3 ms·

This is also the biggest surprise for me, but I'd frame it as people largely just handwaving in the assumption of a (at scale) isotropic universe, even though t

This is also the biggest surprise for me, but I'd frame it as people largely just handwaving in the assumption of a (at scale) isotropic universe, even though that's highly questionable. I think the practical issue is that that assumption let a lot more work get done than would have been possible otherwise. Of course if it turns out the universe is not isotropic then most all of that work is worth less than nothing. So publish or perish strikes again?

programjames 2y ago
It is somewhat surprising, because one of the most famous papers in chaos theory, "The Applicability of the Third Integral of Motion" (Henon & Heiles), basically starts by saying a similar assumption isn't true, that stars aren't ergodically distributed in the axial/radial directions. If you have five equations of motion in a six-dimensional universe (3 space + 3 velocity coordinates), you can compute the future trajectory of each point. Two equations come from constant energy & angular momentum, and these constrain where in phase-space the trajectories can go. Another two equations are do not make any such constraints, which implies stars are at least ergodically distributed in a 2D phase-space. Since none of these equations constrain the axial/radial velocity, you would expect the dispersions to be equal for both directions. However, this turns out to not be the case. This means there must be a third isolating equation of motion out there, and the surprising thing Henon & Heiles find is it's chaotic! Sometimes it constrains points to 2D regions of phase-space (i.e. concentric circles of orbits), and other times it lets them move in a 3D region (i.e. chaotic trajectories filling the space).