Bound on photon circular orbits in general relativity and beyond

Abstract

Existence of a photon circular orbit can tell us a lot about the nature of the underlying spacetime, since it plays a pivotal role in the understanding of the characteristic signatures of compact objects, namely the quasi-normal modes and shadow radius. For this purpose, determination of the location of the photon circular orbit is of utmost importance. In this work, we derive bounds on the location of the photon circular orbit around compact objects within the purview of general relativity and beyond. As we have explicitly demonstrated, contrary to the earlier results in the context of general relativity, the bound on the location of the photon circular orbit is not necessarily an upper bound, rather depending on the matter content it is possible to arrive at a lower bound as well. This have interesting implications for the quasi-normal modes and shadow radius, the two key observables related to the strong field tests of gravity. Besides discussing the bound for higher dimensional general relativity, we have also considered how the bound on the photon circular orbits gets modified in the braneworld scenario, for pure Lovelock and general Lovelock theories of gravity. Implications of these results for compact objects have also been discussed.

Invited contribution, 25 pages, no figures

References in corpus (20)

• Dynamics of dark energy
• Unified cosmic history in modified gravity: from F(R) theory to Lorentz non-invariant models
• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• The Confrontation between General Relativity and Experiment
• Dark energy in modified Gauss-Bonnet gravity: late-time acceleration and the hierarchy problem
• Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole
• Quasinormal modes and Strong Cosmic Censorship
• Solar system constraints on f(T) gravity
• Black hole mimicker hiding in the shadow: Optical properties of the $\gamma$-metric
• TASI Lectures on Extra Dimensions and Branes
• Understanding photon sphere and black hole shadow in dynamically evolving spacetimes
• How does a dark compact object ringdown?
• Upper bound on the radii of black-hole photonspheres
• Rotating regular black holes in conformal massive gravity
• Spherically symmetric brane spacetime with bulk $f(\mathcal{R})$ gravity
• Light rings of stationary spacetimes
• Constraining some Horndeski gravity theories
• On the Size of a Black Hole: The Schwarzschild is the Biggest
• Can a light ray distinguish charge of a black hole in nonlinear electrodynamics?
• Lower bound on the radii of black-hole photonspheres