Mustapha Ishak

Detection of large scale intrinsic ellipticity-density correlation from the sloan digital sky survey and implications for weak lensing surveys

The power spectrum of weak lensing shear caused by large-scale structure is an emerging tool for precision cosmology, in particular for measuring the effects of dark energy on the growth of structure at low redshift. One potential source of systematic error is intrinsic alignments of ellipticities of neighbouring galaxies [the intrinsic ellipticity-intrinsic ellipticity (II) correlation] that could mimic the correlations due to lensing. A related possibility pointed out by Hirata & Seljak is correlation between the intrinsic ellipticities of galaxies and the density field responsible for gravitational lensing shear [the gravitational shear-intrinsic ellipticity (GI) correlation]. We present constraints on both the II and GI correlations using 265 908 spectroscopic galaxies from the Sloan Digital Sky Survey (SDSS) and using galaxies as tracers of the mass in the case of the GI analysis. The availability of redshifts in the SDSS allows us to select galaxies at small radial separations, which both reduces noise in the intrinsic alignment measurement and suppresses galaxy-galaxy lensing (which otherwise swamps the GI correlation). While we find no detection of the II correlation, our results are none the less statistically consistent with recent detections found using the SuperCOSMOS survey. Extrapolation of these limits to cosmic shear surveys at z ∼ 1 suggests that the II correlation is unlikely to have been a significant source of error for current measurements of σ 8 with ∼ 10 per cent accuracy, but may still be an issue for future surveys with projected statistical errors below the 1 per cent level unless eliminated using photometric redshifts. In contrast, we have a clear detection of GI correlation in galaxies brighter than L. that persists to the largest scales probed (60 h -1 Mpc) and with a sign predicted by theoretical models. This correlation could cause the existing lensing surveys at z ∼ 1 to underestimate the linear amplitude of fluctuations by as much as 20 per cent depending on the source sample used, while for surveys at z ∼ 0.5 the underestimation may reach 30 per cent. The GI contamination is dominated by the brightest galaxies, possibly due to the alignment of brightest cluster galaxies (BCGs) with the cluster ellipticity due to anisotropic infall along filaments, although other sources of contamination cannot be excluded at this point. We propose that cosmic shear surveys should consider rejection of BCGs from their source catalogues as a test for GI contamination. Future high-precision weak lensing surveys must develop methods to search for and remove this contamination if they are to achieve their promise.

Probing cosmic acceleration beyond the equation of state: Distinguishing between dark energy and modified gravity models

If general relativity is the correct theory of physics on large scales, then there is a differential equation that relates the Hubble expansion function, inferred from measurements of angular diameter distance and luminosity distance, to the growth rate of large scale structure. For a dark energy fluid without couplings or an unusual sound speed, deviations from this consistency relationship could be the signature of modified gravity on cosmological scales. We propose a procedure based on this consistency relation in order to distinguish between some dark energy models and modified gravity models. The procedure uses different combinations of cosmological observations and is able to find inconsistencies when present. As an example, we apply the procedure to a universe described by a recently proposed 5-dimensional modified gravity model. We show that this leads to an inconsistency within the dark energy parameter space detectable by future experiments.

Intrinsic galaxy alignments from the 2SLAQ and SDSS surveys: Luminosity and redshift scalings and implications for weak lensing surveys

Correlations between intrinsic shear and the density field on large scales, a potentially important contaminant for cosmic shear surveys, have been robustly detected at low redshifts with bright galaxies in Sloan Digital Sky Survey (SDSS) data. Here we present a more detailed characterization of this effect, which can cause anticorrelations between gravitational lensing shear and intrinsic ellipticity (GI correlations). This measurement uses 36 278 luminous red galaxies (LRGs) from the SDSS spectroscopic sample with 0.15 3σ detections of the effect on large scales (up to 60 h−1 Mpc) for all galaxy subsamples within the SDSS LRG sample; for the 2SLAQ sample, we find a 2σ detection for a bright subsample, and no detection for a fainter subsample. Fitting formulae are provided for the scaling of the GI correlations with luminosity, transverse separation and redshift (for which the 2SLAQ sample, while small, provides crucial constraints due to its longer baseline in redshift). We estimate contamination in the measurement of σ8 for future cosmic shear surveys on the basis of the fitted dependence of GI correlations on galaxy properties. We find contamination to the power spectrum ranging from −1.5 per cent (optimistic) to −33 per cent (pessimistic) for a toy cosmic shear survey using all galaxies to a depth of R = 24 using scales l ≈ 500, though the central value of predicted contamination is −6.5 per cent. This corresponds to a bias in σ8 of Δσ8 = −0.004 (optimistic), −0.02 (central) or −0.10 (pessimistic). We provide a prescription for inclusion of this error in cosmological parameter estimation codes. The principal uncertainty is in the treatment of the L ≤ L blue galaxies, for which we have no detection of the GI signal, but which could dominate the GI contamination if their GI amplitude is near our upper limits. Characterization of the tidal alignments of these galaxies, especially at redshifts relevant for cosmic shear, should be a high priority for the cosmic shear community.

The Intrinsic Alignment of Galaxies and its Impact on Weak Gravitational Lensing in an Era of Precision Cosmology

The wealth of incoming and future cosmological observations will allow us to map out the structure and evolution of the observable universe to an unprecedented level of precision. Among these observations is the weak gravitational lensing of galaxies, e.g., cosmic shear that measures the minute distortions of background galaxy images by intervening cosmic structure. Weak lensing and cosmic shear promise to be a powerful probe of astrophysics and cosmology, constraining models of dark energy, measuring the evolution of structure in the universe, and testing theories of gravity on cosmic scales. However, the intrinsic alignment of galaxies -- their shape and orientation before being lensed -- may pose a great challenge to the use of weak gravitational lensing as an accurate cosmological probe, and has been identified as one of the primary physical systematic biases in cosmic shear studies. Correlations between this intrinsic alignment and the lensing signal can persist even for large physical separations, and isolating the effect of intrinsic alignment from weak lensing is not trivial. A great deal of work in the last two decades has been devoted to understanding and characterizing this intrinsic alignment, which is also a direct and complementary probe of structure formation and evolution in its own right. In this review, we report in a systematic way the state of our understanding of the intrinsic alignment of galaxies, with a particular emphasis on its large-scale impact on weak lensing measurements and methods for its isolation or mitigation. (Abridged)

Contribution of the cosmological constant to the relativistic bending of light revisited

We study the effect of the cosmological constant

Stability of transparent spherically symmetric thin shells and wormholes

\ensuremath{\Lambda}

Remarks on the Formulation of the Cosmological Constant/Dark Energy Problems

on the bending of light by a concentrated spherically symmetric mass. Contrarily to previous claims, we show that, when the Schwarzschild-de Sitter geometry is taken into account,

A new independent limit on the cosmological constant/dark energy from the relativistic bending of light by Galaxies and clusters of Galaxies

\ensuremath{\Lambda}

Probing decisive answers to dark energy questions from cosmic complementarity and lensing tomography

does indeed contribute to the bending.

Weak lensing and CMB: Parameter forecasts including a running spectral index

The stability of transparent spherically symmetric thin shells (and wormholes) to linearized spherically symmetric perturbations about static equilibrium is examined. This work generalizes and systematizes previous studies and explores the consequences of including the cosmological constant. The approach shows how the existence (or not) of a domain wall dominates the landscape of possible equilibrium configurations.