Cup products, the Johnson homomorphism and surface bundles over surfaces with multiple fiberings

Abstract

Let $\Sigma_g \to E \to \Sigma_h$ be a surface bundle over a surface with monodromy representation $\rho: \pi_1 \Sigma_h \to \operatorname{Mod}(\Sigma_g)$ contained in the Torelli group $\mathcal{I}_g$. In this paper we express the cup product structure in $H^*(E, \mathbb{Z})$ in terms of the Johnson homomorphism $\tau: \mathcal{I}_g \to \wedge^3 (H_1 (\Sigma_g, \mathbb{Z}))$. This is applied to the question of obtaining an upper bound on the maximal $n$ such that $p_1: E \to \Sigma_{h_1}, ..., p_n: E \to \Sigma_{h_n}$ are fibering maps realizing $E$ as the total space of a surface bundle over a surface in $n$ distinct ways. We prove that any nontrivial surface bundle over a surface with monodromy contained in the Johnson kernel $\mathcal{K}_g$ fibers in a unique way.