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Published April 2012 | Published
Journal Article Open

Measuring the Geometry of the Universe from Weak Gravitational Lensing behind Galaxy Groups in the HST COSMOS Survey


Gravitational lensing can provide pure geometric tests of the structure of spacetime, for instance by determining empirically the angular diameter distance-redshift relation. This geometric test has been demonstrated several times using massive clusters which produce a large lensing signal. In this case, matter at a single redshift dominates the lensing signal, so the analysis is straightforward. It is less clear how weaker signals from multiple sources at different redshifts can be stacked to demonstrate the geometric dependence. We introduce a simple measure of relative shear which for flat cosmologies separates the effect of lens and source positions into multiplicative terms, allowing signals from many different source-lens pairs to be combined. Applying this technique to a sample of groups and low-mass clusters in the COSMOS survey, we detect a clear variation of shear with distance behind the lens. This represents the first detection of the geometric effect using weak lensing by multiple, low-mass groups. The variation of distance with redshift is measured with sufficient precision to constrain the equation of state of the universe under the assumption of flatness, equivalent to a detection of a dark energy component Ω_ X at greater than 99% confidence for an equation-of-state parameter –2.5 ≤ w ≤ –0.1. For the case w = –1, we find a value for the cosmological constant density parameter Ω_Λ = 0.85^(+0.044)_( –0.19) (68% CL) and detect cosmic acceleration (q_0 < 0) at the 98% CL. We consider the systematic uncertainties associated with this technique and discuss the prospects for applying it in forthcoming weak-lensing surveys.

Additional Information

© 2012 The American Astronomical Society. Received 2011 July 29; accepted 2012 February 10; published 2012 March 30. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA Inc. under the NASA contract NAS 5-26555; the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the European Southern Observatory under the Large Program 175.A-0839, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. The authors gratefully acknowledge helpful discussions with Niayesh Afshordi, Hendrik Hildebrandt, Mike Hudson, Fabian Schmidt, and Kris Sigurdson. J.E.T. is supported by a NSERC Canada Discovery Grant. R.M. is supported by STFC Advanced Fellowship PP/E006450/1 and ERC grant MIRG-CT-208994. A.L. acknowledges support from the Chamberlain Fellowship at LBNL and from the Berkeley Center for Cosmological Physics. The work of J.R. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. T.K. was supported by a RAS 2010 Fellowship. This work was supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan. The HST COSMOS Treasury program was supported through the NASA grant HST-GO-09822. We thank Tony Roman, Denise Taylor, and David Soderblom for their assistance in planning and scheduling of the extensive COSMOS observations. We gratefully acknowledge the contributions of the entire COSMOS collaboration consisting of more than 70 scientists. More information on the COSMOS survey is available at http://cosmos.astro.caltech.edu. It is also a pleasure the acknowledge the excellent services provided by the NASA IPAC/IRSA staff (Anastasia Laity, Anastasia Alexov, Bruce Berriman, and John Good) in providing online archive and server capabilities for the COSMOS data sets.

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