Research Papers (TP)

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Subject: search.filters.subject.Large scale structure of the universe

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    Semi-analytic approach to understanding the distribution of neutral hydrogen in the Universe
    (Wiley-Blackwell, 2000-05-15) Choudhury, T. Roy; Padmanabhan, T.; Srianand, R.
    Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the intergalactic medium (IGM) are presented, assuming that the non-linear baryonic mass density distribution in the IGM is lognormal. This ansatz was used earlier by Bi & Davidsen to perform one-dimensional simulations of lines of sight and analyse the properties of absorption systems. We have taken a completely analytic approach, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations to constrain various cosmological and IGM parameters, whenever possible. Two kinds of correlation functions are defined: (i) along the line of sight (LOS); and (ii) across the transverse direction. We find that the effects on the LOS correlation owing to changes in cosmology and the slope of the equation of state of the IGM, γ, are of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γ and its evolution using the observed LOS correlation function at different epochs provided that one knows the background cosmology. We suggest that the constraints on the evolution of γ obtained using the LOS correlation can be used as an independent tool to probe the reionization history of the Universe. From the transverse correlation function, we obtain the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ. We find that this excess probability is always less than 1 per cent for redshifts greater than 2. Our models also reproduce the observed column density distribution for neutral hydrogen, and the shape of the distribution depends on γ. Our calculations suggest that one can rule out γ>1.6 for z≃2.31 using the column density distribution. However, one cannot rule out higher values of γ at higher redshifts.
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    Nonlinear gravitational clustering: Dreams of a paradigm
    (American Astronomical Society, 1997-09-08) Padmanabhan, T.; Engineer, Sunu
    We discuss the late-time evolution of the gravitational clustering in an expanding universe, based on the nonlinear scaling relations (NSR) that connect the nonlinear and linear two-point correlation functions. The existence of critical indices for the NSR suggests that the evolution may proceed toward a universal profle that does not change its shape at late times. We begin by clarifying the relation between the density profles of the individual halos and the slope of the correlation function, and we discuss the conditions under which the slopes of the correlation function at the extreme nonlinear end can be independent of the initial power spectrum. If the evolution should lead to a profle that preserves the shape at late times, then the correlation function should grow as a2 (in a )\1 universe), even at nonlinear scales. We prove that such exact solutions do not exist ; however, there exists a class of solutions ("psuedolinear profles") that evolve as a2 to a good approximation. It turns out that pseudolinear profles are the correlation functions that arise if the individual halos are assumed to be isothermal spheres. They are also confgurations of mass in which the nonlinear effects of gravitational clustering are a minimum, and hence they can act as building blocks of the nonlinear universe. We discuss the implications of this result.
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    Nonlinear evolution of density perturbations using approximate constancy of gravitational potential
    (Royal Astronomical Society, 1993-07-28) Bagla, J. S.; Padmanabhan, T.
    During the evolution of density inhomogeneties in an Ω=1, matter dominated universe, the typical density contrast changes from δ≃ 10-4 to δ≃ 102. However, during the same time, the typical value of the gravitational potential generated by the perturbations changes only by a factor of order unity. This significant fact can be exploited to provide a new, powerful, approximation scheme for studying the formation of nonlinear structures in the universe. This scheme, discussed in this paper, evolves the initial perturbation using a Newtonian gravitational potential frozen in time. We carry out this procedure for different intial spectra and compare the results with the Zeldovich approximation and the frozen flow approximation (proposed by Mattarrese et al. recently). Our results are in far better agreement with the N-body simulations than the Zeldovich approximation. It also provides a dynamical explanation for the fact that pancakes remain thin during the evolution. While there is some superficial similarity between the frozen flow results and ours, they differ considerably in the velocity information. Actual shell crossing does occur in our approximation; also there is motion of particles along the pancakes leading to further clumping. These features are quite different from those in frozen flow model. We also discuss the evolution of the two-point correlation function in various approximations.
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    Nonlinear evolution of density perturbations
    (Indian Academy of Sciences, 1995-02-28) 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 a Friedmann universe with Ω= 1. However, during the same epoch the typical gravitational potential has grown only by a factor of order unity. This fact can be exploited to provide a new, powerful, approximation scheme to study the formation of nonlinear structures in the universe by evolving the initial distribution of matter using a gravitational potential frozen in time. We carry out this scheme for several standard models and discuss the results.
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    Neutral hydrogen at high redshifts as a probe of structure formation – III. Radio maps from N-body simulations.
    (Wiley-Blackwell, 1997-04-04) Bagla, J. S.; Nath, B. B.; Padmanabhan, T.
    Large inhomogeneities in neutral hydrogen in the universe can be detected at redshifts z 10 using the redshifted 21cm line emission. We use cosmological N-Body simulations for dark matter and a simple model for baryonic collapse to estimate the signal expected from structures like proto-clusters of galaxies at high redshifts.We study : (i) the standard CDM model, (ii) a modified CDM model with less power at small scales, and (iii) a +CDM model in a universe with 0 + = 1. We show that it should be possible for the next generation radio telescopes to detect such structures at the redshift 3.34 with an integration of about 100 hours. We also discuss possible schemes for enhancing signal to noise ratio to detect proto-condensates at high redshifts.