1995 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/2812
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Item Estimation of parameters of gravitational waves from coalescing binaries(2015-01-25) Balasubramanian, R.; Sathyaprakash, B.S.; Dhurandhar, S.V.Item Faint star count and the milky way structure(2015-01-25) Ojha, DevendraThe 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.Item UV spectrum of λ boo(2015-01-25) Gerbaldi, M.; Gulati, R.K; Faraggiana, R.; Kurucz, R.L.Abstract. By using stellar computed atmospheric models with ATLAS9 and ATLAS12 codes, we compare fluxes in the ultra-violet domain with the one observed in low dispersion mode of the IUE satellite for the star λ Boo. We derive the chemical abundance of zinc and chromium from high dispersion IUE data by applying spectrum synthesis technique.Item 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 scalesItem 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.Item The thick disc of the galaxy: Sequel of merging event(2015-01-25) Robin, A.C.; Haywood, Misha; Creze, Michel; Ojha, Devendra; Bienayme, OlivierItem 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.Item 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.Item 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.Item The role of shear in expanding cylindrical perfect fluid models(2015-01-27) Dadhich, Naresh; Patel, L.K.For orthogonal cylindrically symmetric expanding perfect fluid spacetime we prove that vanishing of shear implies vanishing of acceleration which further renders spacetime homogeneous. That means inhomogeneous spacetimes must always be shearing and anisotropic. Non-singular spacetimes will thus be both inhomogeneous and anisotropic.Item Cylindrically symmetric cosmological models in the Kaluza-Klein space time(2015-01-27) Patel, L.K.; Dadhich, NareshWe 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.Item On the evolution of magnetic moment of pulsars(2015-01-27) Pandey, U.S.; Prasad, S.S.Item On the weakest falloff conditions in the metric for an isolated system(2015-01-27) Petrov, A.N.The weakest asymptotic behavior in the metric for defining an asymptotically fiat spacetime at spatial infinity is obtained. The technique of the field formulation of general relativity developed earlier in the Lagrangian description is used. The properties of the latter are similar to those of an ordinary gauge field theory in a fixed background spacetime. The role of the auxiliary background is plaid by Minkowski space. Integrals Of motion are defined with the help of a stress-energy tensor of the gravitational field together with its sources and Killing vectors of the background spacetime. It is shown that the weakest asymptotics of gauge transformations, which conserve values of the integrals of motion, defines the weakest falloff conditions in the field gravitational potentials (the same, in the dynamic metric of general relativity if the ordinary geometrical formulation is used). The results are compared with the some known ones.Item A class of solutions for a rotating perfect fluid in general relativity(2015-01-27) Prasad, S.S.Item 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 discussedItem Plane symmetric perfect fluid distributions admitting a one-parameter- group of conformal motion(2015-01-27) Prasad, S.S.; Pandey, U.S.Some exact analytic solutions of Einstein's equations with perfect fluid source have been found, under the assumptions of (i) plane symmetry and (ii) the existence of a one- parameter group of conformal motions, with the generator in the hypersurface. The solutions are algebraically special (Petrov type D) and belong to class I of Wainwright classifications. They are non- static with non- vanishing shear. First of the solutions represent an expanding homogeneous distribution of matter, which evolves from a singular state at t=O. Second one is conformally fiat and represents homogeneous density, but inhomogeneous pressure distributions with shear - free motions. The last solution is inhomogeneous in density as well as pressureItem String-dust perfect fluid in singularity-free models(2015-01-27) Padmanabhan, T.We have attempted to incorporate string-dust in perfect fluid singularity free inhomogeneous models. It turns out the stiff fluid can only sustain inclusion of string-dustItem On the interpretation of the He II absorption in the line of sight of Q0302-003(2015-01-27) Nath, B. B.; Sethi, Shiv K.We point out the peculiarities of the line of sight of Q0302-003 which was recently used to estimate the He II opacity at z∼3.285. This line of sight contains a large void in Lyman α clouds, in the region used for the He II opacity observation, and the void is thought to be caused by the ionizing radiation from a foreground quasar (Dobrzycki and Bechtold 1991). If this is true, then the radiation from the foreground quasar is more relevant in the region used for the estimation of the He II opacity than the diffuse UV background radiation. We argue that in that case the observed opacity should not be used to put constraint on the diffuse UV background radiation. Using a typical spectral index of quasars, we estimate that most of the observed He II opacity must be due to Gunn-Peterson effect and suggest a H I Gunn-Peterson test along this line of sight. We further discuss the clumpiness of the intergalactic medium in the vicinity of Q0302-003 as suggested by observationsItem String-dust in Einstein and godel universes(2015-01-27) Dadhich, Naresh; Patel, L.K.We consider the mixture of perfect fluid and string-dust and obtain string-dust generalization of the Einstein and godel universes.