What are the three possible fates of the universe? How do these relate to the "critical" density? How do these relate to the three possible "geometries" of the universe, as indicated by Einstein's General Relativity. What do light rays do in the three geometries? We think our universe is a low-density universe but has a flat geometry. What are the main observational pieces of evidence pointing to this result? (a) ages of globular clusters, (b) spot sizes in the CMB, (c ) distances to far-away supernovae. How does the Hubble constant relate to the age of the universe? How does the matter density relate to the age of the universe? Know how age scales with these two different properties. For H0, know the equation. For matter density, just know which direction things go