Random maximal isotropic subspaces and Selmer groups

We rst develop a notion of quadratic form on a locally compact abelian group. Under suitable hypotheses, we construct a probability measure on the set of closed maximal isotropic subspaces of a locally compact quadratic space over Fp. A random subspace chosen with respect to this measure is discrete with probability 1, and the dimension of its intersection with a xed compact open maximal isotropic subspace is a certain nonnegativeinteger-valued random variable. We then prove that the p-Selmer group of an elliptic curve is naturally the intersection of a discrete maximal isotropic subspace with a compact open maximal isotropic subspace in a locally compact quadratic space over Fp. By modeling the rst subspace as being random, we can explain the known phenomena regarding distribution of Selmer ranks, such as the theorems of Heath-Brown and Swinnerton-Dyer for 2-Selmer groups in certain families of quadratic twists, and the average size of 2and 3-Selmer groups as computed by Bhargava and Shankar. The only distribution on Mordell-Weil ranks compatible with both our random model and Delaunay's heuristics for p-torsion in Shafarevich-Tate groups is the distribution in which 50% of elliptic curves have rank 0, and 50% have rank 1. We generalize many of our results to abelian varieties over global elds. Along the way, we give a general formula relating self cup products in cohomology to connecting maps in nonabelian cohomology, and apply it to obtain a formula for the self cup product associated to the Weil pairing.