Steiner gives a lecture on the topology of the universe. He starts his lecture with the question “Do we live in a small universe?”, with which he means: is the universe infinite or finite, and if it is finite, what is its topology? To determined to overall geometry of the universe, we need to use observational data. Steiner explains how the energy density of the universe determines the curvature, and how the CMB radiation can be used to estimate the curvature. He finished with a discussion of simulations of the universe.
Since there is no supporting material for this lecture, it might be difficult to follow if the listener does not have prior knowledge of cosmology.