Weak Gravitational Lensing

Weak Gravitational Lensing


The phenomenon of light deflection in the presence of massive objects is denoted as gravitational lensing. On extra-galactic scales, galaxies, clusters of galaxies, and even the filamentary structure of the cosmic web act as gravitational lenses. The tidal fields of those mass distributions cause images of high-redshift galaxies to be distorted. If the distortions are very small, they cannot be detected on individual galaxies, but only statistically, by averaging over a large number of galaxies, we speak of weak lensing.

llustration of lensing correlations on background galaxy images induced by the cosmic web.

 


Non-Gaussian structures

 

On small scales, the structures of the cosmic web are non-Gaussian. This information is not captured by traditional second-order statistics such as the weak-lensing two-point correlation function or power spectrum.  Peaks in weak-lensing maps, defined as local maxima of the lensing convergence, are tracers of over-dense regions, and provide a means to extract higher-order, non-Gaussian information. We devised a new model for WL peaks, that was studied in a handful of publications.

Weak-lensing peaks as maximum in the convergence map.

 


Mass mapping

 

Weak lensing allows us to map the dark matter that comprises the cosmic web. Missing data however, due to masking out bright foreground stars and galaxies, or CCD defects and gaps, makes the technique of weak-lensing mass mapping challenging. We have developed advanced statistical and image processing techniques to cope with this ill-defined problem.

Simulated weak-lensing data with a real mask pattern (black regions on the left), inpainted and reconstructed mass map (on the right).

 


Cosmological parameters

 

Weak lensing is a powerful tool to constrain the model of our Universe, its expansion history, and the evolution of of the cosmic web. It allows to measure parameters of our cosmological model such as the total matter density, and the "clumpiness" of the cosmic web. This probe also is very promising to test theories the standard model of cosmology, and to measure properties of dark energy or to constrain the laws of gravity on very large scales. CosmoStat scientists have contributed to the cosmological analysis of many large wide-field optical surveys, including CFHTLS, COSMOS, CFHTLenS, DES. We are strongly involved in upcoming very large surveys such as CFIS and Euclid.

Allowed region for the matter density parameter (Omega_m) and the parameter quantifying the clumpiness of the cosmic web (sigma_8), from the CFHTlenS survey