Massive particle surfaces, partial umbilicity and circular orbits

TL;DR

This paper introduces Massive Particle Surfaces (MPS) as a generalization of photon spheres, using Riemannian geometry to analyze trajectories of massive particles and characterize circular orbits in curved spacetimes.

Contribution

It provides a new geometric framework for defining and analyzing massive particle surfaces and extends existing results on photon spheres and circular orbits.

Findings

Derived conditions for the existence of MPS in asymptotically flat spacetimes

Characterized event horizons using Riemannian curvature

Calculated radii of photon spheres and ISCOs

Abstract

The generalization of photon spheres by considering the trajectories of massive particles leads to the definition of Massive Particle Surfaces (MPS). These surfaces are built with the trajectories of massive particles, and have a partial umbilicity property. Using the geodesic and Gaussian curvature of the Jacobi metric (a Riemannian metric) we derive a general condition for the existence of a Massive Particle Surface defined for an asymptotically flat spacetime metric. Our results can be applied to the worldlines of charged massive particle surfaces. We provide a simple characterization for timelike and null trajectories using a Riemannian geometric approach. We are able to recover the results for the existence of Light Rings (LR's) and timelike circular orbits (TCO's). We show how an event horizon gets characterized using the curvatures of a Riemannian metric. We discuss several examples, where we derive conditions for the existence of photon sphere and a massive particle surface. We calculate the radius of the photon sphere and the radius of the Innermost Stable Circular Orbits (ISCO).