Laplacians in various dimensions and the swampland

A bstract The species cutoff is a moduli-dependent quantity signaling the onset of quantum gravitational phenomena, whose form can be oftentimes determined from higher-derivative and higher-curvature corrections within low-energy gravitational EFTs. In this work, we point out that these Wilson coefficients are eigenfunctions of an appropriate second-order elliptic operator defined over moduli space in theories with more than four supercharges. This was already known to be the case for the leading $$ {\mathcal{R}}^4 $$ R 4 -correction to the two-derivative (bosonic) action of maximal supergravity in d ≤ 10. Here, we reconsider this fact from the Swampland point of view and show how, in d = 10, 9, 8, solving a Laplace equation imposes non-trivial restrictions on the species hull vectors. We further argue that this property is also satisfied in settings with less supersymmetry. In particular, we focus on the $$ {\mathcal{R}}^4 $$ R 4 -operator in minimal supergravity theories in d = 10, 9, and on the leading $$ {\mathcal{R}}^2 $$ R 2 -term in setups with 8 supercharges in d = 6, 5, 4. Finally, we provide a symmetry-based criterion for determining when the relevant elliptic operator should be the Laplacian. A bottom-up rationale for this constraint remains to be fully understood, and we conclude by outlining some compelling possibilities.