We consider analytic, vacuum spacetimes that admit compact, non-degenerate Cauchy horizons. Many years ago Vince Moncrief and I proved that, if the null geodesic generators of such a horizon were all closed curves, then the enveloping spacetime would necessarily admit a non-trivial, horizon-generating Killing vector field.
I will describe my recent works, some joint with M.\,Disconzi and J.\,Luk, on the compressible Euler equations and their relativistic analog. The works concern solutions with non-vanishing vorticity and entropy, under any equation of state.
The starting point is our new formulations of the equations exhibiting miraculous geo-analytic structures, including