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Gravitational lensing (Schneider 2006; Bartelmann 2010), the deflection of light from distant "sources" due to the bending of space-time by masses ("lenses") along the line of sight, is a useful cosmological probe because it is sensitive to all matter, whether baryonic or dark. Weak lensing is gravitational lensing in the limit that the deflections are very small, causing tiny distortions, or "shear", in galaxy shapes (Bartelmann & Schneider 2001; Refregier 2003). In the absence of lensing, galaxy orientations are assumed to be random, so they should not exhibit statistically significant, coherent alignments. Lensing induces small (1%) but coherent shears in background galaxy images, which have typical ellipticities that are far larger (RMS ellipticity ~0.36). Thus, weak lensing is detected statistically by averaging over many lensed galaxies, for which shapes have been measured in a way that eliminates contributions from observational effects that also distort galaxy shapes (e.g., the point-spread function, or PSF).
Large-scale structure of dark matter (orange) from cosmological simulations. Superimposed are the shapes of galaxies (blue), showing how the observed galaxy ellipticity is distorted by the gravitational field of the dark matter. Image credit: CFHT group, University of Hawaii.
Measurements of the growth of large-scale structure of the dark matter ﬁeld via weak lensing are sensitive to dark energy, since the accelerated expansion of the Universe that is caused by dark energy opposes the gravitational attraction that would otherwise lead to increased clumping of dark matter structures. Because of its sensitivity to both dark matter and dark energy, weak lensing is one of the four methods advocated by the Dark Energy Task Force, and is a major driver of survey design for LSST.
Conveners: Joe Zuntz (Manchester), [joseph dot zuntz at manchester dot ac dot uk], Michael Schneider (LLNL) [schneider42 at llnl dot gov]