Homogeneous and flow-invariant geometry on the unit tangent bundle of hyperbolic space

Abstract

We construct the Sasaki metric on the unit tangent bundle $U_g M$ of a Riemannian manifold $(M , g)$ and describe the unit tangent bundle $U\mathbb{H}^n$ of real hyperbolic space as a homogeneous space, both under $SO_0 (1, n)$ and under the larger group $SO_0 (1, n) \times SO_0 (1, 1)$, yielding explicit of $G$-invariant metrics. Using Hopf coordinates and Busemann functions, we then construct a Riemannian metric $g_{Hopf}$ on $U\mathbb{H}^n$ that is invariant under the geodesic flow, and we identify the horospherical cylinders as totally geodesic leaves of a natural foliation associated to a Busemann function, with respect to an explicit metric connection with torsion.