1995 (IPP)
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Item Dynamical history of Ly α clouds(2015-02-07) Srianand, R.Item Analysis of low z absorbers in the QSO spectra(2015-02-07) Srianand, R.Item The thermo-optical coupling in optical resonators(2015-02-07) Dhurandhar, S.V.; Hello, P.; Sathyaprakash, B.S.Interferometric detectors of gravitational waves employ long baseline FabryPerot cavities with stored power of the order of 10 kW. The mirrors have a high reflectivity with absorption coefficient of a few parts in a million. The laser beam therefore acts as a source of heat creating a thermal gradient in the substrate and the consequent deformation in the mirror which in turn modifies the intra-cavity light field. The problem is thus coupled and nonlinear. Though the effect is expected to be negligible in the case of initial interferometers future interferometers are expected to employ much higher powers and it is necessary to ascertain thermo-elastic deformations and their effect on the stability of the laser field in the cavity. In this paper, which is first in a series to study instabilities in giant high power laser cavities, we have analytically solved the coupled problem of thermo-elastic deformations and their effect on the laser field, perturbatively and we show that within the realm of our (physically reasonable) assumptions there are no instabilities in the frequency range of 1 Hz-1 kHz.Item Analysis of Ly α absorption lines in the vicinity of QSOS(2015-02-07) Srianand, R.; Khare, PushpaItem Crisis in cosmology : observational constraints on Ω and H0(2015-02-07) Bagla, J. S.; Padmanabhan, T.; Naralikar, J.V.Two decades ago, in an article in Nature, Gunn and Tinsley1 had reviewed the then available data in cosmology to conclude: " New Data on the Hubble diagram, combined with constraints on the density of the universe and the ages of galaxies, suggest that the most plausible cosmological models have a positive cosmological constant, are closed, too dense to make deuterium in the big bang, and will expand for ever ... ". Thanks to new technology of observations and fresh inputs from particle physics, cosmology has since advanced on both observational and theoretical fronts. The standard hot big bang model has, if at all, become more deeply rooted in cosmology today than in 1975. It is therefore opportune that we take fresh stock of the cosmological situation today and examine the observational and theoretical constraints as they are now. Not surprisingly, some of the issues discussed by Gunn and Tinsley [ op. cit.] continue to be relevant today whereas fresh ones have replaced the rest. The purpose of this article is to carry out a similar exercise in the modern cosmological framework. The bottom line in this review is that despite the availability of the cosmological constant as an extra parameter for flat Friedmann models, the allowed parameter space for such models has shrunk drastically. The observations that we will consider here include the ages of globular clusters, measurement of Hubble's constant, abundance of rich clusters of galaxies, fraction of mass contributed by baryons in rich clusters and abundance of high red shift objects. We begin with a brief description of the theoretical models in standard cosmology. For the notation the reader may refer to standard textbooks2 •Item Inhomogeneous cosmological models with heat flux(2015-02-07) Patel, L.K.; Tikekar, R.; Dadhich, NareshWe 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 givenItem How empty must empty space be ?(2015-02-07) Dadhich, NareshWe propose a generalised definition of empty space in general relativity which is characterised by vanishing of gravitational charge density Rikuiuk = 0 instead of Rik = 0. It can be argued that so long as there exists energy distribution outside the empty space region, the generalised definition seems to be more appropriate for its description.Item Behaviour of lagrangian approximations in spherical voids(2015-02-07) Sahni, Varun; Shandarin, S.F.We study the behaviour of spherical voids in Lagrangian perturbation theories L(n), of which the Zel'dovich approximation is the lowest order solution L(1). We find that at early times higher order L(n), give an increasingly accurate picture of void expansion. However, at late times particle trajectories in L (2) begin to turnaround and converge leading to the contraction of a void, a sign of pathological behaviour. By contrast particle trajectories in L(3) are well behaved and this approximation gives results in excellent agreement with the exact top-hat solution as long as the void is not too underdense. For very underdense voids, L (3) evacuates the void much too rapidly, leading us to conclude that the Zel'dovich approximation L(1) remains the best approximation to apply to the late-time study of voids. The behaviour of high-order Lagrangian approximations in spherical voids is typical for asymptotic (semiconvergent) series and may be generic for Lagrangian perturbation theory.Item Comparative performance of artificial neural networks for UV spectral classification(2015-02-07) Mukherjee, Soma; Bhattacharya, Ujwal; Parui, S.K; Gupta, Ranjan; Gulati, R.KIn this paper we present an application of an artificial neural network model based on a multi-layered back propagation algorithm for spectral classification of UV data from the International Ultraviolet Explorer (IUE) low dispersion spectra reference atlas. The model used is similar to that of von Rippel et al. (1994), and is found to reduce the classification error as compared to .the recently reported results on the same data set (Gulati et al. 1994b ). The improved version of the network is much simpler in structure and the training time is reduced by a factor of almost 20. Such networks will prove very useful in efficient classification of large databasesItem Structure and kinematical properties of the galaxy at intermediate galactic latitudes.(2015-02-07) Ojha, D.K.; Bienayme, Olivier; Robin, A.C.; Creze, Michel; Mohan, V.