A spectral version of the Moore problem for bipartite regular graphs

Let $b(k,\theta )$ be the maximum order of a connected bipartite $k$-regular graph whose second largest eigenvalue is at most $\theta$. In this paper, we obtain a general upper bound for $b(k,\theta )$ for any $0\leq \theta< 2\sqrt{k-1}$\mathrm{.}Ourboundgivestheexactvalueof$b(k,\theta)$ whenever there exists a bipartite distance-regular graph of degree $k$, second largest eigenvalue $\theta$, diameter $d$ and girth $g$ such that $g\ge 2d-2$. For certain values of $d$, there are infinitely many such graphs of various valencies $k$. However, for $d=11$ or $d\ge 15$, we prove that there are no bipartite distance-regular graphs with $g\ge 2d-2$.