Minimal Surfaces and Weak Gravity

We show that the Weak Gravity Conjecture (WGC) implies a nontrivial upper bound on the volumes of the minimal-volume cycles in certain homology classes that admit no calibrated representatives. In compactification of type IIB string theory on an orientifold $X$ of a Calabi-Yau threefold, we consider a homology class $[Σ] \in H_4(X,\mathbb{Z})$ represented by a union $Σ_{\cup}$ of holomorphic and antiholomorphic cycles. The instanton form of the WGC applied to the axion charge $[Σ]$ implies an upper bound on the action of a non-BPS Euclidean D3-brane wrapping the minimal-volume representative $Σ_{\mathrm{min}}$ of $[Σ]$. We give an explicit example of an orientifold $X$ of a hypersurface in a toric variety, and a hyperplane $\mathcal{H} \subset H_4(X,\mathbb{Z})$, such that for any $[Σ] \in H$ that satisfies the WGC, the minimal volume obeys $\mathrm{Vol}(Σ_{\mathrm{min}}) \ll \mathrm{Vol}(Σ_{\cup})$: the holomorphic and antiholomorphic components recombine to form a much smaller cycle. In particular, the sub-Lattice WGC applied to $X$ implies large recombination, no matter how sparse the sublattice. Non-BPS instantons wrapping $Σ_{\mathrm{min}}$ are then more important than would be predicted from a study of BPS instantons wrapping the separate components of $Σ_{\cup}$. Our analysis hinges on a novel computation of effective divisors in $X$ that are not inherited from effective divisors of the toric variety.

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