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Naresh Dadhich

Publications and source records attributed to Naresh Dadhich.

At least 109 records · Page 6Linked to original sources
arXiv2010

Energetics of a rotating charged black hole in 5-dimensional supergravity

We investigate the properties of the event horizon and static limit for a charged rotating black hole solution of minimal supergravity theory in (1 + 4) dimension. Unlike the four-dimensional case, there are in general two rotations, and they couple to both mass and charge. This gives rise to much richer structure to ergosphere leading to energy extraction even for axial fall. Another interesting feature is that the metric in this case is sensitive to the sign of the Maxwell charge.

hep-th↗
arXiv2009

Why do we live in four dimension?

We perceive the dimension of physical spacetime we live in through physical experiments and hence it is pertinent to probe the dimension in which the fundamental physical forces exist and act? In this context we shall investigate the two classical fields of gravitation and electromagnetism and argue that four dimension is necessary for spacetime but may not be sufficient. Some motivation for higher dimension would also be discussed.

gr-qc↗
arXiv2008

Focusing versus defocusing properties of truly naked black holes

We study the properties of the congruence of null geodesics propagating near the so-called truly naked horizons (TNH) - objects having finite Kretschmann scalar but with diverging tidal acceleration for freely falling observers. The expansion of outgoing rays near the future horizon always tends to vanish for the non-extremal case but may be non-zero for the distorted (ultra)extremal one. It tends to diverge for the ingoing ones if the the null energy condition (NEC) is satisfied in the vicinity of the horizon outside. However, it also tends to zero for NEC violating cases except the remote horizons. We also discuss the validity of test particle approximation for TNHs and find the sufficient condition for backreaction remaining small.

gr-qc↗
arXiv2008

Entropy Bound and Causality Violation in Higher Curvature Gravity

In any quantum theory of gravity we do expect corrections to Einstein gravity to occur. Yet, at fundamental level, it is not apparent what the most relevant corrections are. We argue that the generic curvature square corrections present in lower dimensional actions of various compactified string theories provide a natural passage between the classical and quantum realms of gravity. The Gauss-Bonnet and $({\rm Riemann})^2$ gravities, in particular, provide concrete examples in which inconsistency of a theory, such as, a violation of microcausality, and a classical limit on black hole entropy are correlated. In such theories the ratio of the shear viscosity to the entropy density, $η/s$, can be smaller than for a boundary conformal field theory with Einstein gravity dual. This result is interesting from the viewpoint that the nuclear matter or quark-gluon plasma produced (such as at RHIC) under extreme densities and temperatures may violate the conjectured bound $η/s\ge 1/4π$, {\it albeit} marginally so.

hep-th↗
arXiv2008

On Kaluza-Klein spacetime in Einstein-Gauss-Bonnet gravity

By considering the product of the usual four dimensional spacetime with two dimensional space of constant curvature, an interesting black hole solution has recently been found for Einstein-Gauss-Bonnet gravity. It turns out that this as well as all others could easily be made to radiate Vaidya null dust. However there exists no Kerr analogue in this setting. To get the physical feel of the four dimensional black hole spacetimes, we study asymptotic behavior of stresses at the two ends, $r\to0$ and $r\to\infty$.

gr-qc↗
arXiv2007

Formation of a black hole from an AdS spacetime

We shall first discuss motivation for higher dimension even for classical description of gravitational dynamics and then construct a black hole out of an anti-deSitter (AdS) spacetime by prescribing a coupling between Gauss-Bonnet parameter, constant curvature of extra dimensional space and $Λ$. This is a creation of pure curvature which establishes the fundamental reciprocity between matter and gravity/curvature.

gr-qc↗
arXiv2007

Origin of matter out of pure curvature

We propose a mechanism for origin of matter in the universe in the framework of Einstein-Gauss-Bonnet gravity in higher dimensions. The recently discovered new static black hole solution by the authors \cite{md2006} with the Kaluza-Klein split up of spacetime as a product of the usual ${\ma M}^4$ with a space of negative constant curvature is indeed a pure gravitational creation of a black hole which is also endowed with a Maxwell-like {\it gravitational charge} in four-dimensional vacuum spacetime. Further it could be envisioned as being formed from anti-de Sitter spacetime by collapse of radially inflowing charged null dust. It thus establishes the remarkable reciprocity between matter and gravity - as matter produces gravity (curvature), gravity too produces matter.

hep-th↗
arXiv2007

Singularity: Raychaudhuri Equation once again

I first recount Raychaudhuri's deep involvement with the singularity problem in general relativity. I then argue that precisely the same situation has arisen today in loop quantum cosmology as obtained when Raychaudhuri discovered his celebrated equation. We thus need a new analogue of the Raychaudhuri equation in quantum gravity.

gr-qc↗
arXiv2007

Matter without matter: novel Kaluza-Klein spacetime in Einstein-Gauss-Bonnet gravity

We consider Einstein-Gauss-Bonnet gravity in $n(\ge 6)$-dimensional Kaluza-Klein spacetime ${\ma M}^{4} \times {\ma K}^{n-4}$, where ${\ma K}^{n-4}$ is the Einstein space with negative curvature. In the case where ${\ma K}^{n-4}$ is the space of negative constant curvature, we have recently obtained a new static black-hole solution (Phys. Rev. D {\bf 74}, 021501(R) (2006), hep-th/0605031) which is a pure gravitational creation including Maxwell field in four-dimensional vacuum spacetime. The solution has been generalized to make it radially radiate null radiation representing gravitational creation of charged null dust. The same class of solutions though exists in spacetime ${\ma M}^{d} \times {\ma K}^{n-d}$ for $d=3,4$, however the gravitational creation of the Maxwell field is achieved only for $d=4$. Also, Gauss-Bonnet effect could be brought down to ${\ma M}^d$ only for $d=4$. Further some new exact solutions are obtained including its analogue in Taub-NUT spacetime on ${\ma M}^4$ for $d=4$.

