Rationally simply connected varieties and pseudo algebraically closed fields

Abstract

The cohomological dimension of a field is the largest degree with non-vanishing Galois cohomology. Serre's "Conjecture II" predicts that for every perfect field of cohomological dimension $2$, every torsor over the field for a semisimple, simply connected algebraic group is trivial. A field is perfect and "pseudo algebraically closed" (PAC) if every geometrically irreducible curve over the field has a rational point. These have cohomological dimension $1$. Every transcendence degree $1$ extension of such a field has cohomological degree $2$. We prove Serre's "Conjecture II" for such fields of cohomological degree $2$ provided either the field is of characteristic $0$ or the field contains primitive roots of unity for all orders $n$ prime to the characteristic. The method uses "rational simple connectedness" in an essential way. With the same method, we prove that such fields are $C_2$-fields, and we prove that "Period equals Index" for the Brauer groups of such fields. Finally, we use a similar method to reprove and extend a theorem of Fried-Jarden: every perfect PAC field of positive characteristic is $C_2$