Lyapunov spectrum via boundary theory I

This paper is concerned with the Lyapunov spectrum for measurable cocycles over an ergodic pmp system taking values in semi-simple real Lie groups. We prove simplicity of the Lyapunov spectrum and its continuity under certain perturbations for a class systems that includes many familiar examples. Our framework uses some soft qualitative assumptions, and does not rely on symbolic dynamics. We use ideas from boundary theory that appear in the study of super-rigidity to deduce our results. This gives a new perspective even on the most studied case of random matrix products. The current paper introduces the general framework and contains the proofs of the main results and some basic examples. In a follow up paper we discuss further examples.