Lyapunov exponents, holomorphic flat bundles and de Rham moduli space

TL;DR

This paper refines bounds on Lyapunov exponents for flat bundles over hyperbolic curves, relating them to geometric degrees and exploring their behavior in various moduli spaces, including the de Rham and Shatz strata.

Contribution

It improves existing inequalities for Lyapunov exponents, establishes conditions for equality, and analyzes their properties on different moduli space strata, especially in rank two cases.

Findings

Refined lower bounds for sum of Lyapunov exponents.

Equality conditions for compact base curves.

Unbounded Lyapunov exponents on the maximal Shatz stratum.

Abstract

We consider Lyapunov exponents for flat bundles over hyperbolic curves defined via parallel transport over the geodesic flow. We refine a lower bound obtained by Eskin, Kontsevich, Moeller and Zorich showing that the sum of the first k exponents is greater or equal than the sum of the degree of any rank k holomorphic subbundle of the flat bundle and the asymptotic degree of its equivariant developing map. We also show that this inequality is an equality if the base curve is compact. We moreover relate the asymptotic degree to the dynamical degree defined by Daniel and Deroin. We then use the previous results to study properties of Lyapunov exponents on variations of Hodge structures and on Shatz strata of the de Rham moduli space. In particular we show that the top Lyapunov exponent function is unbounded on the maximal Shatz stratum, the oper locus. In the final part of the work we specialize to the rank two case, generalizing a result of Deroin and Dujardin about Lyapunov exponents of holonomies of projective structures.