Week 7: Gravitational Lensing

According to general relativity, the trajectories of light rays can be bent by gravitational fields. Massive objects can therefore gravitationally “lens” the images of objects behind them. A cosmological examples of gravitational lensing is lensing of galaxies or quasars at redshifts z ∼ 1 by elliptical galaxies or galaxy clusters along the line of sight. If the lensing is “strong,” then there will be multiple images of the source. If the lensing is “weak, ” then there will be no extra images, but the images we see will be distorted by lensing. Weak lensing is now a common and powerful tool to measure the mass distribution in galaxy clusters. There is also “gravitational microlensing” of stars in the Galactic bulge or LMC by other stars along the line of sight. In gravitational microlensing, there are multiple images, but they are so close together that they cannot be resolved. Instead, the combined light from the two unresolved images makes the source star appear brighter than it would otherwise. There is also “cosmic shear,” weak gravitational lensing of galaxies at z ∼ 1 by the large-scale distribution of mass in the Universe. Large-scale correlations in the mass induce large-scale correlations to distant-galaxy images.