The discussion centers on understanding how calculations demonstrate that the observable universe is flat, particularly through the analysis of cosmic microwave background (CMB) fluctuations. The apparent size of these fluctuations, approximately 1 degree, is linked to the actual size of the sound horizon, which must match to confirm a flat universe. The scale factor, denoted as "a," relates to the Hubble constant and helps describe the universe's expansion over time. The conversation also touches on the speed of sound in the primordial plasma and its implications for understanding these measurements. Ultimately, the relationship between the CMB fluctuations and the geometry of the universe is crucial for confirming its flatness.