• Naresh Dadhich

      Articles written in Pramana – Journal of Physics

    • Spectral features of radiation from Nordström and Kerr-Newman white holes

      Naresh Dadhich

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      Unlike the Schwarzschild white hole, Nordström and Kerr-Newman white holes cannot explode right down from the space time singularityR=0. For example a charged white hole has to commence explosion (i.e., comes into existence) with a radiusR0=Rc(2−Rc/Rb)−1 whereRc is the ‘classical radius’ andRb is the final radius attained when the stationary state is reached. That means charged and rotating black holes also cannot hit the singularityR=0 and perish.

      Here the explosion is decelerated by the presence of charge and rotation and hence the radiation emitted would be not as energetic as in the Schwarzschild case where its energy is infinitely large for emission fromR=0.

    • Timelike and null geodesics in the Nordström field

      Naresh Dadhich P P Kale

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      The timelike and null geodesics are investigated in the Nordström geometry and it is found that incoming geodesics always encounter a turning point at a finite radial distance. The limits for escape, bound and stable orbits are obtained and they are closer to the source as compared to their counterparts in the Schwarzschild’s field.

    • On singularity-free spacetimes

      Naresh Dadhich L K Patel R Tikekar

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      We consider here the metric for the singularity-free family of fluid models. The metric is unique for cylindrically symmetric space-time with metric potentials being separable functions of radial and time coordinates in the comoving coordinates. It turns out that fluid models separate out into two classes, withρµp in general butρ = 3p in particular andp =ρ. It is shown that in both the cases radial heat flow can be incorporated without disturbing the singularity-free character of the spacetime. The geodesics of the singularity-free metric are studied and the geodesic completeness is established. Several previously known solutions are derived as particular cases.

    • A class of stationary rotating string cosmological models

      L K Patel Naresh Dadhich

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      We obtain a one parameter class of stationary rotating string cosmological models of which the well-known Gödel universe is a particular case. By suitably choosing the free parameter function, it is always possible to satisfy the energy conditions. The rotation of the model hinges on the cosmological constant which turns out to be negative. String-dust distribution in Gödel-type universes is also briefly discussed.

    • Inhomogeneous cosmological models with heat flux

      L K Patel Ramesh Tikekar Naresh Dadhich

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      We present a general class of inhomogeneous cosmological models filled with non-thermalized perfect fluid by assuming that the background spacetime admits two space-like commuting Killing vectors and has separable metric coefficients. The singularity structure of these models depends on the choice of the parameters and the metric functions. A number of previously known perfect fluid models follow as particular cases of this general class. Physical and geometrical features of these models are studied and the general expression for temperature distribution is given.

    • Isothermal fluid sphere: Uniqueness and conformal mapping

      Naresh Dadhich

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      We prove the theorem: A necessary and sufficient condition for a spacetime to represent an isothermal fluid sphere (linear equation of state with density falling off as inverse square of the curvature radius) without boundary is that it is conformal to a spacetime of zero gravitational mass (‘minimally’ curved).

    • Schwarzschild black hole with global monopole charge

      Naresh Dadhich K Narayan U A Yajnik

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      We derive the metric for a Schwarzschild black hole with global monopole charge by relaxing asymptotic flatness of the Schwarzschild field. We then study the effect of global monopole charge on particle orbits and the Hawking radiation. It turns out that existence, boundedness and stability of circular orbits scale up by (1−8πη2)−1, and the perihelion shift and the light bending by (1−8πη2)−3/2, while the Hawking temperature scales down by (1−8πη2)2 the Schwarzschild values. Hereη is the global charge.

    • On spacetimes dual to spherically symmetric solutions

      Naresh Dadhich L K Patel

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      By defining a duality transformation which implies interchange of active and passive electric parts of gravitational field, it is possible to construct spacetimes dual to solutions of the Einstein equation. Under the duality transformation a fluid spacetime maps into a fluid spacetime with density and pressure transforming as ρ→ (ρ + 3p)/2 andp → (ρ -p)/2. On the other hand a vacuum solution will acquire a global monopole charge. The remarkable feature of spherically symmetric solutions is that it is possible to give a general prescription for writing dual solutions. We demonstrate its application by writing dual solutions to the McVittie solution for a Schwarzschild particle in an expanding universe and to Vaidya’s radiating star solution.

    • Quantifying energy condition violations in traversable wormholes

      Sayan Kar Naresh Dadhich Matt Visser

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      The ‘theoretical’ existence of traversable Lorentzian wormholes in the classical, macroscopic world is plagued by the violation of the well-known energy conditions of general relativity. In this brief article we show: (i) how the extent of violation can be quantified using certain volume integrals and (ii) whether this ‘amount of violation’ can be minimised for some specific cut-and-paste geometric constructions. Examples and possibilities are also outlined.

    • Summary of classical general relativity workshop

      Naresh Dadhich Narayan Banerjee

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      This is a summary of the presentations at the parallel session in the classical general relativity workshop of the ICGC-2004.

    • Preface

      Naresh Dadhich Pankaj Joshi Probir Roy

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    • Singularity: Raychaudhuri equation once again

      Naresh Dadhich

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      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.

    • Characterization of the Lovelock gravity by Bianchi derivative

      Naresh Dadhich

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      We prove the theorem: The second-order quasilinear differential operator as a second-rank divergence-free tensor in the equation of motion for gravitation could always be derived from the trace of the Bianchi derivative of the fourth-rank tensor, which is a homogeneous polynomial in curvatures. The existence of such a tensor for each term in the polynomial Lagrangian is a new characterization of the Lovelock gravity.

    • On the enigmatic 𝛬 – A true constant of spacetime

      Naresh Dadhich

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      Had Einstein followed the Bianchi differential identity for the derivation of his equation of motion for gravitation, 𝛬 would have emerged as a true new constant of spacetime on the same footing as the velocity of light? It is then conceivable that he could have perhaps made the most profound prediction that the Universe may suffer accelerated expansion some time in the future! Further we argue that its identification with the quantum vacuum energy is not valid as it should have to be accounted for like the gravitational field energy by enlarging the basic framework of spacetime and not through a stress tensor. The acceleration of the expansion of the Universe may indeed be measuring its value for the first time observationally.

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