An algebraic variant of the Fischer-Grauert Theorem

Abstract

A well-known theorem of W. Fischer and H. Grauert states that analytic fiber spaces with all fibers isomorphic to a fixed compact connected complex manifold are locally trivial. Motivated by this result, we show that if $k$ is an algebraically closed field of infinite transcendence degree over its prime field, then every smooth projective family over a reduced $k$-scheme of finite type with isomorphic fibers having reduced automorphism group schemes is locally trivial in the étale topology. We do so by reducing the problem to the case when the base is a smooth integral curve, and then, using the vanishing of the Kodaira-Spencer map, we prove formal triviality of such families at every geometric point of the base. We also provide examples of smooth projective fibrewise trivial families in positive characteristic whose Kodaira-Spencer map are nowhere vanishing.