Note on fundamental physics tests from black hole imaging: Comment on "Hunting for extra dimensions in the shadow of Sagittarius A$^*$"

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Visinelli, Luca 

Several works over the past years have discussed the possibility of testing fundamental physics using Very Long Baseline Interferometry horizon-scale black hole (BH) images, such as the Event Horizon Telescope (EHT) images of M87 and Sagittarius A (Sgr A), using the size rsh and deviation from circularity ΔC of the BH shadow. For the case of the EHT image of Sgr A, limits on ΔC are not available due to the sparse interferometric coverage of the 2017 observations, alongside the short variability timescale of Sgr A compared to M87. Concerning this point, we comment on the results of a recent preprint which purports to have derived new limits on extra dimensions using the deviation from circularity of Sgr A's shadow. The latter is quoted to be ≲10% as with M87, based on the "similarity" of the two shadows: however, this is an incorrect assumption, invalidating the subsequent results. In the immediate future, the simplest tests of fundamental physics from Sgr A's image will therefore mostly have to rely on rsh, whereas additional observables such as the photon ring and azimuthal angle lapse should soon be available and allow for novel tests.

astro-ph.GA, astro-ph.GA, astro-ph.CO, gr-qc, hep-ph, hep-th
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