Research Papers (TP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/3
Browse
9 results
Search Results
Item 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.Item 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.Item Inflation for astronomers(Annual Reviews Inc., 1991-03-12) Narlikar, J. V.; Padmanabhan, T.Item Horizon problem and inflation(Indian Academy of Sciences, 1987-06-24) Padmanabhan, T.; Seshadri, T. R.We show that, the part of the universe that is observable today (in principle), could not have evolved out of a domain which was causally connected in the past. This and other issues related to horizon problem in inflationary models are discussed.Item Dynamical scenarios with unstable neutrinos(American Astronomical Society, 1987-04-15) Padmanabhan, T.; Vasanthi, M. M.We describe a cosmological scenario with an unstable warm dark matter candidate (a heavy neutrino νH with mass of ~120 eV) which decays into a “hot” particle (a light nuetrino νL with mass of ~6 eV) and a relativistic boson. Theoretical and observational constraints on such a model are discussed. We note that (1) decay of νH disrupts the condensates made of primordial νL, lowering their mass to acceptable values of ~1012 M⊙. (2) The relativistic boson can contribute nearly 0.25 to the total density in a “recent” decay. The model predicts two prominent scales in dark matter distribution: (i) a mass of about ~4 × 1012M⊙ around the galaxies, distributed over about ~200 kpc, and (ii)a smoother density of ~10-27 g cm-3 distributed over ~1 Mpc. We argued that the model agrees with observations at all scales.Item Gaussian states in the De-Sitter spacetime and the evolution of semiclassical density perturbations: 1. Homogeneous mode(Indian Academy of Sciences, 1989-08-08) Seshadri, T. R.; Padmanabhan, T.The evolution of Gaussian quantum states in the de Sitter phase of the early universe is investigated. The potential is approximated by that of an inverted oscillator. We study the origin and magnitude of the density perturbations with special emphasis on the nature of the semiclassical limits.Item Quantum fluctuations and the non-avoidance of singularities in bianchi type I cosmologies(Springer, 1980-04-28) Padmanabhan, T.An effective metric is defined and used for analyzing the quantum fluctuations in a classical geometry. Early work showing that quantum conformal fluctuations avoid the classical singularity in the case of spherically symmetric collapse is briefly reviewed it is shown that this result does not extend to anisotropic Bianchi type I cosmology. Here the dispersion in the fluctuations increase to slowly to quench the classical singularity. The singularity persists in the space-time describe by the effective metric.Item Quantum cosmology as a cure for three ailments of classical cosmology(Astronomical Society of India, 1983-08-12) Narlikar, J. V.; Padmanabhan, T.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.Item Patterns in non-linear gravitational clustering: a numerical investigation(American Astronomical Society, 1996-02-15) Padmanabhan, T.; Cen, Renyue; Ostriker, Jeremiah P.; Summers, F. J.The nonlinear clustering of dark matter particles in an expanding universe is usually studied by N-body simulations. One can gain some insight into this complex problem if simple relations between physical quantities in the linear and nonlinear regimes can be extracted from the results of N-body simulations. Hamilton and coworkers and Nityananda & Padmanabhan have made an attempt in this direction by relating the mean relative pair velocities to the mean correlation function in a useful manner. We investigate this relation and other closely related issues in detail for six different power spectra: power laws with spectral indexes n = -2 and -1; cold dark matter (CDM) and hot dark matter models with density parameter Ω = t1 a CDM model including a cosmological constant (Α) with ΩCDM = 0.4 and ΩΑ = 0.6; and an n = -1 model with Ω = 0.1. We find the following: (t) Power-law spectra lead to self-similar evolution in an Ω = 1 universe. (2) Stable clustering does not hold in an Ω = 1 universe to the extent that our simulations can ascertain. (3) Stable clustering is a better approximation in the case of an Ω < 1 universe in which structure formation freezes out at some low redshift. (4) The relation between the dimensionless pair velocity and the mean correlation function, ξ, is only approximately independent of the shape of the power spectrum. At the nonlinear end, the asymptotic value of the dimensionless pair velocity decreases with increasing small-scale power because the stable clustering assumption is not universally true. (5) The relation between the evolved ξ and the linear regime ξ is also not universal but shows a weak spectrum dependence. We present simple theoretical arguments for these conclusions.