Tests of Loop Quantum Gravity from the Event Horizon Telescope Results of Sgr A*

Abstract

The Event Horizon Telescope (EHT) collaboration’s image of the compact object at the Galactic center is the first direct evidence of the supermassive black hole (BH) Sgr A*. The shadow of Sgr A* has an angular diameter d _sh = 48.7 ± 7 μ as with fractional deviation from the Schwarzschild BH shadow diameter $\delta =-{0.08}_{-0.09}^{+0.09},-{0.04}_{-0.10}^{+0.09}$ (for the VLTI and Keck mass-to-distance ratios). Sgr A*'s shadow size is within 10% of Kerr predictions, equipping us with yet another tool to analyze gravity in the strong-field regime, including testing loop quantum gravity (LQG). We use Sgr A*'s shadow to constrain the metrics of two well-motivated LQG-inspired rotating BH (LIRBH) models characterized by an additional deviation parameter L _q , which recover the Kerr spacetime in the absence of quantum effects ( L _q → 0). When increasing the quantum effects through L _q , the shadow size increases monotonically, while the shape gets more distorted, allowing us to constrain the fundamental parameter L _q . We use the astrophysical observables shadow area A and oblateness D to estimate the BH parameters. It may be useful in extracting additional information about LIRBHs. While the EHT observational results completely rule out the wormhole region in LIRBH-2, a substantial parameter region of the generic BHs in both models agrees with the EHT results. We find that the upper bounds on L _q obtained from the shadow of Sgr A*— L _q ≲ 0.0423 and L _q ≲ 0.0821 for the two LIRBHs, respectively—are more stringent than those obtained from the EHT image of M87*.

Keywords

• Astrophysical black holes
• Black hole physics
• Galactic center
• Gravitation
• Gravitational lensing