Skew-symmetric complex matrices, pure spinors, the twistor space of the conformal $2n$-sphere, and the Fano variety of linear $n$-folds of a non-singular complex quadric hypersurface in $\mathbb{P}^{2n+1}$

TL;DR

This paper explores the complex geometry of twistor spaces of conformal spheres, revealing their structure as closures in Grassmannians, stratifications, and explicit foliations, especially focusing on the 6-sphere case.

Contribution

It provides a detailed description of the twistor space's stratification and explicit foliation for the 6-sphere, connecting complex geometry with Riemannian structures.

Findings

Twistor space of conformal 2n-sphere is a Zariski closure in Grassmannian.

The twistor space of S^6 is a smooth complex quadric hypersurface.

Constructed a foliation of the quadric hypersurface with quotient isomorphic to S^6.

Abstract

For $n \geq 1$, the twistor space $\mathfrak{Z}(\mathbb{S}^{2n})$ of the conformal $2n$-sphere is biholomorphic to the Zariski closure, taken in the complex Grassmannian manifold $\mathbf{G}(n+1, 2n+2)$, of the set of graphs of skew-symmetric linear endomorphism of $\mathbb{C}^{n+1}$. We use this fact to describe a natural stratification of the twistor space $\mathfrak{Z}(\mathbb{S}^{2n})$ with $n \geq 3$, in terms of what we have called {\it generalised complex orthogonal Stiefel manifolds} of $\mathbb{C}^{n+1}$. In particular, the twistor space $\mathfrak{Z}(\mathbb{S}^{6})$ is biholomorphic to a non-singular complex quadric hypersurface in $\mathbb{P}^{7}$. We explicitly construct a real-analytic foliation, by linear 3-folds, of this quadric hypersurface such that the quotient space is isomorphic to the 6-sphere with its standard metric. This foliation is Riemannian with respect to the Fubini-Study metric and isometrically equivalent to the twistor fibration over the 6-sphere.