Hello, I was enjoying Zee's book on GR when I noticed the location of this "a(t)" thing in the metric sound quite disturbing to me. BTW: I experience the same annoyance and went down to the same conclusions, when I watched a related Theoretical Minimum lesson... Here's the setup, the flat Minkowsky spacetime (with the c=1 convention): Here's the disturbing variation, a metric with "a scale factor": now: can you see "where" that "a(t)" is, in the equation, and what also there should be nearby, when dropping the c=1 convention? so: is it just me, or a(t) is just the inverse of the speed of light, changing in time? This view would be consistent with what those people are saying, right? Let alone it's really what the equations say: I'm not reading the equations wrong, am I? PS: That's just using Susskind's "math autopilot", isn't it? Cheers Corrado.