Scaling in Gravitational Clustering, 2D and 3D Dynamics

Perturbation Theory (PT) applied to a cosmological density field with Gaussian initial fluctuations suggests a specific hierarchy for the correlation functions when the variance is small. In particular quantitative predictions have been made for the moments and the shape of the one-point probability distribution function (PDF) of the top-hat smoothed density. In this paper we perform a series of systematic checks of these predictions against N-body computations both in 2D and 3D with a wide range of featureless power spectra. In agreement with previous studies, we found that the reconstructed PDF-s work remarkably well down to very low probabilities, even when the variance approaches unity. Our results for 2D reproduce the features for the 3D dynamics. In particular we found that the PT predictions are more accurate for spectra with less power on small scales. The nonlinear regime has been explored with various tools, PDF-s, moments and Void Probability Function (VPF). These studies have been done with unprecedented dynamical range, especially for the 2D case, allowing in particular more robust determinations of the asymptotic behaviour of the VPF. We have also introduced a new method to determine the moments based on the factorial moments. Results using this method and taking into account the finite volume effects are presented.Comment: 13 pages, Latex file, 9 Postscript Figure

Non-local scaling in two-dimensional gravitational clustering

Using an ensemble of high resolution 2D numerical simulations, we explore the scaling properties of cosmological density fluctuations in the non-linear regime. We study the scaling behaviour of the usual $N$--point volume-averaged correlations, and also examine the scaling of the entire probability density function (pdf) of the fluctuations. We focus on two important issues: (i) whether the scaling behaviour of 2D clustering is consistent with what one infer from radial collapse arguments; and (ii) whether there is any evidence from these high-resolution simulations that a regime of stable clustering is ever entered. We find that the answers are (i) yes and (ii) no. We further find that the behaviour of the highly non-linear regime in these simulations suggests the existence of a regime where the correlation function is independent of the initial power spectrum.Comment: 5 pages, Latex file, 2 Postscript Figure

Cosmology with weak lensing surveys

Weak gravitational lensing is responsible for the shearing and magnification of the images of high-redshift sources due to the presence of intervening mass. Since the lensing effects arise from deflections of the light rays due to fluctuations of the gravitational potential, they can be directly related to the underlying density field of the large-scale structures. Weak gravitational surveys are complimentary to both galaxy surveys and cosmic microwave background observations as they probe unbiased nonlinear matter power spectra at medium redshift. Ongoing CMB experiments such as WMAP and future Planck satellite mission will measure the standard cosmological parameters with unprecedented accuracy. The focus of attention will then shift to understanding the nature of dark matter and vacuum energy: several recent studies suggest that lensing is the best method for constraining the dark energy equation of state. During the next 5 year period ongoing and future weak lensing surveys such as the Supernova Anisotropy Probe (SNAP), Large-aperture Synoptic Survey Telescope (LSST) will play a major role in advancing our understanding of the universe in this direction. In this review article we describe various aspects of probing the matter power spectrum and the bispectrum and other related statistics with weak lensing surveys. This can be used to probe the background dynamics of the universe as well as the nature of dark matter and dark energy

Screening of a Moving Parton in the Quark-Gluon Plasma

The screening potential of a parton moving through a quark-gluon plasma is calculated using the semi-classical transport theory. An anisotropic potential showing a minimum in the direction of the parton velocity is found. As consequences possible new bound states and J/psi dissociation are discussed.Comment: 4 pages, 2 figures, final, extended version, to be published in Phys.Rev.

Sersiclets - A Matched Filter Extension of Shapelets for Weak Lensing Studies

The precision study of dark matter using weak lensing by large scale structure is strongly constrained by the accuracy with which one can measure galaxy shapes. Several methods have been devised but none have demonstrated the ability to reach the level of precision required by future weak lensing surveys. In this Letter we explore new avenues to the existing Shapelets approach, combining a priori knowledge of the galaxy profile with the power of orthogonal basis function decomposition. This Letter discusses the new issues raised by this matched filter approach and proposes promising alternatives to shape measurement techniques. In particular it appears that the use of a matched filter (e.g. Sersic profile) restricted to elliptical radial fitting functions resolves several well known Shapelet issues.Comment: 6 pages, 6 figures. MNRAS Accepte

Gravitational instability in the strongly nonlinear regime: A study of various approximations

We study the development of gravitational instability in the strongly non-linear regime. For this purpose we use a number of statistical indicators such as filamentary statistics, spectrum of overdense/underdense regions and the void probability function, each of which probes a particular aspect of gravitational clustering. We use these statistical indicators to discriminate between different approximations to gravitational instability which we test against N-body simulations. The approximations which we test are, the truncated Zel'dovich approximation (TZ), the adhesion model (AM), and the frozen flow (FF) and linear potential (LP) approximations. Of these we find that FF and LP break down relatively early, soon after the non-linear length scale exceeds $R_*$ -- the mean distance between peaks of the gravitational potential. The reason for this break down is easy to understand, particles in FF are constrained to follow the streamlines of the initial velocity field. Shell crossing is absent in this case and structure gradually freezes as particles begin to collect near minima of the gravitational potential. In LP particles follow the lines of force of the primordial potential, oscillating about its minima at late times when the non-linear length scale $k_{\rm NL}^{-1}\simeq R_*$. Unlike FF and LP the adhesion model (and to some extent TZ) continues to give accurate results even at late times when $k_{\rm NL}^{-1} \ge R_*$. This is because both AM and TZ useComment: mn.sty (Latex), 20 pages + 11 figures (not included: hardcopy available on request from [email protected]), Submitted to MNRAS, IUCAA-24/9