After reviewing the basic equation for an expanding universe, Professor Susskind solves the equation explicitly for a zero energy universe, and then extends the derivation to universes with non-zero energy. These universes can take two forms: matter-dominated universes, and radiation-dominated universes. The radiation-dominated form characterizes our early universe, and the matter-dominated form characterizes our universe today - or so cosmologists believed until observations led to theories of dark energy in the 1990s.