Research Publications

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Author: search.filters.author.Padmanabhan, T.End date: 2009

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Now showing 1 - 10 of 242
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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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    Temperature fluctuations of cosmic microwave background induced by gravitational lensing
    (Elsevier Science Publishers, 1986-06-26) Chitre, S. M.; Narlikar, J. V.; Padmanabhan, T.
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    On a nonlinear and lorentz-invariant version of newtonian gravitation-II
    (Indian Academy of Sciences, 1985-08-06) Narlikar, J. V.; Padmanabhan, T.
    This paper gives a full nonlinear version of Newtonian gravity in which the gravitational energy acts as a source of the gravitational field. The generalized field equation for the scalar gravitational potential is solved for a spherically symmetric localized distribution of matter. It is shown that the perihelia of orbits of test particles in such a field precess steadily. The effect is, however, too small to account for the observed shift in the perihelion of planet Mercury. Further, the bending of light in this theory is zero. It is suggested that these inadequacies of the quasi-Newtonian framework call for more sophisticated approaches to gravity.
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    Creation field cosmology : A possible solution to singularity, horizon and flatness problems
    (American Physical Society, 1985-10-15) Narlikar, J. V.; Padmanabhan, T.
    A solution of Einstein s equations which admits radiation and a negative-energy massless scalar creation field as a source is presented. It is shown that the cosmological model based on this solu- tion satisfies all the observational tests and thus is a viable alternative to the standard big-bang model. The present model is free from singularity and particle horizon and provides a natural ex- planation for the flatness problem. We argue that these features make the creation-field cosmologi- cal model theoretically superior to the big-bang model.
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    Quantum cosmology via path integrals
    (Elsevier Science Publishers, 1983-05-05) Narlikar, J. V.; Padmanabhan, T.
    The main purpose of this article is to report the progress of the path integral approach to quantum cosmology. Since quantum cosmology is an interdisciplinary field involving inputs from quantum theory, general relativity and cosmology, we begin with a brief survey of classical geometrodynamics and classical cosmology as well as an outline of the problems faced by any quantum theory of gravity. It is against this background that the authors’ approach described in sections 3—5 is to be viewed and assessed. The Feynman path integral formalism to the extent necessary for following this approach is described first in section 2. In section 3 it is shown that the limited goal of quantizing only the conformal part of the space-time metric can be reached with the help of path integral techniques. A case is made as to why this limited approach is still of relevance to quantum cosmology. Explicit examples are worked Out to show how meaningful conclusions can be drawn about quantum uncertainty at the classical singularity, the likelihood of singularity-free and horizon-free models in quantum cosmology and the limits on the validity of classical relativity close to the big bang. In section 4 the existence of stationary states of the universe is discussed. It is shown how the quantization of the conformal degree of freedom leads to stationary states for the quantum analogues of the classical models. The results are generalized and discussed in the framework of the superspace metric. The difficult problem of the back reaction of quantum conformal fluctuations on the space-time metric is tackled in a semiclassical fashion in section 5. In this approach the conformal part of the metric is treated classically while the conformal fluctuations are replaced by their expectation values. The resulting field equations are solved in a few simple cases and physically interpreted. This preliminary work holds promise for a more complete theory of the future. In the end a solution to the flatness problem of classical cosmology is suggested within the framework of conformal fluctuations.
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    Quantum cosmology as a cure of three ailments of classical cosmology
    (Astronomical Society of India, 1983-08-12) Padmanabhan, T.; Narlikar, J. V.
