Symmetries of Cosmological Cauchy Horizons with Non-Closed Orbits

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

I. A sharp decomposition of the flow into geometric ``wave parts'' and ``transport-div-curl parts;''

II. Null form source terms;

III. Structures that allow one to propagate one additional degree of differentiability (compared to standard estimates) for the entropy and vorticity.

I will then describe a main application: the study of stable shock formation, without symmetry assumptions, in more than one spatial dimension. I will emphasize the role that nonlinear geometric optics plays in the analysis and highlight how the new formulations allow for its implementation. Finally, I will describe some important open problems, and I will connect the results to the broader goal of obtaining a rigorous mathematical theory that models the long-time behavior of solutions in the presence of shock singularities.