On the Growth of Torsion in the Cohomology of Arithmetic Groups

Let G be a semisimple Lie group with associated symmetric space D, and let Γ ⊂ G be a cocompact arithmetic group. Let L be a lattice inside a ZΓ-module arising from a rational finite-dimensional complex representation of G. Bergeron and Venkatesh recently gave a precise conjecture about the growth of the order of the torsion subgroup Hi(Γk;L )tors as Γk ranges over a tower of congruence subgroups of Γ. In particular they conjectured that the ratio log |Hi(Γk;L )tors|/[Γ : Γk] should tend to a nonzero limit if and only if i = (dim(D)− 1)/2 and G is a group of deficiency 1. Furthermore, they gave a precise expression for the limit. In this paper, we investigate computationally the cohomology of several (non-cocompact) arithmetic groups, including GLn(Z) for n = 3, 4, 5 and GL2(O) for various rings of integers, and observe its growth as a function of level. In all cases where our dataset is sufficiently large, we observe excellent agreement with the same limit as in the predictions of Bergeron–Venkatesh. Our data also prompts us to make two new conjectures on the growth of torsion not covered by the Bergeron–Venkatesh conjecture.