Couple minor additions: geodesics are physical in that (i) they are the paths taken by objects in free-fall and (ii) light (massless waves, etc etc) in free spa

Couple minor additions: geodesics are physical in that (i) they are the paths taken by objects in free-fall and (ii) light (massless waves, etc etc) in free space is always in free-fall: all allowable null curves are null geodesics. Massive objects and light may be accelerated (the direction part of the velocity vector changes with scattering even if the magnitude part does not) and during acceleration will not be bound to a geodesic because they aren't in free-fall. Even though I agree with the way you put it, I'll say it's probably a bit too strong to write that stress-energy is the (sole) source of curvature given the extremely curved spacetime of our expanding universe. However, Einstein was OK with Schrödinger taking that view <https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338> (a super-interesting bit of science history), and we don't know enough about the cosmological constant vs dark energy, nor cosmic inflation.