Abstract
Weak gravitational lensing, or cosmic shear, is a valuable tool for obtaining the mass distribution in halos, total mass and the bias between dark and visible matter. It is also possible to observe lensing effects, and hence shear, on the cosmological background. The deviation of light rays from this vestigial Big Bang background provides diagnostic information on dark energy and modified gravity. Clusters also serve as gravitational telescopes, enabling us to determine the physics of galaxies at high redshift, impossible without amplification. As sensitivity and spatial resolution increase, so do the details of critical lines in the image plane (where the magnification factor tends towards infinity), or caustics, equivalent in the source plane. Nearby, the strong lens phenomenon produces an odd number of images on either side of the caustic, which will be symmetrical in a mirror (change of parity), allowing them to be recognized in the confusion.
The deflections of cosmic microwave background photons can be calculated statistically: on average, there are around 50 deflections for a lens size of 300 Mpc.A random walk will give a final deflection of 2 arcmin. Shear will dominate small-scale structures. On the main acoustic peak (1°), 2 arcmin corresponds to 3 % distortion. This distortion also introduces polarization into the radiation, which is important to take into account in order to better determine the initial polarization, a tracer of primordial gravitational waves (evidence of inflation). In fact, the polarization added by shearing is of the B type(rotational type), which is precisely the trace of primordial waves. Note that the ISW (Integrated Sachs-Wolf) effect in the cosmic background due to large structures and dark energy, has been detected in correlation with the Sloan maps. There is also a good correlation of deflections with the infrared background (by stacking).
Finally, using clusters as gravitational telescopes, it has been possible to study molecular gas and cloud physics at large z, and to show that there is a drop in the number of massive galaxies at z = 8. Their study will have to wait for the JWST.
