In Neil DeGrass Tyson's epic video, at 2:26:50
He mentions how if net energy is negative, the spacetime curvature is spherical, and if it's net positive, saddle-shaped.
He uses the terms "flat" and "saddle-shaped" which are 2d, but he's actually just using that as an analogy for 3d curvature, which we can't really visualize, correct? If so, what does it mean for 3d space to be "saddle-shaped" or "spherical"?
1) II'm guessing spherical implies that if we keep traveling in one direction in space, it would be possible to end up back where you started? (kind of like how you can end up in the same position traversing the surface of a planet if the 2d plane is curved?
2) If net energy is 0. But they also say there is far more dark energy and dark matter than energy and matter. So isn't this a contradiction? How does the accounting work out?