ASTR 104. Cosmology in the Twentieth Century.The twentieth century saw the emergence of a coherent scientific understanding of the physical universe as a whole. According to this understanding, the universe has evolved from a hot, dense, and rapidly expanding soup of elementary particles into the system of galaxies we see today. But the picture is not complete, and topping the list of unresolved puzzles is the identity of the so-called dark matter. We cannot see the dark matter (hence its name), but we do measure its gravitational influences on matter we can see. The disconcerting conclusion is that there is much more dark matter than visible matter. This course examines the development of modern cosmology, with attention to both that which seems to be well understood and that which is not yet understood. Enrollment limited to 64. [S] [Q] Normally offered every year. E. Wollman.