Algebraic hull of maximal measurable cocycles of surface groups into Hermitian Lie groups

Abstract

Following the work of Burger, Iozzi and Wienhard for representations, in this paper we introduce the notion of maximal measurable cocycles of a surface group. More precisely, let mathbf {G} be a semisimple algebraic {mathbb {R}}-group such that G=mathbf {G}({mathbb {R}})^{circ } is of Hermitian type. If Gamma le L is a torsion-free lattice of a finite connected covering of mathrm{PU}(1,1), given a standard Borel probability Gamma -space (Omega ,mu _Omega ), we introduce the notion of Toledo invariant for a measurable cocycle sigma :Gamma times Omega rightarrow G. The Toledo invariant remains unchanged along G-cohomology classes and its absolute value is bounded by the rank of G. This allows to define maximal measurable cocycles. We show that the algebraic hull mathbf {H} of a maximal cocycle sigma is reductive and the centralizer of H=mathbf {H}({mathbb {R}})^{circ } is compact. If additionally sigma admits a boundary map, then H is of tube type and sigma is cohomologous to a cocycle stabilizing a unique maximal tube type subdomain. This result is analogous to the one obtained for representations. In the particular case G=mathrm{PU}(n,1) maximality is sufficient to prove that sigma is cohomologous to a cocycle preserving a complex geodesic. We conclude with some remarks about boundary maps of maximal Zariski dense cocycles.