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

Abstract

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 r sh and deviation from circularity ∆C of the BH shadow. For the case of the EHT image of Sgr A * , limits on ∆C 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 r sh , whereas additional observables such as the photon ring and azimuthal angle lapse should soon be available and allow for novel tests.