Spontaneous Lorentz symmetry-breaking constraints in Kalb-Ramond gravity

TL;DR

This paper investigates how Lorentz symmetry-breaking in Kalb-Ramond gravity affects black hole geodesics and observational signatures, providing constraints on the symmetry-breaking parameter using astrophysical data from EHT, GP-B, and S2 star orbits.

Contribution

It introduces new constraints on the Lorentz symmetry-breaking parameter in Kalb-Ramond gravity based on multiple astrophysical observations and analyzes the impact on geodesic properties and black hole shadows.

Findings

Parameter bounds from EHT data: -0.0700225 to 0.189785

Tighter constraints from Gravity Probe B: approximately ±3.9×10^{-12}

Impact on photon sphere and shadow radius consistent with GR in the limit l→0

Abstract

In this work, we study timelike and lightlike geodesics in Kalb-Ramond (KR) gravity around a black hole with the goal of constraining the Lorentz symmetry-breaking parameter $l$. The analysis involves studying the precession of the S2 star periastron orbiting Sgr A* and geodesic precession around the Earth. The ratio of precession frequencies for General Relativity (GR) and KR gravity is computed, with Event Horizon Telescope (EHT) results providing a parameter range for the spontaneous symmetry-breaking of $-0.185022 \leq l \leq 0.060938$. Utilizing the geodesic precession frequency from the Gravity Probe B (GP-B), the $l$ parameter is further constrained to $-6.30714 \times 10^{-12} \leq l \leq 3.90708 \times 10^{-12}$, which is consistent with the Schwarzschild limits. Moreover, for timelike geodesics, the innermost circular orbit (ICO) and innermost stable circular orbit (ISCO) are determined and analyzed to illustrate the impact of the symmetry breaking term. Zoom-whirl obstructions are compared with the Schwarzschild solution. Lower and upper limits of the photon sphere for lightlike geodesics are established to demonstrate the influence of KR gravity on the photon sphere. Additionally, the shadow radius is determined for two observers, one situated at a finite distance from the KR black hole, and the other located at an infinite distance, to constrain the symmetry-breaking parameter $l$, with comparisons made to EHT results. The bounds for $l$ derived from constraints on the photon sphere radius for lightlike geodesics yield $-0.0700225 \leq l \leq 0.189785$ using EHT data. The findings of this paper align with experimental results in the $l \rightarrow 0$ limit.