Higher Kazhdan property and unitary cohomology of arithmetic groups

Abstract: Notions of higher Kazhdan property can be defined in terms of vanishing of unitary group cohomology in higher degrees. Garland's theorem for simple groups over non-archimedean fields provides the first examples of a higher Kazhdan property. We prove a version of Garland's theorem for simple Lie groups and their lattices. We generalize theorems of Borel and Borel-Yang about the invariance of the cohomology of lattices in semisimple Lie groups and adelic groups by improving the stability range and allowing for arbitrary unitary representations as coefficients. A novelty of our approach is the use of methods from geometric group theory and -- in the case of rank 1 -- from Clozel's work on the spectral gap property.