Null and timelike circular orbits from equivalent 2D metrics

TL;DR

This paper uses auxiliary 2D metrics to analyze null and timelike circular geodesics in static, spherically symmetric spacetimes, providing insights into light rings and stable orbits through curvature analysis.

Contribution

It introduces a method to study circular geodesics via equivalent 2D metrics, extending known results to include stable timelike orbits using Gaussian curvature analysis.

Findings

Number of light rings determined by optical manifold curvature

Existence of marginally stable timelike circular orbits characterized

Method applicable to generic static, spherically symmetric spacetimes

Abstract

The motion of particles on spherical $1 + 3$ dimensional spacetimes can, under some assumptions, be described by the curves on a 2-dimensional manifold, the optical and Jacobi manifolds for null and timelike curves, respectively. In this paper we resort to auxiliary 2-dimensional metrics to study circular geodesics of generic static, spherically symmetric, and asymptotically flat $1 + 3$ dimensional spacetimes, whose functions are at least $C^2$ smooth. This is done by studying the Gaussian curvature of the bidimensional equivalent manifold as well as the geodesic curvature of circular paths on these. This study considers both null and timelike circular geodesics. The study of null geodesics through the optical manifold retrieves the known result of the number of light rings (LRs) on the spacetime outside a black hole and on spacetimes with horizonless compact objects. With an equivalent procedure we can formulate a similar theorem on the number of marginally stable timelike circular orbits (TCOs) of a given spacetime satisfying the previously mentioned assumption