1992 (IPP)

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(2014-01-12)
Density perturbations, gravity waves and the cosmic microwave background
(2014-07-30) Souradeep, Tarun; Sahni, Varun
The soft x-ray excess in quasars
(2014-11-22) Kembhavi, A.K.; Fabian, A.C.
A soft x-ray excess in the energy range of a few tenths of a ke V has been detected in some quasars , and there are indications in the x-ray data which point to the excess being a feature universal to all quasars. The poor energy resolution in the concerned energy band make it impossible at the present to observationally constrain the excess quantitatively. We show in this paper that the shape of the excess sensitively determines the surface density therefore allows an assessment of the nature of the excess
Post-COBE prediction for inflationary gravity wave and density perturbation spectra
(2014-11-22) Sahni, Varun;Souradeep,Tarun
We assess the relative contribution to the COBE - measured microwave anisotropy arising both from relic gravity waves as well as primordial density perturbations originating during inflation. We show that the gravity wave contribution to the CMBR anisotropy depends sensitively upon n -- the primordial spectral index ($\dk^2 propto k^n$), increasing as n deviates from a Harrison - Zeldovich spectrum (n = 1). As a result, for n<0.84 the contribution from gravity waves towards δT/T is greater than the corresponding contribution from density perturbations, whereas for n>0.84 the reverse is true. (n=0.84 corresponds to an expansion index p=13.5 in models with power-law inflation a∝tp. ) Our results show that for a scale-invariant Harrison-Zeldovich spectrum generated by chaotic inflation, gravity waves contribute approximately 24% to the CMBR anisotropy measured by COBE. Applying our results to the cold dark matter scenario for galaxy formation, we find that in general CDM models with tilted power spectra (n<1), require the biasing parameter to be greater than unity, on scales of 16h−150Mpc. We also obtain an expression for the COBE - normalised amplitude and spectrum of the stochastic gravity wave background and compare it with the sensitivity of planned laser-interferometer gravity wave detectors.
Neutral hydrogen at high redshifts as a probe of structure formation : 1. Post COBE analysis of CDM and HDM models
(2014-11-23) Subramanian, Kandaswamy; Padmanabhan, T.
On gravitational charge of stationary spacetimes
(2014-11-23) Dadhich, Naresh
We point out that ours is a definition of gravitational charge and not of quasilocal mass [1] we have defined gravitational charge as the flux of effective gravitational acceleration across a closed 2- surface. It turns out that tidal force can change its sense from convergence to divergence outside the horizon for suitable range of values of rotation and electric charge parameters of the black hole.
A parallel algorithm for filtering gravitational waves from coalescing binaries
(2014-11-23) Sathyaprakash, B.S.; Dhurandhar, S.V.
Coalescing binary stars are perhaps the most promising sources for the observation of gravitational waves with laser interferometric gravity wave detectors. The waveform from these sources can be predicted with sufficient accuracy for matched filtering techniques to be applied. In this paper we present a parallel algorithm for detecting signals from coalescing compact binaries by the method of matched filtering. We also report the details of its implementation on a 256-node connection machine consisting of a network of transputers. The results of our analysis indicate that parallel processing is a promising approach to on-line analysis of data from gravitational wave detectors to filter out coalescing binary signals. The algorithm described is quite general in that the kernel of the algorithm is applicable to any set of matched filters.
Spectroscopy of the recurrent nova V3890 sagittarii 18 days after the 1990 outburst
(2014-11-23) Anupama, G. C.; Sethi, S.
The spectrum of the recurrent nova V3890 Sgr, obtained from the Vainu Bappu Observatory 18 d after the 1990 outburst maximum, is presented. The nova was in the coronal line phase. The spectrum is similar to that of the recurrent nova RS Oph. An extinction E(B - V) = 1.1 is derived from the B - V colours, and from Balmer and He I line ratios. From the maximum magnitude-rate-of-decline relations for a nova, MV = -8.6 mag is estimated, which places the nova at a distance of about 5 kpc. Balmer line fluxes are used to derive the density, ˜109 cm-3, and the mass of the ejected ionized shell, ˜10-7 Msun. The temperature and radius estimates for the ionizing source are 3 × 105 K and 0.06 Rsun. A helium abundance of 0.23 is estimated. Applying expansion models in which the fast moving nova eject interact with a slow moving pre-outburst wind, we conclude that the eject expanded into an inhomogeneous stellar wind.
Singularity free inhomogeneous viscous fluid cosmological models
(2014-11-23) Patel, L.K.; Dadhich, Naresh
We obtain two new classes of singularity free inhomogeneous viscous fluid cosmological solutions, which reduce to the Senovilla [1] class when viscosity is switched off. It is interesting to note that inclusion of viscosity does not disturb the singularity free character and general behavior of the models remains qualitatively same.
On active and passive gravitational mass of fluid sphere
(2014-11-23) Dadhich, Naresh
Nonlinear sigma models for inflation scenarios
(2014-11-23) Chervon, S.V.