    The standard big bang models of classical cosmology are known to possess three defects. The oldest known defect is spacetime singularity whose existence seems inevitable within the classical framework. The second defect is the existence of a particle horizon which severely limits communications across the distant parts of the universe whose observed homogeneity therefore becomes inexplicable. Recently a third defect has been highlighted, viz., the required fine tuning of the early universe close to the flat spatial model in order to account for the present range of its mean density. We show before that the injection of quantum ideas holds out hope of a cure for all the three ailments described above. Using a simple path integral formalism for quantum cosmology we present arguments which suggest that (i) it is extremely unlikely that the universe evolved to the present state from quantum states of singularity and particles horizon;(ii) of all the possible Robertson-Walker models that could evolve our of quantum fluctuations of the empty Minkowski universe the flat model is overwhelmingly probable.
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    Problems of singularity, particle horizon and flatness in quantum cosmology
    (Elsevier Science Publishers, 1983-03-14) Narlikar, J. V.; Padmanabhan, T.
    Classical relativistic cosmology is known to have the space-time singularity as an inevitable feature The standard big bang models have very small particle horizons in the early stages which make it difTicult to understand the observed homogeneity in the universe. The relatively narrow range of the observed matter density in the neighbourhood of closure density requires highly fine tuning of the early universe. In this paper it is argued that these three problems can be satisfactorily resolved in quantum cosmology. It is shown that it is extremely unlikely that the universe evolved to the present state from quantum states with singularity and particle horizon. Similarly, it is shown that of all possible states the Robertson-Walker model of flat spatial sections is the most likely state for the universe to evolve out of a quantum fluctuation. To demonstrate these results a suitable formalism for quantum cosmology is first developed.
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    Quantum conformal fluctuations in a singular spacetime
    (Nature Publishing Group, 1982-02-25) Padmanabhan, T.; Narlikar, J. V.
    The cosmological solutions of Einstein's general relativistic equations lead inevitably to space-time singularities. However, general relativity is only an approximation to a fully quantized theory of gravity and we need to consider whether singularity persists in the quantum domain. Although a full quantum theory of gravity has not yet been developed, we show here that the above question can be tackled in a simplified model where only the conformal degree of freedom is quantized. Previous applications of this technique had shown that in specific cases the quantum conformai fluctuations (QCF) from the classical solutions diverge at the classical singularity, thus rendering the classical solution physically meaningless. Recently one of us (J.V.N. ref. 4) has generalized this result to cover all dust cosmologies. Here we show that this conclusion is applicable to even more general types of cosmological singularities.
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    Standard cosmology and alternatives : A critical appraisal
    (Annual Reviews Inc., 2001-10-20) Narlikar, J. V.; Padmanabhan, T.
    This review takes a critical look at the cosmological scenario at the turn of the century by examining the available cosmological models in the light of the present observational evidence. The center stage is held by the big bang models, which are collectively referred to here as standard cosmology (SC) and its extensions. SC itself is characterized by a seven parameter set of models based on Einstein’s general theory of relativity. The seven parameters are H0;ÄB ;ÄDM ;Ä3;ÄR (describing the background universe, and A, n (specifying the amplitude and power law index of initial fluctuation spectrum). The extended SC includes extrapolations of the SC to earlier epochs when the mean energies of the particles were greater than about 100 GeV. The strength of the SC is seen to lie in its successful prediction of the expansion of the universe, the abundance of light nuclei, and the spectrum and anisotropies of the cosmic microwave background (CMBR). The SC has led to a whole class of theories of structure formation, which are, in principle, testable observationally. The subject of twentieth century cosmology gained considerably from occasional ideas different from the SC; some of these are briefly outlined and placed in historical perspective. Currently there is only one alternative cosmology, the quasi steady state cosmology (QSSC), that has been developed to a stage where it can be compared with observations and also with the SC. Although the SC does appear quite successful, there are still many unresolved issues that keep the cosmological scene fairly open.
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    Stationary states in a quantum gravity model
    (Elsevier Science Publishers, 1981-05-05) Padmanabhan, T.; Narlikar, J. V.
    A model theory for quantized gravity is discussed whereonly selected degrees of freedomare quantized. The concept of stationary states is introduced. It is shown that the Planck length arises as a lower bound to the space—time length scale in a natural way.