Expected values of eigenfunction periods

Abstract

Let $(M,g)$ be a compact Riemannian surface. Consider a family of $L^2$ normalized Laplace-Beltrami eigenfunctions, written in the semiclassical form $-h_j^2\Delta_g \phi_{h_j} = \phi_{h_j}$, whose eigenvalues satisfy $h h_j^{-1} \in (1, 1 + hD]$ for $D>0$ a large enough constant. Let $\mathbf{P}_h$ be a uniform probability measure on the $L^2$ unit-sphere $S_h$ of this cluster of eigenfunctions and take $u \in S_h$. Given a closed curve $\gamma \subset M$, there exists $C_{1}(\gamma, M), C_{2}(\gamma, M) > 0$ and $h_0>0$ such that for all $h \in (0, h_0],$ \begin{equation*} C_1 h^{1/2} \leq \mathbf{E}_{h} \bigg[ \big| \int_{\gamma} u \, d \sigma \big| \bigg] \leq C_2 h^{1/2} . \end{equation*} This result contrasts the deterministic $\mathcal{O}(1)$ upperbounds obtained by Chen-Sogge \cite{CS}, Reznikov \cite{Rez}, and Zelditch \cite{Zel}. Furthermore, we treat the higher dimensional cases and compute large deviation estimates. Under a measure zero assumption on the periodic geodesics in $S^*M$, we can consider windows of small width $D=1$ and establish a $\mathcal{O}(h^{1/2})$ estimate. Lastly, we treat probabilistic $L^q$ restriction bounds along curves.