Isotropic random spin weighted functions on $S^2$ vs isotropic random fields on $S^3$

TL;DR

This paper explores the differences between isotropic random fields on $SU(2)$ and $S^3$, revealing the stronger conditions on isotropy on the sphere and analyzing the structure of such fields through harmonic polynomials and spin-weighted functions.

Contribution

It clarifies the distinction between isotropy on groups and spheres, and characterizes isotropic random fields on $S^3$ using spin-weighted functions and harmonic polynomials.

Findings

Isotropic fields on $SU(2)$ are not necessarily isotropic on $S^3$.

Any isotropic field on $S^3$ decomposes into uncorrelated harmonic polynomials with spin weights.

For fixed degree, each spin weight appears with equal magnitude.

Abstract

We show that an isotropic random field on $SU(2)$ is not necessarily isotropic as a random field on $S^3$, although the two spaces can be identified. The ambiguity is due to the fact that the notion of isotropy on a group and on a sphere are different, the latter being much stronger. We show that any isotropic random field on $S^3$ is necessarily a superposition of uncorrelated random harmonic homogeneous polynomials, such that the one of degree $d$ is necessarily a superposition of uncorrelated random spin weighted functions of every possible spin weight in the range $\{-\frac{d}{2},\dots,\frac{d}{2}\}$, each of which is isotropic in the sense of $SU(2)$. Moreover, for a random field of fixed degree, each spin weight appears with the same magnitude, in a sense to be specified. In addition we will give an overview of the theory of spin weighted functions and Wigner $D$-matrices, with the purpose of gathering together many different points of view and adding ours. As a byproduct of this survey we will prove some new properties of the Wigner matrices and a formula relating the operators $\eth\overline{\eth}$ and the horizontal Laplacian of the Hopf fibration $S^3\to S^2$.