[Paper Review] Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$

Sunny Vagnozzi, Rittick Roy+13 more|arXiv (Cornell University)|May 16, 2022

Astrophysical Phenomena and Observations32 citations

TL;DR

The paper uses the EHT image of Sgr A* to constrain a wide range of gravity theories and fundamental-physics scenarios by comparing observed ring/shadow sizes with theoretical predictions, leveraging the mass-to-distance ratio and a calibration between ring and shadow sizes.

ABSTRACT

Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. We consider a wide range of well-motivated deviations from classical General Relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A$^*$ (Sgr A$^*$), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of Sgr A$^*$'s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BHs, string-inspired space-times, violations of the no-hair theorem driven by additional fields, alternative theories of gravity, novel fundamental physics frameworks, and BH mimickers including well-motivated wormhole and naked singularity space-times. We demonstrate that the EHT image of Sgr A$^*$ places particularly stringent constraints on models predicting a shadow size larger than that of a Schwarzschild BH of a given mass, with the resulting limits in some cases surpassing cosmological ones. Our results are among the first tests of fundamental physics from the shadow of Sgr A$^*$ and, while the latter appears to be in excellent agreement with the predictions of GR, we have shown that a number of well motivated alternative scenarios, including BH mimickers, are far from being ruled out at present.

Motivation & Objective

• Assess how the EHT image of Sagittarius A* constrains deviations from general relativity across diverse gravity theories and BH mimickers.
• Link the observed bright ring angular size to the theoretical BH shadow size for various space-time metrics.
• Quantify bounds on curvature-potential and other fundamental-physics parameters using the Sgr A* mass-to-distance ratio.
• Explore the regime differences probed by Sgr A* versus M87* in strong-field gravity tests.

Proposed method

• Compare the observed angular radius of the bright ring in the Sgr A* EHT image with shadow radii computed in various space-time metrics.
• Utilize the mass-to-distance ratio of Sgr A* from Keck and VLTI measurements to calibrate the shadow size predictions.
• Define and use the fractional deviation delta between the inferred shadow radius and the Schwarzschild shadow radius: delta = r_sh/(3√3 M) − 1.
• Restrict analysis to static spherically symmetric metrics to avoid spin-related degeneracies on shadow size.
• Adopt a calibration factor for ring-to-shadow size mapping and propagate uncertainties from mass, distance, and emission modeling.

Experimental results

Research questions

• RQ1 What are the constraints on non-Schwarzschild space-times and BH mimickers given the Sgr A* shadow size?
• RQ2 How does the Sgr A* shadow size limit models with larger-than-Schwarzschild shadow radii at a given mass?
• RQ3 How do the Sgr A* measurements compare to M87* in probing strong-field gravity regimes?
• RQ4 What is the impact of the mass-to-distance calibration and emission physics on bounds of alternative gravity theories?

Key findings

• EHT data constrain models predicting a shadow larger than the Schwarzschild value 3√3 M, with some bounds surpassing cosmological ones.
• Average δ from Keck and VLTI indicates a slight preference for a shadow marginally smaller than Schwarzschild, within 1σ–2σ ranges.
• Shadow-size bounds translated into 1σ ranges for r_sh/M: 4.55 to 5.22, and 2σ ranges: 4.21 to 5.56.
• Results rely on Sgr A*’s well-measured mass-to-distance ratio, enabling robust tests independent of specific gravity theories.
• The methodology is robust for spherically symmetric metrics and provides first-principles tests of multiple gravity scenarios using the Sgr A* shadow.
• The analysis highlights that Sgr A* probes a different curvature regime than M87*, enabling complementary strong-field tests.