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Item Possible Newtonian interpretation of relativistic cosmological perturbation theory(2015-03-11) Nayeri, Ali; Padmanabhan, T.Cosmological perturbations with wavelengths smaller than Hubble radius can be handled in the context of Newtonian theory with very high accuracy. The application of this Newtonian approximation, however, is restricted to nonrelativistic matter and cannot be used for relativistic matter. Recently, by modifying the continuity equation, Lima, et. a!., extended the domain of applicability of Newtonian cosmology to radiation dominated phase. We adopted this continuity equation to re-examine linear cosmological perturbation theory for a two fluid universe with uniform pressure. We study the evolution equations for density contrasts and their validity in different epochs and on scales larger than Hubble radius and compare the results with the full relativistic approach. The comparison shows the high accuracy of this approximation.Item Crisis in cosmology : observational constraints on Ω and H0(2015-02-07) Bagla, J. S.; Padmanabhan, T.; Naralikar, J.V.Two decades ago, in an article in Nature, Gunn and Tinsley1 had reviewed the then available data in cosmology to conclude: " New Data on the Hubble diagram, combined with constraints on the density of the universe and the ages of galaxies, suggest that the most plausible cosmological models have a positive cosmological constant, are closed, too dense to make deuterium in the big bang, and will expand for ever ... ". Thanks to new technology of observations and fresh inputs from particle physics, cosmology has since advanced on both observational and theoretical fronts. The standard hot big bang model has, if at all, become more deeply rooted in cosmology today than in 1975. It is therefore opportune that we take fresh stock of the cosmological situation today and examine the observational and theoretical constraints as they are now. Not surprisingly, some of the issues discussed by Gunn and Tinsley [ op. cit.] continue to be relevant today whereas fresh ones have replaced the rest. The purpose of this article is to carry out a similar exercise in the modern cosmological framework. The bottom line in this review is that despite the availability of the cosmological constant as an extra parameter for flat Friedmann models, the allowed parameter space for such models has shrunk drastically. The observations that we will consider here include the ages of globular clusters, measurement of Hubble's constant, abundance of rich clusters of galaxies, fraction of mass contributed by baryons in rich clusters and abundance of high red shift objects. We begin with a brief description of the theoretical models in standard cosmology. For the notation the reader may refer to standard textbooks2 •Item Cosmology today-models and constraints(2015-01-17) Padmanabhan, T.Item 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.Item Evolution of the correlation function for a class of processes involving non local self-Replication(2002-07-06) Padmanabhan, T.A large class of evolutionary processes can be modeled by a rule that involves self-replication of some physical quantity with a non local rescaling. We show that a class of such models are exactly solvable — in the discrete as well as continuum limit — and can represent several physical situations as varied from the formation of galaxies in some cosmological models to growth of bacterial cultures. This class of models, in general, has no steady state solution and evolve unstably as t → ∞ for generic initial conditions. They can however exhibit (unstable) power law correlation function in the continuum limit, for an intermediate range of times and length scales.Item Cosmology with tachyon field as dark energy(2011-07-06) Bagla, J. S.; Jassal, H. K.; Padmanabhan, T.We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with non-relativistic matter and radiation, concentrating on the inverse square potential and the expo- nential potential for the tachyonic scalar field. A distinguishing feature of these models (compared to other cosmological models) is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with a(t) ∝ t 2/3 as t → ∞. This elimi- nates the future event horizon present in ΛCDM models and is an attractive feature from the string theory perspective. A comparison with supernova Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts and are also consistent with requirements of structure formation. They do require fine tuning of parameters but not any more than in the case of ΛCDM or quintessence models.Item Understanding Our Universe : Current Status and Open Issues(2005-03-01) Padmanabhan, T.Last couple of decades have been the golden age for cosmology. High quality data confirmed the broad paradigm of standard cosmology but have thrusted upon us a preposterous composition for the universe which defies any simple explanation, thereby posing probably the greatest challenge theoretical physics has ever faced. Several aspects of these developments are critically reviewed, concentrating on conceptual issues and open questions.