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Testing General Relativity with Black Holes

Cosimo Bambi

TL;DR

The paper surveys tests of General Relativity in the strong-field regime near black holes using gravitational waves, X-ray spectroscopy, and imaging, with a focus on X-ray techniques that rely on disk-corona physics to probe spacetime geometry. It emphasizes continuum-fitting and relativistic reflection spectroscopy, discusses disk-corona models and tools like relxill_nk and nkbb for testing non-Kerr spacetimes, and presents current constraints on deviations from Kerr, notably via the Johannsen deformation parameter α13. The results indicate all measurements are consistent with Kerr, with X-ray methods often providing the strongest bounds and serving as a complement to gravitational-wave and imaging tests. The author also discusses a speculative interstellar mission to the nearest black hole as a potential route to ultra-precise tests of the Kerr hypothesis, contingent on future technological advances in propulsion and costs.

Abstract

The theory of General Relativity has successfully passed a large number of observational tests. The theory has been extensively tested in the weak-field regime with experiments in the Solar System and observations of binary pulsars. The past 10 years have seen significant advancements in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and black hole imaging. Here I summarize the state-of-the-art of the tests of General Relativity with black hole X-ray data and I briefly discuss the long-term vision of the possibility of an interstellar mission to the closest black hole for more precise and accurate tests.

Testing General Relativity with Black Holes

TL;DR

The paper surveys tests of General Relativity in the strong-field regime near black holes using gravitational waves, X-ray spectroscopy, and imaging, with a focus on X-ray techniques that rely on disk-corona physics to probe spacetime geometry. It emphasizes continuum-fitting and relativistic reflection spectroscopy, discusses disk-corona models and tools like relxill_nk and nkbb for testing non-Kerr spacetimes, and presents current constraints on deviations from Kerr, notably via the Johannsen deformation parameter α13. The results indicate all measurements are consistent with Kerr, with X-ray methods often providing the strongest bounds and serving as a complement to gravitational-wave and imaging tests. The author also discusses a speculative interstellar mission to the nearest black hole as a potential route to ultra-precise tests of the Kerr hypothesis, contingent on future technological advances in propulsion and costs.

Abstract

The theory of General Relativity has successfully passed a large number of observational tests. The theory has been extensively tested in the weak-field regime with experiments in the Solar System and observations of binary pulsars. The past 10 years have seen significant advancements in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and black hole imaging. Here I summarize the state-of-the-art of the tests of General Relativity with black hole X-ray data and I briefly discuss the long-term vision of the possibility of an interstellar mission to the closest black hole for more precise and accurate tests.

Paper Structure

This paper contains 7 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Testing Black Holes with X-Ray Data
  3. Disk-Corona Model
  4. Testing New Physics
  5. Results
  6. An interstellar mission to the closest black hole?
  7. Concluding remarks

Figures (2)

  • Figure 1: Left panel: Disk-corona model. Right panel: Examples of possible coronal geometries. Figure from Ref. Bambi:2024hhi.
  • Figure 2: Summary of current 3-$\sigma$ constraints on the Johannsen deformation parameter $\alpha_{13}$ with X-ray reflection spectroscopy (error bars in green for stellar-mass black holes and error bar in cyan for supermassive black holes), continuum-fitting method (error bar in magenta), combination of X-ray reflection spectroscopy and continuum-fitting method (error bars in blue), gravitational waves (error bar in red), and black hole imaging (error bars in gray). See the text and Ref. Bambi:2022dtw for more details.