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# The current ability to test theories of gravity with black hole shadows

## Abstract

Our Galactic Centre, Sagittarius A*, is believed to harbour a supermassive black hole, as suggested by observations tracking individual orbiting stars1,2. Upcoming submillimetre very-long baseline interferometry images of Sagittarius A* carried out by the Event Horizon Telescope collaboration (EHTC)3,4 are expected to provide critical evidence for the existence of this supermassive black hole5,6. We assess our present ability to use EHTC images to determine whether they correspond to a Kerr black hole as predicted by Einstein’s theory of general relativity or to a black hole in alternative theories of gravity. To this end, we perform general-relativistic magnetohydrodynamical simulations and use general-relativistic radiative-transfer calculations to generate synthetic shadow images of a magnetized accretion flow onto a Kerr black hole. In addition, we perform these simulations and calculations for a dilaton black hole, which we take as a representative solution of an alternative theory of gravity. Adopting the very-long baseline interferometry configuration from the 2017 EHTC campaign, we find that it could be extremely difficult to distinguish between black holes from different theories of gravity, thus highlighting that great caution is needed when interpreting black hole images as tests of general relativity.

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## Acknowledgements

We thank M. Moscibrodzka, T. Bronzwaer and J. Davelaar for fruitful discussions. This research is supported by the ERC synergy grant ‘BlackHoleCam: Imaging the Event Horizon of Black Holes’ (grant number 610058). Z.Y. acknowledges support from an Alexander von Humboldt Fellowship. H.O. is supported in part by a CONACYT-DAAD scholarship. The simulations were performed on LOEWE at the CSC-Frankfurt and Iboga at ITP Frankfurt.

## Author information

Authors

### Contributions

Y.M. performed the GRMHD simulations and wrote the manuscript. Z.Y. performed the GRRT calculations and authored the GRRT code BHOSS. C.M.F. calculated the synthetic black hole shadow images. Z.Y. and C.M.F. helped write the manuscript. O.P. authored the GRMHD code BHAC. M.D.L. provided information on alternative theories of gravity. H.O. helped to perform the GRMHD simulations. H.F. and M.K. provided insight into the scientific interpretation of the results. L.R. initiated and closely supervised the project, and wrote the manuscript. All authors discussed the results and commented on all versions of the manuscript.

### Corresponding author

Correspondence to Yosuke Mizuno.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary text, Supplementary Figures 1–9, Supplementary Tables 1–5

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### Cite this article

Mizuno, Y., Younsi, Z., Fromm, C.M. et al. The current ability to test theories of gravity with black hole shadows. Nat Astron 2, 585–590 (2018). https://doi.org/10.1038/s41550-018-0449-5

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