Strong lensing, cosmology, and the line of sight

Strong gravitational lensing is a powerful observational tool in astrophysics and cosmology, which has been used to study the mass and light properties of galaxies and quasars, the scales and distributions of dark matter subhaloes, and the expansion history of the Universe. Matter inhomogeneties between the observer, lens galaxy, and source galaxy can complicate strong lensing cosmology, but also offer novel competitive constraints on the matter distribution of the universe. I will introduce strong lensing as an observational tool, briefly illustrate the consequences of the line of sight for measurements of the Hubble constant and the dark energy equation of state, and then present the use of the strong lensing line-of-sight shear as a new, complementary observable for constraining the matter power spectrum.