1995 (IPP)

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Analysis of low z absorbers in the QSO spectra
(2015-02-07) Srianand, R.
Analysis of Ly α absorption lines in the vicinity of QSOS
(2015-02-07) Srianand, R.; Khare, Pushpa
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.
A class of solutions for a rotating perfect fluid in general relativity
(2015-01-27) Prasad, S.S.
Coincidence detection of broadband signals by networks of the planned interferometric gravitational wave detectors
(2015-01-25) Bhawal, Biplab; Dhurandhar, S.V.
We describe how the six planned detectors (2 LIGOs, VIRGO, GEO, AIGO, TAMA) can be used to perform coincidence experiments for the detection of broadband signals from either coalescing compact binaries or burst sources. We make comparisons of the achievable sensitivities of these detectors under different optical configurations and find that a meaningful coincidence experiment for the detection of coalescing binary signals can only be performed by a network where the LIGOs and VIRGO are operated in power recycling mode and other medium scale detectors are operated in dual recycling mode. For the model of burst waveform considered by us (i.e. uniform power upto 2000Hz), we find that the relative sensitivity of the power-recycled VIRGO is quite high as compared to others with their present design parameters and thus coincidence experiment performed by including VIRGO in the network would not be a meaningful one. We also calculate optimized values for the time-delay window sizes for different possible networks. The effect of filtering on the calculation of thresholds has also been discussed. We set the thresholds for different detectors and find out the volume of sky that can be covered by different possible networks and the corresponding rate of detection of coalescing binaries in the beginning of the next century. We note that a coincidence experiment of power-recycled LIGOs and VIRGO and dual-recycled GEO and AIGO can increase the volume of the sky covered by 3.2 times as compared with only the power-recycled LIGO detectors and by 1.7 times the sky covered by the power-recycled LIGO-VIRGO network. These values are far less than the range that can be covered by only the LIGO-VIRGO network with dual recycling operation at a later stage, but the accuracy in the determination of direction, distance and other source parameters will be much better in a coincidence experiment in which other detectors and especially AIGO take part.
Comparative performance of artificial neural networks for UV spectral classification
(2015-02-07) Mukherjee, Soma; Bhattacharya, Ujwal; Parui, S.K; Gupta, Ranjan; Gulati, R.K
In 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 databases
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 •
Cylindrically symmetric cosmological models in the Kaluza-Klein space time
(2015-01-27) Patel, L.K.; Dadhich, Naresh
We consider a non-diagonal cylindrically symmetric metric in the Kaluza-Klein spacetime. We obtain a number of homogeneous and inhomogeneous perfect fluid cosmological models, which include the 5-dimensional analogue of the recently found 4-dimensional non-singular stiff fluid model. Amongst the homogeneous models, which are all as expected big-bang singular, there is the 5-dimensional version of the Friedman-Robertson-Walker flat model.
Dynamical history of Ly α clouds
(2015-02-07) Srianand, R.
Estimation of parameters of gravitational waves from coalescing binaries
(2015-01-25) Balasubramanian, R.; Sathyaprakash, B.S.; Dhurandhar, S.V.
Exact solutions the field equations in the quasi-steady state cosmological model and relation to observations
(2015-01-25) Sachs, R.
Faint star count and the milky way structure
(2015-01-25) Ojha, Devendra
The milky way galaxy offers a unique opportunity for testing theories of galaxy formation and evolution. I discuss how large surveys, both photometric and astrometric, of galactic stars are the keystones of investigation in to such fundamental problems as the merging history and future of galaxy. This work features a sample survey plan to produce probes of stellar populations in the milky way. Objectives of this work are to trace the fine structure of our galaxy through the statistical study of the stellar distribution according to their luminosity, colors and proper motions. The work has two step: first acquiring a new photometric and astrometric sample survey in various galactic direction; secondly analyzing data using a model of population synthesis and determining the properties of population in the galaxy and constraints on the scenario of formation and evolution.
Gravitational dynamics in an expanding universe
(2015-01-27) Padmanabhan, T.
The dynamical evolution of collisionless particles in an expanding background is described. After discussing qualitatively the key features, the gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is indeed possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models. Several implications of the result are discussed
How empty must empty space be ?
(2015-02-07) Dadhich, Naresh
We 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.
Inhomogeneous cosmological models with heat flux
(2015-02-07) Patel, L.K.; Tikekar, R.; Dadhich, Naresh
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
Limits on the validity of the semiclassical theory
(2015-01-25) Sriramkumar, L.
For want of a more natural proposal, it is generally assumed that the back-reaction of a quantised matter field on a classical metric is given by the expectation value of its energy-momentum tensor, evaluated in a specified state. This proposal can be expected to be quite sound only when the fluctuations in the energy-momentum tensor of the quantum field are negligible. Based on this condition, a dimensionless criterion has been suggested earlier by Kuo and Ford for drawing the limits on the validity of this semiclassical theory. In this paper, we examine this criterion for the case of a toy model, constructed with two degrees of freedom and a coupling between them that exactly mimics the behaviour of a scalar field in a Friedmann universe. To reproduce the semiclassical regime of the field theory, in the toy model, one of degrees of freedom is assumed to be classical and the other quantum mechanical. Also the back reaction is assumed to be given by the expectation values of the quantum operators involved in the equations of motion for the classical system. Motivated by the same physical reasoning as Kuo and Ford, we, here, suggest another criterion, one which will be shown to perform more reliably as we evaluate these criterions for different states of the quantum system in the toy model. Finally, from the results obtained we conclude that the semiclassical theory being considered for the toy model is reliable, during all stages of its evolution, only if the quantum system is specified to be in coherent like states. The implications of these investigations on field theory are discussed.
Modelling the nonlinear gravitational clustering in the expanding universe
(2015-01-25) Padmanabhan, T.
The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is indeed possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in heirarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. The ideas presented here will also serve as a powerful anlytical tool to investigate nonlinear clustering in different models. Several implications of the result are discussed.
Motion of test particle around monopoles
(2015-02-07) Chakraborty, Subenoy
We present a detail analysis of the motion of test particles around gauge and global monopoles using the Hamilton-Jacabi (H-J) formalism. We find that particles cannot be trapped by gauge monopoles while there may exist bound orbits for global monopoles under certain conditions.
A new indicator of nonlinear gravitational clustering
(2015-01-25) Bagla, J. S.
Alignment of velocity and acceleration before shell crossing, and later misalignment are used to define velocity contrast, an indicator of dynamical state of matter undergoing gravitational collapse. We use this study bias in clustering properties of dynamically nonlinear mass.
A new statistical indicator to study nonlinear gravitational clustering and structure formation
(2015-01-25) Bagla, J. S.; Padmanabhan, T.
In an Ω = 1 universe dominated by nonrelativistic matter, velocity field and gravitational force field are proportional to each other in the linear regime. Neither of these quantities evolve in time and these can be scaled suitably so that the constant of proportionality is unity and velocity and force field are equal. The Zeldovich approximation extends this feature beyond the linear regime, until formation of pancakes. Nonlinear clustering which takes place after the breakdown of Zeldovich approximation, breaks this relation and the mismatch between these two vectors increases as the evolution proceeds. We suggest that the difference of these two vectors could form the basis for a powerful, new, statistical indicator of nonlinear clustering. We define an indicator called velocity contrast, study its behaviour using N-Body simulations and show that it can be used effectively to delineate the regions where nonlinear clustering has taken place. We discuss several features of this statistical indicator and provide simple analytic models to understand its behaviour. Particles with velocity contrast higher than a threshold have a correlation function which is biased with respect to the original sample. This bias factor is scale dependent and tends to unity at large scales