1999 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/2909

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    Topology of the universe: Theory and observation
    (2015-03-01) Luminet, Jean-pierre; Roukema, B. F.
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    Galaxy clustering at Z~2 and halo radii
    (2015-03-01) Roukema, B. F.; Valls-Gabaud, D.; Mobasher, B.; Bajtlik, Stanislaw
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    Oscillating non-singular relativistic spherical model
    (2015-03-01) Dadhich, Naresh; Raychaudhuri, A.K.
    A particular choice of the time function in the recently presented spherical solution by Dadhich [1] leads to a singularity free cosmological model which oscillates between two regular states. The energy-stress tensor involves anisotropic pressure and a heat flux term but is consistent with the usual energy conditions (strong, weak and dominant). By choosing the parameters suitably one can make the model consistent with observational data. An interesting feature of the model is that it involves blue shifts as in the quasi steady state model [2] but without violating general relativity.
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    Global monopole as dual-vacuum solution in kaluza-klein spacetime
    (2015-03-01) Dadhich, Naresh; Patel, L.K.; Tikekar, R.
    By application of the duality transformation, which implies interchange of active and passive electric parts of the Riemann curvature (equivalent to interchange of Ricci and Einstein tensors) it is shown that the global monopole solution in the Kaluza-Klein spacetime is dual to the corresponding vacuum solution. Further we also obtain solution dual to flat space which would in general describe a massive global monopole in 4-dimensional Euclidean space and would have massless limit analogus to the 4-dimensional dual-flat solution.
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    New classes of black hole spacetime in 2+1 gravity
    (2015-03-01) Bose, Sukanta; Dadhich, Naresh; Kar, Syan
    New multi-parameter families of black holes in three-dimensional (3D) gravity are obtained. We apply the electrogravity transformation (which implies an exchange of the Ricci and Einstein tensors) to the 3D field equations to obtain these solutions. Several properties of these geometries, including the nature of the matter that threads them, are discussed. Some of these properties are found to be strikingly different from known black holes in (2+1) dimensions.
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    A novel approach to particle production in an uniform electric field
    (2015-03-01) Srinivasan, K.; Padmanabhan, T.
    We outline a different method of describing scalar field particle production: in a uniform electric field. In the standard approach, the (analytically continued) harmonic oscillator paradigm is important in describing particle production. In the gauges normally considered, in which the four vector potential depends only on the time or space coordinate, the system reduces to a non-relativistic effective Schri:idinger equation with an inverted oscillator potential. The Bogolubov coefficients are determined by tunnelling in this potential. In the Schwinger proper time method of determining the effective Lagrangian, the analytically continued propagator for the usual oscillator system is regarded as the correct propagator for the inverted oscillator system and is used to obtain the gauge invariant result. However, there is another gauge in which the particle production process has striking similarities with the one used to describe Hawking radiation in black holes. This gauge we use to describe the electric field in is the lightcone gauge, so named because the mode functions for a scalar field are found to be singular on the lightcone. We use these modes in evaluating the effective Lagrangian using the proper time technique. The key feature of this analysis is that these modes can be explicitly "normalized" by using the criterion that they reduce to the usual flat space modes in the limit of the electric field tending to zero. This normalization procedure allows one to determine the Schwinger proper time kernel without using the analytical continuation of the harmonic oscillator kernel that is resorted to in the standard analysis. We find that the proper time kernel is not the same as the analytically continued oscillator kernel though the effective Lagrangian is the standard result as it should be. We also consider an example of a confined electric field system using the lightcone gauge modes that has several features of interest. In particular, our analysis indicates that the Bogolubov coefficients, in taking the limit to the uniform electric field case, are multiplied by energy dependent boundary factors that have not been taken into account before.
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    Magnetic fields of neutron stars
    (2015-03-01) Konar, Sushan; Bhattacharya, Dipankar
    The evolution of the magnetic field is investigated for isolated as well as binary neutron stars. The overall nature of the field evolution is seen to be similar for an initial crustal field and an expelled flux. The major uncertainties of the present models of field evolution and the directions in which further investigation are required are also discussed in detail.
