Time change for unipotent flows and rigidity

Abstract: We prove a dichotomy regarding the behavior of one-parameter unipotent flows on quotients of semisimple lie groups under time change. We show that if $u^{(1)}_t$ acting on $G_{1}/\Gamma_1$ is such a flow it satisfies exactly one of the following: (1) The flow is loosely Kronecker, and hence isomorphic after an appropriate time change to any other loosely Kronecker system. (2) The flow exhibits the following rigid behavior: if the one-parameter unipotent flow $u^{(1)} _ t$ on $G_1/\Gamma_1$ is isomorphic after time change to another such flow $u^{(2)} _ t$ on $G_2/\Gamma _ 2$, then $G_1/\Gamma_1 $ is isomorphic to $G_2/ \Gamma_2$ with the isomorphism taking $u^{(1)} _ t$ to $u^{(2)} _ t$ and moreover the time change is cohomologous to a trivial one. The full details will appear in [LW23].