Automorphic Cohomology and the Limits of Algebraic Cycles

Abstract: This paper establishes an explicit obstruction to constructing algebraic cycles from automorphic cohomology classes on Shimura varieties. We produce a rational Hodge class $Ω_E$ in the intersection cohomology of the Baily-Borel compactification of a Shimura variety for $\text{SO}(2,26)$, arising from a stable residual automorphic representation via theta lift from the weight-$2$ newform of conductor $11$. While $Ω_E$ is automorphic and of pure Hodge type, we prove it is non-interior and hence cannot be obtained from special cycles, theta lifts, endoscopic transfers, or boundary pushforwards, all of which yield interior classes. The result is unconditional, relying only on Arthur's classification, Vogan-Zuckerman theory, the fundamental lemma, and the Zucker conjecture (proven by Looijenga-Saper-Stern), and it highlights a fundamental asymmetry between automorphic cohomology and geometric access to algebraic cycles, refining the Hodge conjecture from a question of existence to one of constructive tractability.