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    Transverse galaxy velocities from multiple topological images
    (2015-03-01) Roukema, B.F.; Bajtlik, Stanislaw
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    On the electrogravity-dual solution to stringy charged black holes
    (2015-03-01) Bose, Sukanta; Dadhich, Naresh
    By resolving the Riemann curvature tensor relative to a timelike unit vector into electric and magnetic parts, electrogravity-duality transformation is defined. Such a transformation interchanges the "active" and "passive" electric parts of this tensor and can be used to obtain spacetime metrics dual to known solutions of any geometric theory of gravity, such as Einstein gravity. The vacuum field equations of general relativity are invariant under such a transformation. It is possible to break this symmetry by introducing matter terms in such a way that the characteristic vacuum solution is still obtained as a special case of that matter distribution. Such a possibility exists for all stationary black hole solutions. Interestingly, solutions exist even for the dual equations with the matter terms. Here, we extend this formalism to study the static, charged black hole solutions of a four-dimensional low-energy effective action of heterotic string theory. We show that analogous to general relativity, the dual solution is itself a similar black hole spacetime, endowed with a global monopole charge.
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    Reconstructing the cosmic equation of state from supernova distances
    (2015-03-01) Saini, Tarun Deep; Raychaudhuri, Somak; Sahni, Varun; Strarobinsky, A.A.
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    Stellar orbits in triaxial clusters around black holes in galactic nuclei
    (2015-03-01) Sambhus, Niranjan; Sridhar, S.
    We investigate the orbital structure of a model triaxial star cluster, centered around a supermassive black hole (BH), appropriate to galactic nuclei. Sridhar and Touma {1999) proved that the presence of the BH enforces some regularity in the dynamics within the radius of influence of the BH. We employ their averaging method to reduce the degrees of freedom from three to two. Numerical orbit integrations, together with Poincare surfaces of section allow us to draw a global portrait of the orbital structure; in our calculations we employ a model cluster potential that is triaxial and harmonic. The averaged dynamics of the axisymmetric case is integrable, and we present a detailed comparison of orbits in oblate and prolate axisymmetric potentials. Both cases support resonant orbits with fixed values of eccentricity, inclination, and periapse, whose lines of nodes rotates steadily. These occur for all values of oblateness, but only for axis ratio greater than two, in the prolate case; we identify this phenomenon with the (in)stability of the long axis orbit. We then systematically explore significantly triaxial potentials, possessing small oblateness, or prolateness. Resonant orbits and their families are studied both numerically, and through secular perturbation theory. Chaos is highly suppressed for all the cases we studied, and we obtain effective third integrals. Some of the orbits appear to reinforce the shape of the potential; we provide phase space, as well as real space portraits of these orbits. A particularly promising resonant orbit exists in highly prolate, triaxial potentials.
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    A simple shear-free-non-singular spherical model with heat flux
    (2015-03-01) Dadhich, Naresh; Patel, L.K.
    We obtain an exact simple solution of the Einstein equation describing a spherically symmetric cosmological model without the bigbang or any other kind of singularity. The matter content of the model is shear free isotropic fluid with radial heat flux and it satisfies the weak and strong energy conditions. It is pressure gradient combined with heat flux that prevents occurrence of singularity. So far all known non-singular models have non-zero shear. This is the first shear free non-singular model, which is also spherically symmetric.
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    The quasi- steady state cosmology: A study of angular size against redshift
    (2015-03-01) Banerjee, S.K.; Narlikar, J.V.
    The data on angular sizes and redshifts of ultracompact radio sources used by Jackson and Dodgson in a recent paper have been applied to the various theoretical models in the framework of the quasi-steady state cosmology proposed by Hoyle, Burbidge and Narlikar. It is found that although acceptable fits to the data are available for the fiat models, those with a negative curvature of spatial sections provide a better fit. These latter models require low densities of matter and as such do not demand too high a proportion of dark matter. A comparison is made with the work of Jackson and Dodgson and theoretical implications of the test are discussed.
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    New BVR photometry of six prominent RS CVn Binaries
    (2015-03-01) Padmakar; Pandey, S. K.
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    A study o f quasar radio emission from the VLA first survey
    (2015-03-01) Wadadekar, Yogesh; Kembhavi, A.K.
    Using the most recent (1998) version of the VLA FIRST survey radio catalog, we have searched for radio emission from 1704 quasars taken from the most recent (1993) version of the Hewitt and Burbidge quasar catalog. These quasars lie in the "'5000 square degrees of sky already covered by the VLA FIRST survey. Our work has resulted in positive detection of radio emission from 389 quasars of which 69 quasars have been detected for the first time at radio wavelengths. We find no evidence of correlation between optical and radio luminosities for optically selected quasars. We find indications of a bimodal distribution of radio luminosity, even at a low flux limit of 1 mJy. We show that radio luminosity is a good discriminant between radio loud and radio quiet quasar populations, and that it may be inappropriate to make such a division on the basis of the radio to optical luminosity ratio. We discuss the dependence of the radio loud fraction on optical luminosity and redshift.