1993 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/2786

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Start date: 2015End date: 2015

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Now showing 1 - 10 of 22
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    On the smallness of the cosmological constant
    (2015-01-17) Masafumi, Seriu,
    We investigate the possible values of the cosmological constant allowed by quantum cosmology. If we formulate quantum cosmology respecting the causal nature of fundamental equations in the semiclassical regime of the universe, then any classical universe should have at least one symmetric surface on which every component of the extrinsic curvature vanishes. Combined with the Hamiltonian constrain, this implies that the allowed values of the cosmological constant are bounded from above. Applying this argument to the Robertson- Walker Universe, we obtain the theoretical upperbound for the cosmological constant, being of order (H0 / c)2 . This upperbound can also be interpreted as being determined by the adiabatic Schwarzshild radius of the whole universe. In this way, the question as to why the cosmological constant is so small is reduced to the question, why there is so much matter in our universe.
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    The evolution voids in the adhesion approximation
    (2015-01-17) Sahni, Varun; Sathyaprakash, B.S.; Shandarin, S.F.
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    Cosmology today-models and constraints
    (2015-01-17) Padmanabhan, T.
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    Nonlinear sigma model for inflation scenarios.II
    (2015-01-17) Chervon, S.V.
    In this paper the self –gravitating nonlinear sigma model is considered in the context of inflation scenarios as an alternative to the self-interacting scalar field theory. The complete set of new exact solutions for the two-component sigma model in the frame work of spatially flat friendmann- Robertson- Walker and de sitter universes is obtained.
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    A model for the declining rotation curves of cluster spirals
    (2015-01-17) Valluri, Monica
    Numerical studies have shown that mean tidal field in clusters of galaxies can cause a significant increase in the planar velocity dispersions of stars, and gas clouds in disks of spiral galaxies. Since the perturbation caused by cluster tidal field are generally weak and fairly symmetric, the stellar distribution of galaxies would not necessary show obvious signs of tidal disturbance. In this paper it is shown that the increased non-circular velocities of stars can result in symmetric decline (at large radii) of the rotation curves of the perturbed spiral galaxies. This decline result from the well known phenomenon of the “asymmetric drift” a high velocity dispersion stellar population. This suggest that the observation of the declining rotation curves of spiral galaxies in clusters do not necessarily imply that their dark matter halos have been tidally truncated as is generally assumed.
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    Signal analysis of the gravitational waveform of pulsars
    (2015-01-17) Jotania, Kanti; Dhurandhar, S.V.; Valluri, S.R.
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    Nonlinear evolution of density perturbation
    (2015-01-17) Bagla, J. S.; Padmanabhan, T.
    From the epoch of recombination ( Z ≈103 ) till today, the typical density contrasts have grown by a factor of about 106 in Friedmann universe with Ω = 1. However, during the same epoch the typical gravitational potential has grown only by a factor of order unity. We present theoretical arguments explaining the origin of this approximate constancy of gravitational potential. This fact can be exploited to provide a new, powerful, approximation scheme to study the formation of nonlinear structures in the universe. The essential idea of this method is to evolve the initial distribution of particles using a gravitational potential frozen in time. We carry out this scheme for several standard models including the CDM and HDM and show that the results match quite well with those obtained by exact Numerical simulations. We compute different statistical measures of clustering and compare them for the description of nonlinear evolution. This approximation also provides valuable insight into understanding various features of nonlinear evolution; for example, it provides a simple explanation as to why pancakes remain thin during the evolution even in the absence of any artificial, adhesion-like, damping terms. We also compare this approximation with other schemes like Zeldovich approximation and frozen-flow. Our procedure has a far greater range of validity than the Zeldovic h approximation since it can handle motion across (and inside) caustic properly. Unlike in frozen-flow, actual shell-crossing does occur in the frozen-potential approximation; hence it provides a far more accurate description of the velocity field compared to frozen flow approximation.
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    Performance of Newtonian filters in the detection of gravitational radiation from coalescing binaries
    (2015-01-13) Balasubramanian, R.; Dhurandhar, S.V.
    Post – Newtonian corrections to the gravitational waveform emitted by coalescing binaries have been found to lead to a secular phase accumulation error as compared with the signals calculated in the Newtonian approximation. The matched filtering process which relies on the correlation between the correlation between the signal and the filter is extremely sensitive to errors in phase. We explore the possibility of compensating for the phase difference caused by the post-Newtonian terms by allowing for a shift in the Newtonian filter parameters. We find that, on the average, we lose by about 30% in the correlation.