In this paper new implementation from nonlinear sigma models are presented for cosmological inflationary scenarios. It is shown that the scalar field models with any form of a potential of self –interaction can be derived from the appropriate nonlinear sigma model. The complete set of new exact salutation for the two- component sigma model in the spatially flat Friedmann-Robertson-Walker and de sitter universes is obtained.
On the missing interstellar comets
(2014-11-23) Sen, A.K.; Rana, N. C.
Comets are supposed to have been born in the outer part of the solar nebula and later thrown into their present location of the Oort cloud by perturbations of the giant planets. This process was inefficient and the majority of the comets were totally lost into the interstellar medium, instead of being trapped in the Oort cloud. Assuming stars in the solar neighbourhood to have formed in the same way as the Sun, one can find the number of interstellar comets and also estimate the expected number of detectable interstellar comets (McGlynn & Chapman 1989). In this work, using our present day knowledge of the local interstellar medium, we first calculate the population of interstellar comets within the past 150 year or so, is therefore nothing unusual and fully consistent with the existing models of the oort cloud and solar system formation.
Constraints on the shape of the density spectrum from COBE and galaxy surveys
(2014-11-23) Padmanabhan, T.; Narasimha, D.
Zeldovich approximation and the probability distribution for the smoothed density field in the nonlinear regime
(2014-11-23) Padmanabhan, T.; Subramanian, Kandaswamy
The study of large-scale structure in the Universe is often based on the observed density distribution of matter smoothed by a suitable filter function. The probability distribution for this smoothed density field in the nonlinear regime is studied using the Zeldovich approximation. When the shear term of the velocity field is not too large, one can obtain a reasonably good analytic approximation to this probability distribution. The properties of this distribution are discussed and compared with other attempts along similar lines.
Gravity without metric
(2014-11-25) Kshirsagar, A.K.
A recent development in gravity theories is described. It is shown that a complete description of gravity is possible without explicit reference to metric at all, instead in term of spin-connection variable. This represent a natural covariantisation of Ashtekars new variables approach to gravity. The gravity without metric action is particularly suitable for a Yang-Mills type quantization. I describe this theory in some detail, in a non-technical manner.
Losses in pendular suspension due to centrifugal coupling
(2014-11-25) Pitre, Sangita. N.; Dhurandhar, S.V.; Blair, D.G.; Li, Ju
We present an analysis of the centrifugal coupling of simple pendulum to a dissipative support. We show that such a coupling leads to an amplitude dependent quality factor. For amplitudes which could be present in laser interferometer gravitational wave detector suspension, this mechanism could limit the quality factor of the test mass suspension significantly to 1010 and should be considered in the design of advanced LIGO type detectors.
Cylindrical universes with heat and null radiation flow
(2014-11-25) Patel, L.K.; Dadhich, Naresh
In paper-I(Patel and Dadhich ,1992) we have discussed cylindrically symmetric viscous fluid models with the Kasnerian time evolution. In this paper we incorporate heat flow and null radiation flow with the perfect fluid. Here again, in the case of heat flow the Kasner spacetime is the matter-free limit of the model. We establish a general result that a static perfect fluid distribution will on Kasnerization (introduce t4/3, t4/3, and t−2/3 in the coefficients of dr2, dΦ2, and dz2) yield a time-dependent distribution with the same equation of state and with or without heat flow.
Choice of filters for the detection of gravitational waves from coalescing binaries II: detection in coloured noise
(2014-11-25) Dhurandhar, S.V.; Sathyaprakash, B.S.
We discuss the problem of detecting gravitational waves signals embedded in coloured noise from coalescing binary systems. The signal is assumed to be Newtonian and matched filtering techniques are employed to filter out the signal. The problem is discussed at first for a general power spectral density of the noise and then specific numerical results are obtained for the standard recycling case. Since the signal parameters are unknown, a bank of filters is needed to carry out the signal detection. The number of filters in a bank , the spacing between filter etc. is obainted for different values of minimum strength of the signal relative to the threshold. We also present an approximate analytical formula which relates the spacing between filter to the minimum strength. Finally we discuss the problem of detection probabilities given a data train.
The search for gravitational waves
(2014-11-26) Dhurandhar, S.V.
The direct detection of gravitational waves is one of the most challenging problems in experimental physics today. Sustained efforts have led to impressive advances on both theoretical and experimental fronts. This article first deals with the theoretical aspects of gravitational waves and then continues on to describe the detectors in vogue, the present status and various gravitational waves sources.
Generation of seed perturbations from quantum cosmology
(2014-11-26) Souradeep, Tarun
The origin of seed perturbations in the Universe is studied within the framework of a specific mini super space model. It is shown that the ‘creation’ of the Universe as a result of a quantum transition from a flat empty space time would lead to a flat FLRW (Friedmann Lemaitre Robertson- Walker) Universe with weak inhomogeneous perturbations at large wavelengths. The power spectrum of these perturbations is found to be scale invariant at horizon crossing (i.e., the Harrison- Zeldovich spectrum). It is also recognised that the seed perturbations generated in our model would be generically of the isocurvature kind.