Smooth Fields of Hilbert Spaces, Hermitian bundles and Riemannian Direct Images

Abstract

Given a field of Hilbert spaces there are two ways to endow it with a smooth structure: the standard and geometrical notion of Hilbert (or Hermitian) bundle and the analytical notion of smooth field of Hilbert spaces. We study the relationship between these concepts in a general framework. We apply our results in the following interesting example called Riemannian direct images: Let $M,N$ be Riemannian oriented manifolds, $\rho:M\to N$ be a submersion and $\pi:E\to M$ a finite dimensional vector bundle. Also, let $M_\lambda=\rho^{-1}(\lambda)$ and fix a suitable measure $\mu_\lambda$ in $M_\lambda$. Does the field of Hilbert spaces $\mathcal{H}(\lambda)=L^2(M_\lambda,E)$ admits a smooth field of Hilbert space structure? or a Hilbert bundle structure? In order to provide conditions to guarantee a positive answer for these questions, we develop an interesting formula to derivate functions defined on $N$ as a integral over $M_\lambda$.