hep-th↗
arXiv2006

Kaluza-Klein black hole with negatively curved extra dimensions in string generated gravity models

We obtain a new exact black-hole solution in Einstein-Gauss-Bonnet gravity with a cosmological constant which bears a specific relation to the Gauss-Bonnet coupling constant. The spacetime is a product of the usual 4-dimensional manifold with a $(n-4)$-dimensional space of constant negative curvature, i.e., its topology is locally ${\ma M}^n \approx {\ma M}^4 \times {\ma H}^{n-4}$. The solution has two parameters and asymptotically approximates to the field of a charged black hole in anti-de Sitter spacetime. The most interesting and remarkable feature is that the Gauss-Bonnet term acts like a Maxwell source for large $r$ while at the other end it regularizes the metric and weakens the central singularity.

hep-th↗
arXiv2006

On the Gauss-Bonnet Gravity

We argue that propagation of gravitational field in the extra dimension is motivated by physical realization of second iteration of self interaction of gravity and it is described by the Gauss-Bonnet term. The most remarkable feature of the Gauss-Bonnet gravity is that at high energy it radically transforms radial dependence from inverse to proportionality as singularity is approached and thereby making it weak. Similar change over also occurs in approach to singularity in loop quantum gravity. It is analogous to Planck's law of radiation where similar change occurs for high and low energy behavior. This is how it seems to anticipate in qualitative terms and in the right sense the quantum gravity effect in 5 dimensions where it is physically non-trivial. The really interesting question is, could this desirable feature be brought down to the $4-$dimensional spacetime by dilatonic coupling to the Gauss-Bonnet term or otherwise?

hep-th↗
arXiv2006

A Unified View of the Basic Forces

In this essay we wish to seek a unifying thread between the basic forces. We propose that there exists a universal force which is shared by all that physically exists. Universality is characterized by the two properties: (i) universal linkage and (ii) long range. They uniquely identify Einstein gravity as the unversal force. All other forces then arise as these properties are peeled off. For instance, relaxing (i) but retaining (ii) will lead to Maxwell electromagnetic force. This unified outlook makes interesting suggestions and predictions: if there exists a new force, it can only be a short range non-abelian vector or a scalar field, and there should exist in an appropriate space duality relations between weak and electric, and between strong and gravity.

gr-qc↗
arXiv2006

A Lighthouse Falls

Professor A. K. Raychaudhuri suddenly passed away on 18th June 2005. I have attempted to give a glimpse of his life and work, and more specifically his role as a great teacher and inspiring force to three generations of students of the Presidency College, Kolkata.

physics.hist-ph↗
arXiv2005

Space-time curvature coupling of spinors in early universe: Neutrino asymmetry and a possible source of baryogenesis

It is well known that when a fermion propagates in curved space-time, its spin couples to the curvature of background space-time. We propose that this interaction for neutrinos propagating in early curved universe could give rise to a new set of dispersion relations and then neutrino asymmetry at equilibrium. We demonstrate this with the Bianchi models which describe the homogeneous but anisotropic and axially symmetric universe. If the lepton number violating processes freeze out at 10^{-37} second when temperature T ~ 10^{15} GeV, neutrino asymmetry of the order of 10^{-10} can be generated. A net baryon asymmetry of the same magnitude can thus be generated from this lepton asymmetry either by a GUT B-L symmetry or by the electro-week sphaleron processes which have B+L symmetry.

hep-ph↗
arXiv2005

Curvature driven acceleration : a utopia or a reality ?

The present work shows that a combination of nonlinear contribution from the Ricci curvature in Einstein field equations can drive a late time acceleration of expansion of the universe. The transit from the decelerated to the accelerated phase of expansion takes place smoothly without having to resort to a study of asymptotic behaviour. This result emphasizes the need for thorough and critical examination of models with nonlinear contribution from the curvature.

astro-ph↗
arXiv2005

Why Einstein (Had I been born in 1844!)?

In his monumental discoveries, the driving force for Einstein was, I believe, consistency of concept and principle rather than conflict with experiment. In this spirit, I would like to look at the journey from the classical to the relativistic world as a simple and direct exercise first in recognition of universal character of universal entities and then carrying out the universalization. By this process not only the relativistic world follows most naturally but I would like to conjecture that if Einstein were born in 1844 (or had I been born in 1844 and had followed this line of thought as I do now!) it would have in fact been predicted including existence of a wave with universal constant velocity. That would have indeed been not only the greatest but most amazing and remarkable feat of human thought. Beating further on the same track of principle and concept driven ideas, we ponder over to see beyond Einstein, and ask the questions: in how many dimensions does gravity live, how many basic forces are there in nature and what are the basic building blocks of space-time?

physics.gen-ph↗
arXiv2004

A dynamical symmetry of the spherical dust collapse

By linearly scaling the initial data set (mass and kinetic energy functions) together with the initial area radius of a collapsing dust sphere, we find a symmetry of the collapse dynamics. That is, the linear transformation defines an equivalence class of data sets which lead to the same end result as well as its evolution all through. In particular, the density and shear remain invariant initially as well as during the collapse. What the transformation is exhibiting is an interesting scaling relationship between mass, kinetic energy and the size of the collapsing dust sphere.

gr-qc↗