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

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${}^{\ast }$ and Sagittarius A${}^{\ast }$ (Sgr A${}^{\ast }$), using the size $r_{\mathrm{sh}}$ and deviation from circularity $\hat{I}”\mathcal{C}$ of the BH shadow. For the case of the EHT image of Sgr A${}^{\ast }$, limits on $\hat{I}”\mathcal{C}$ are not available due to the sparse interferometric coverage of the 2017 observations, alongside the short variability timescale of Sgr A${}^{\ast }$ compared to M87${}^{\ast }$. 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${}^{\ast }$'s shadow. The latter is quoted to be $\lesssim 10\%$ as with M87${}^{\ast }$, 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${}^{\ast }$'s image will therefore mostly have to rely on $r_{\mathrm{sh}}$, whereas additional observables such as the photon ring and azimuthal angle lapse should soon be available and allow for novel tests.

3 pages, no figures