Hodge bundle

In mathematics, the Hodge bundle, named after W. V. D. Hodge, appears in the study of families of curves, where it provides an invariant in the moduli theory of algebraic curves. Furthermore, it has applications to the theory of modular forms on reductive algebraic groups¹ and string theory.²

Let ${\displaystyle {\mathcal {M}}_{g}}$ be the moduli space of algebraic curves of genus g curves over some scheme. The Hodge bundle ${\displaystyle \Lambda _{g}}$ is a vector bundle^{note 1} on ${\displaystyle {\mathcal {M}}_{g}}$ whose fiber at a point C in ${\displaystyle {\mathcal {M}}_{g}}$ is the space of holomorphic differentials on the curve C. To define the Hodge bundle, let ${\displaystyle \pi \colon {\mathcal {C}}_{g}\rightarrow {\mathcal {M}}_{g}}$ be the universal algebraic curve of genus g and let ${\displaystyle \omega _{g}}$ be its relative dualizing sheaf. The Hodge bundle is the pushforward of this sheaf, i.e.,³

${\displaystyle \Lambda _{g}=\pi _{*}\omega _{g}}$.