Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$ (2205.07787v3)

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.

• The paper demonstrates that several modified gravity theories predict shadow sizes consistent with EHT observations of Sagittarius A*.
• It leverages precise mass-to-distance measurements and horizon-scale imaging to rigorously test departures from General Relativity.
• The findings constrain no-hair theorem violations and offer insights into wormhole configurations and quantum gravity effects.

Overview of "Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A*"

This paper presents a comprehensive investigation into the potential deviations from classical General Relativity (GR) in the vicinity of supermassive black holes, focusing on the Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the black hole at the center of the Milky Way. By employing horizon-scale images of black holes, the paper aims to test various theories of gravity and novel physics that propose deviations from the standard black hole solutions obtained from GR.

Methodology and Scope

The paper hinges on comparing the observed angular radius of the emission ring surrounding Sgr A*'s shadow with predictions derived from various modified gravity theories. These theories suggest deviations from GR parameters by altering horizon properties and introducing a new set of potential observables. The authors leverage the precise mass-to-distance ratio provided by stellar dynamics around Sgr A* to accomplish this task, which is essential for linking the observed angular size to theoretical models.

A range of metrics inspired by regular black holes, alternative gravity theories, and wormhole configurations are explored. These cover an array of scenarios where the classical singularity at a black hole’s core is resolved or altered through new physics, often inspired by ideas from string theory, quantum gravity, or cosmology.

Key Results and Implications

1. Consistent Scenarios: The paper finds multiple instances where modifications to black hole metrics are consistent with observations. For instance, some regular black hole solutions, when accompanied by suitably defined parameters, predict shadow sizes within EHT's observational constraints.
2. Constrained Deviations: Several deviations predict shadow sizes larger than the classical Schwarzschild black hole: these are particularly constrained by the EHT data. The effect of additional parameters often results in observable predictions about the extent of these deviations, offering a window into testing theories like those incorporating additional scalar and tensor fields.
3. No-hair Theorem Violations: The observational data serves as a testbed for violations of the no-hair theorem. This theorem typically constrains black holes to be described completely by three parameters (mass, charge, and spin), but alternative metrics suggest additional "hair," or parameters.
4. Wormholes: Some wormhole configurations are permitted under the observed shadow size, suggesting intriguing possibilities about the nature of space-time. Constraints provided by the EHT may serve to exclude certain parameter spaces in wormhole physics.
5. Fundamental Down to Planck Scale: The paper addresses how certain aspects of quantum gravity, like generalizations of the uncertainty principle or non-commutative geometry, can influence the shadow's size. Such modifications are tightly constrained, pointing towards minimal impacts at scales probed by the EHT.

Future Directions

The work sets a precedent for utilizing high-resolution images of black holes as tools to examine the frontier of theoretical physics beyond GR. It suggests several directions for future research:

• Improved Precision: Enhancing mass-to-distance measurements and imaging resolution will refine these tests significantly.
• Complementary Observations: Together with gravitational wave detections, these provide independent checks on the same physics, potentially leading to breakthroughs when combined.
• Broader Parameter Studies: As observational technology advances, investigating more diverse classes of modified metrics, including rapidly rotating black holes, is crucial.

The paper underscores the importance of black hole imagery in testing fundamental physics, advocating for continual improvements in observational capabilities. By tethering theoretical predictions to observational data, this line of research bridges the gap between abstract theoretical advancements and empirical validation, paving the way for potential discoveries regarding the true nature of gravity and space-time.