Gravitational lensing by a black hole with torsion

TL;DR

This paper explores how torsion in a generalized Einstein-Cartan-Kibble-Sciama gravity affects gravitational lensing, revealing unique phenomena like horizons without photon spheres and unusual deflection angle behaviors.

Contribution

It introduces a detailed analysis of gravitational lensing in torsion-influenced spacetimes, highlighting novel cases and effects not seen in standard general relativity.

Findings

Existence of horizons without photon spheres in certain torsion parameters.

Deflection angle diverges logarithmically near the horizon in special cases.

In the far-field limit, deflection angles can approach zero from the negative side.

Abstract

In this paper we have investigated the gravitational lensing in a spherically symmetric spacetime with torsion in the generalized Einstein-Cartan-Kibble-Sciama (ECKS) theory of gravity by considering higher order terms. The torsion parameters change the spacetime structure which affects the photon sphere, the deflection angle and the strong gravitational lensing. The condition of existence of horizons is not inconsistent with that of the photon sphere. Especially, there exists a novel case in which there is horizon but no photon sphere for the considered spacetime. In this special case, the deflection angle of the light ray near the event horizon also diverges logarithmically, but the coefficients in the strong-field limit are different from those in the cases with photon sphere. Moreover, in the far-field limit, we find that the deflection angle for certain torsion parameters approaches zero from the negative side, which is different from those in the usual spacetimes.