Stable Moduli Spaces of High Dimensional Handlebodies

We study the moduli space of handlebodies diffeomorphic to $(D^{n+1}\times S^{n})^{\natural g}$,i\mathrm{.}e\mathrm{.}theclassifyingspace$BDiff((D^{n+1}\times S^n)^{\natural g}, D^{2n})$ of the group of diffeomorphisms that restrict to the identity near a $2n$-dimensional disk embedded in the boundary, $\mathrm{\partial }(D^{n+1}\times S^n{)}^{\mathrm{♮}g}$. We construct a map $colim_{g\to\infty}BDiff((D^{n+1}\times S^n)^{\natural g}, D^{2n}) \longrightarrow Q_{0}BO(2n+1)\langle n \rangle_{+}$ and prove that it induces an isomorphism on integral homology in the case that $2n+1\ge 9$. Above, $BO(2n+1)⟨n⟩$ denotes the $n$-connective cover of $BO(2n+1)$. The (co)homology of the space $Q_{0}BO(2n+1)⟨n{⟩}_{+}$ is well understood and so our results enable one to compute the homology groups $H_k(BDiff((D^{n+1}\times S^n{)}^{\mathrm{♮}g},D^{2n}))$ in a range of degrees when $k<<g$. Our main theorem can be viewed as an analogue of the Madsen-Weiss theorem for the moduli spaces of surfaces and the recent theorem of Galatius and Randal-Williams for the moduli spaces of manifolds of dimension $2n\ge 6$.