Gorenstein singularities with ${\mathbb{G}}_m$-action and moduli spaces of holomorphic differentials

Abstract

Given a holomorphic differential on a smooth complex algebraic curve, we associate to it a Gorenstein curve singularity with ${\mathbb{G}}_m$-action via a test configuration. This construction decomposes the strata of holomorphic differentials with prescribed orders of zeros into negatively graded miniversal deformation spaces of such singularities. Additionally, it provides a natural description for the singular curves that appear in the boundary of the miniversal deformation spaces. Our approach leads to a number of applications. We classify the unique Gorenstein singularity with ${\mathbb{G}}_m$-action for each nonvarying stratum of holomorphic differentials and study when these nonvarying strata can be compactified by weighted projective spaces. Moreover, extending the classical results about $ADE$ singularities, we establish the $K(\ddot{I}€,1)$-property for non-hypersurface complete intersection singularities of type $U_7$, $U_8$, $U_9$, and $S_k$. We also study singularities with bounded $\hat{I}\pm$-invariants in the log minimal model program for ${\overline{\mathcal{M}}}_g$ and utilize them to bound the slopes of effective divisors in ${\overline{\mathcal{M}}}_g$. Finally, we show that the loci of subcanonical points with fixed semigroups have trivial tautological rings and provide a criterion to determine whether they are affine varieties.

58 pages, added a new result about the $K(\ddot{I}€,1)$-property for singularities of $S$ and $U$ type