Research Papers (TP)

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End date: 1999

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Now showing 1 - 10 of 127
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    Crisis in cosmology: Observational constraints on Omega and H
    (Overseas Publishers Association, 1996-03-18) Bagla, J. S.; Padmanabhan, T.; Narlikar, J. V.
    This review of recent observations of cosmological interest seeks to take stock of how they constrain the standard hot big bang models with or without inflation. We look at two specific series indicative of this class of models. In one series the flatness condition of inflation requires that the density parameter shall be unity. Of late this statement has been relaxed somewhat to include the cosmological constant also as a contributor to the density parameter. Hence we ha»e used this "generalised" flatness condition. The other series of models does not need (be cosmological constant but assumes that the curvature parameter k = -1. Both these models are currently being pushed as "the" models of the universe. The observational constraints used by us are the measurements of the Hubble constant and the deceleration parameter, the ages of globular clusters, the abundance of primordial deuterium, the abundance of rich clusters, the baryon content of galaxy clusters and the abundance of high rsdshift objects. These constraints essentially limit the allowed values of the cosmological parameters. Our findings are that with measurements within their quoted error bars, the available parameter space has shrunk to negligible proportions. For survival of the standard models, therefore, one needs to take recourse to two normally unpalatable steps: (i) to doubt the existing error bars and hope to expand them and (ii) to fine-tone the theoretical parameters so that they fall within the available space. This is the essence of our perception of the crisis in cosmology.
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    Cosmology Today: Models and constraints
    (Indian Academy of Sciences, 1995-03-12) Padmanabhan, T.
    Cosmological models for structure formation are severely constrained by several of the recent observational results. we now have observations which probe the power spectrum of fluctuations from about 0.5h-1 Mpc. these probes and the constraints they imply on models for structure formation are reviewed.
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    Cosmological N-Body Simulations
    (Indian Academy of Sciences, 1997-08-12) Bagla, J. S.; Padmanabhan, T.
    In this review we discuss Cosmological N-Body codes with a special emphasis on Particle Mesh codes. We present the mathematical model for each component of N-Body codes. We compare alternative methods for computing each quantity by calculating errors for each of the components. We suggest an optimum set of components that can be combined reduce overall errors in N-Body codes.
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    Constraints on unstable heavy neutrinos from cosmology
    (Indian Academy of Sciences, 1985-12-11) Vasanthi, M. M.; Padmanabhan, T.
    Cosmological scenario with massive unstable neutrinos are discussed. Restrictions on the mass and lifetime of the unstable neutrino are derived from (a) age and mass density of the universe and (b) the growth of primordial fluctuations. It will not be possible to accommodate unstable neutrinos with masses above ~ `1 keV in standard cosmology unless they have exceedingly small lifetime
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    Constraints on the shape of the density spectrum from COBE and galaxy surveys
    (Wiley-Blackwell, 1992-10-28) Padmanabhan, T.; Narasimha, D.
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    Conserved quantities from piecewise Killing vectors
    (IOP Publishing, 1988-11-10) Dray, Tevian; Padmanabhan, T.
    In the presence of symmetries, conserved quantities can be obtained by contracting the stress-energy tensor with a killing vector. We generalize this result to piecewise Killing vectors by giving sufficient conditions for the construction of an associated conserved quantity. A typical example namely, namely two stationary space-times joined together in such a way that the resulting space-time is not stationary, is treated in detail.
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    Conformal invariance, gravity and massive gauge theories
    (IOP Publishing, 1985-05-07) Padmanabhan, T.
    A gauge theory is constructed maintaining the invariance of the action of massless scalar fields under local conformal transformations. The theory leads to gravity in a natural fashion and also allows for the gauge field to be massive.
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    Is gravitational lensing a local phenomenon caused by Pop - III objects?
    (Gordon and Breach Science Publishers, 1986-01-31) Padmanabhan, T.; Chitre, S. M.
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    Inverse Compton Scattering – Revisited
    (Indian Academy of Sciences, 1996-11-30) Padmanabhan, T.
    The inverse Compton scattering of high energy electrons by photons is discussed and a simple derivation of the total power radiated is presented. The derivation is completely classical and exhibits clearly why similar formulas are applicable in the case of inverse compton scattering and synchroton radiation.
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    Instability of flat space and origin of conformal fluctuations
    (Elsevier Science Publishers, 1983-01-03) Padmanabhan, T.
    It is shown that conformal fluctuations in the metric can be initiated by the vacuum fluctuations of a scalar field with mass greater than the Planck mass. Flatspace is unstable against such fluctuations.