Weak lensing

The arc lets seen in clusters arise because the image of the background galaxy is strongly deformed. Equation (12.3) shows that as we look at galaxies further from the projected centre of the cluster - and hence as the impact parameter $b$ increases - the distortion will become smaller and smaller. Eventually, we won't be able to detect the deformation for a single galaxy any more, but we could look at many galaxies in a given small patch of sky, and try to determine whether perhaps they are all slightly elongated in the same direction, as would be the case if there were a lot of mass nearby. This is called weak lensing: you try to identify the presence of a large mass in a given direction, from the fact that galaxies nearby in projection, tend to be elongated in a same direction. Astronomers are now searching through stacks of images trying to find whether may be there are some really massive objects that have no galaxies associated with them. Another use of weak lensing is just try to map directly the distribution of matter in the Universe, irrespective of whether it emits light as well.