The Laplace-Beltrami spectrum on Naturally Reductive Homogeneous Spaces

We prove a formula for the spectrum of the Laplace-Beltrami operator on functions for compact naturally reductive homogeneous spaces in terms of eigenvalues of a generalized Casimir operator and spherical representations. We apply this result to a large family of canonical variations of normal homogeneous metrics, thus allowing for the first time to study how the spectrum depends on the deformation parameters of the metric. As an application, we provide a formula for the full spectrum of compact positive homogeneous $3$-$(\alpha,\delta)$-Sasaki manifolds (a family of metrics which includes, in particular, all homogeneous $3$-Sasaki manifolds). The second part of the paper is devoted to the detailed computation and investigation of the spectrum of this family of metrics on the Aloff-Wallach manifold $W^{1,1}=SU(3)/S^{1}$; in particular, we provide a documented Python script that allows the explicit computation in any desired range. We recover Urakawa's eigenvalue computation for the $SU(3)$-normal homogeneous metric on $W^{1,1}$ as a limiting case and cover all the positively curved $SU(3)\times SO(3)$-normal homogeneous realizations discovered by Wilking. By doing so, we complete Urakawa's list of the first eigenvalue on compact, simply conntected, normal homogeneous spaces with positive sectional curvature.