Summing over Seifert Manifolds in Narain Ensemble Averages

The ensemble average of Narain conformal field theories over their moduli space is a real-analytic Eisenstein series, whose Poincaré expansion can be interpreted as a sum over lens spaces weighted by the partition functions of Abelian Chern-Simons theory. In this paper we extend this correspondence from lens spaces to Seifert manifolds, which gives an example of a restricted sum over topologies of the kind that arises in three-dimensional quantum gravity. A Seifert manifold is obtained from a fixed core, a punctured surface times a circle, by gluing solid tori to its boundary tori, and we place a copy of the Narain theory on each of these tori. The ensemble average supplies the sum over the solid tori, or more precisely over marked presentations with fixed base genus, number of fibers and integer Seifert invariant. The resulting sum converges absolutely and is a polynomial in the components of the vector-valued Eisenstein series. When all the boundary tori share a single modulus, the average acquires connected corrections, which we identify with wormholes gluing the core to itself; for two boundary tori, the terms with rational Lagrangian sublattices of the Siegel-Eisenstein series are realized by wormholes containing an exceptional fiber. After reduction along the fiber, the construction closely parallels the two-dimensional topological gravity of Marolf and Maxfield, with the base of the fibration playing the role of their spacetime. We also discuss Dehn fillings of 3-manifolds with torus boundary, non-orientable bases, and the surgery formula. Our results suggest that modularity could be an organizing principle in the summation of geometries in quantum gravity.