The field of moduli of varieties with a structure

TL;DR

This paper studies the field of moduli for varieties with additional algebraic structures, establishing formal results, correspondences, and criteria that advance understanding of when these structures are defined over the field of moduli.

Contribution

It provides a general framework for algebraic structures on varieties, including new correspondences, classification results, and cohomological criteria for their definability over the field of moduli.

Findings

G-structures correspond to twisted forms of quotient stacks

Every algebraic structure can be associated with a 0-cycle under certain conditions

Cohomological criteria determine the existence of structures not defined over the field of moduli

Abstract

If $X$ is a variety with an additional structure $\xi$, such as a marked point, a divisor, a polarization, a group structure and so forth, then it is possible to study whether the pair $(X,\xi)$ is defined over the field of moduli. There exists a precise definition of ``algebraic structures'' which covers essentially all of the obvious concrete examples. We prove several formal results about algebraic structures. There are immediate applications to the study of fields of moduli of curves and finite sets in $\mathbb{P}^{2}$, but the results are completely general. Fix $G$ a finite group of automorphisms of $X$, a $G$-structure is an algebraic structure with automorphism group equal to $G$. First, we prove that $G$-structures on $X$ are in a $1:1$ correspondence with twisted forms of $X/G\dashrightarrow\mathcal{B} G$. Secondly we show that, under some assumptions, every algebraic structure on $X$ is equivalent to the structure given by some $0$-cycle. Third, we give a cohomological criterion for checking the existence of $G$-structures not defined over the field of moduli. Fourth, we identify geometric conditions about the action of $G$ on $X$ which ensure that every $G$-structure is defined over the field of moduli.