Extra dimensions
- Kaluza: electric force ≈ component of momentum in the extra dimension
- Same direction = repel
- Different = attract
- Didn’t cycle in the compact direction: ordinary gravitational attraction
Curvature
- Flat (Euclidean): total energy density = critical
- Near-flat & accelerated expansion: 30% ordinary & dark matter + 70% cosmological constant
- Entire universe is many orders of magnitude bigger - appears flat
- Open (positively curved, like a sphere): > critical
- Closed (negatively curved, like a saddle): < critical
