The $N$-Body Problem in Spaces with Uniformly Varying Curvature

We generalize the curved $N$-body problem to spheres and hyperbolic spheres whose curvature $\kappa$ varies in time. Unlike in the particular case when the curvature is constant, the equations of motion are non-autonomous. We first briefly consider the analogue of the Kepler problem and then investigate the homographic orbits for any number of bodies, proving the existence of several such classes of solutions on spheres. Allowing the curvature to vary in time offers some insight into the effect of an expanding universe, in the context the curved $N$-body problem, when $\kappa$ satisfies Hubble's law. The study of these equations also opens the possibility of finding new connections between classical mechanics and general relativity.