes in D \ge 4 dimensions also. Here, assuming an ansatz similar to McVittie's, we present solutions for uncharged branes which can be thought of as branes in a time dependent universe. We consider their application to the brane antibrane decay process, also referred to as tachyon condensation, and discuss the necessary generalisations required for our ansatz to describe such a process.

hep-th↗

arXiv2005

Multiparameter Brane Solutions by Boosts, S and T dualities

We show that the multiparameter (intersecting) brane solutions of string/M theories given in the literature can all be obtained by a suitable combination of boosts in eleven dimension, S and T dualities. We also describe a duality property of the D dimensional multiparameter solutions describing branes smeared in compact directions. This duality is analogous to the T duality of the string theory but is valid for any value of D.

hep-th↗

arXiv2004

Cosmology and Static Spherically Symmetric solutions in D-dimensional Scalar Tensor Theories: Some Novel Features

We consider scalar tensor theories in D-dimensional spacetime, D \ge 4. They consist of metric and a non minimally coupled scalar field, with its non minimal coupling characterised by a function. The probes couple minimally to the metric only. We obtain vacuum solutions - both cosmological and static spherically symmetric ones - and study their properties. We find that, as seen by the probes, there is no singularity in the cosmological solutions for a class of functions which obey certain constraints. It turns out that for the same class of functions, there are static spherically symmetric solutions which exhibit novel properties: {\em e.g.} near the ``horizon'', the gravitational force as seen by the probe becomes repulsive.

gr-qc↗

arXiv2004

A Description of multi Charged Black Holes in terms of Branes and Antibranes

We describe multicharged black holes in terms of branes and antibranes together with multiple copies of gas of massless excitations. Assuming that energies of these copies of gas are all equal, we find that the entropy of the brane antibrane configuration agrees with that of the multicharged black hole in supergravity approximation, upto a factor X. We find that X = 1 for a suitable normalisation which admits a simple empirical interpretation that the available gas energy is all taken by one single gas which is, in a sense, a certain superposition of the multiple copies; and that the brane tensions are decreased by a factor of 4. This interpretation renders superfluous the assumption of equal energies, which is unnatural from a physical point of view.

hep-th↗

arXiv2004

A Description of Schwarzschild Black Holes in terms of Intersecting M-branes and antibranes

Intersecting M-branes are known to describe multi charged black holes. Using a configuration of such intersecting branes and antibranes, together with massless excitations living on them, we give a description of Schwarzschild black holes following Danielsson, Guijosa, and Kruczenski. We calculate the entropy of these black holes and find that it agrees, upto a numerical factor, with the entropy of the corresponding Schwarzschild black holes in supergravity approximation. We give an empirical interpretation of this factor.

hep-th↗

arXiv2003

Asymptotic Density of Open p-brane States with Zero-modes included

We obtain the asymptotic density of open p-brane states with zero-modes included. The resulting logarithmic correction to the p-brane entropy has a coefficient - \frac{p + 2}{2 p}, and is independent of the dimension of the embedding spacetime. Such logarithmic corrections to the entropy, with precisely this coefficient, appear in two other contexts also: a gas of massless particles in p-dimensional space, and a Schwarzschild black hole in (p + 2)-dimensional anti de Sitter spacetime.

hep-th↗

arXiv2003

Black Hole Entropy from Spin One Punctures

Recent suggestion, that the emission of a quantum of energy corresponding to the asymptotic value of quasinormal modes of a Schwarzschild black hole should be associated with the loss of spin one punctures from the black hole horizon, fixes the Immirzi parameter to a definite value. We show that saturating the horizon with spin one punctures reproduces the earlier formula for the black hole entropy, including the $ln (area)$ correction with definite coefficient (- 3/2) for large area.

gr-qc↗

arXiv2002

Classical Velocity in kappa-deformed Poincare Algebra and a Maximum Acceleration

We study the commutators of the kappa-deformed Poincare Algebra (kappaPA) in an arbitrary basis. It is known that the two recently studied doubly special relativity theories correspond to different choices of kappaPA bases. We present another such example. We consider the classical limit of kappaPA and calculate particle velocity in an arbitrary basis. It has standard properties and its expression takes a simple form in terms of the variables in the Snyder basis. We then study the particle trajectory explicitly for the case of a constant force. Assuming that the spacetime continuum, velocity, acceleration, etc. can be defined only at length scales greater than x_{min} ne 0, we show that the acceleration has a finite maximum.

hep-th↗

arXiv2002

Dynamical Features of Maggiore's Generalised Commutation Relations

We study the dynamical features of Maggiore's generalised commutation relations. We focus on their generality and, in particular, their dependence on the Hamiltonian H. We derive the generalisation of the Planck's law for black body spectrum, study the statistical mechanics of free particles, and study the early universe evolution which now exhibits non trivial features. We find that the dynamical features, found here and in our earlier work, are all generic and vary systematically with respect to the asymptotic growth of the Hamiltonian H.

hep-th↗

arXiv2002

No Go Theorem for Self Tuning Solutions With Gauss-Bonnet Terms

We consider self tuning solutions for a brane embedded in an anti de Sitter spacetime. We include the higher derivative Gauss-Bonnet terms in the action and study singularity free solutions with finite effective Newton's constant. Using the methods of Csaki et al, we prove that such solutions, when exist, always require a fine tuning among the brane parameters. We then present a new method of analysis in which the qualitative features of the solutions can be seen easily without obtaining the solutions explicitly. Also, the origin of the fine tuning is transparent in this method.

hep-th↗