Tuesday, August 09, 2011

Weak Gravitational Lensing

Kind of a follow up to this entry on Compressive Sensing Coded Aperture in Space and Weak Lensing here is a book chapter entitled: Weak Gravitational Lensing by Sandrine Pires, Jean-Luc Starck, Adrienne Leonard and Alexandre Refregier. The introduction answers the question as to why weak (and not strong) gravitational lensing is far more interesting and how blind deconvolution and compressed sensing meet. 
 ...Weak gravitational lensing is believed to be the most promising tool to understand the nature of dark matter and to constrain the cosmological parameters used to describe the Universe because it provides a method to map directly the distribution of dark matter (see [6, 61, 70, 1, 64]). From this dark matter distribution, the nature of dark matter can be better understood and better constraints can be placed on dark energy, which asp ects the evolution of structures. Gravitational lensing is the process by which light from distant galaxies is bent by the gravity of intervening mass in the Universe as it travels towards us. This bending causes the images of background galaxies to appear slightly distorted, and can be used to extract important cosmological information. 
In the beginning of the twentieth century, A. Einstein predicted that massive bodies could be seen as gravitational lenses that bend the path of light rays by creating a local curvature in space-time. One of the rst con rmations of Einstein's new theory was the observation during the 1919 solar eclipse of the detection of light from distant stars by the sun. Since then, a wide range of lensing phenomena have been detected. The gravitational detection of light by mass concentrations along light paths produces magni cation, multiplication, and distortion of images. These lensing e ects are illustrated by Fig. 2, which shows one of the strongest lens observed: Abell 2218, a very massive and distant cluster of galaxies in the constellation Draco. The observed gravitational arcs are actually the magni ed and strongly distorted images of galaxies that are about 10 times more distant than the cluster itself. These strong gravitational lensing e ects are very impressive but they are very rare. Far more prevalent are weak gravitational lensing e ects, which we consider in this chapter, and in which the induced distortion in galaxy images is much weaker. These gravitational lensing e ects are now widely used, but the amplitude of the weak lensing signal is so weak that its detection relies on the accuracy of the techniques used to analyze the data. Future weak lensing surveys are already planned in order to cover a large fraction of the sky with high accuracy, such as Euclid. However improving accuracy also places greater demands on the methods used to extract the available information...

In short the weak lensing is closer to a full random gaussian measurement matrix as opposed to the sparse strong lensing measurement matrix. Since events behind these lenses are somewhat deterministic I wonder how new techniques in low rank approximation will help in the determination of the weak lenses property or how this lens can be calibrated.

. Also while we are on the subject of Dark Matter and assuming it exist, maybe we should wait for a press release on September 6th:

TAUP2011 in Munich - Press conference
Latest results from the CRESST Experiment provide an indication of dark matter

The press conference will be held on 6. September 2011 starting at 2:00 pm.

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