Suppressing GR deviations with rotating black holes in PRL

In a paper published in Physical Review Letters, Gr@v researchers Pedro Cunha and Eugen Radu (in collaboration  with Carlos Herdeiro from IST-Lisbon) provide a new insight on how spin could mask non-GR features in astrophysical black holes.

Black holes are mysterious spacetime regions that populate the Universe. In Einstein’s general relativity they are described by the canonical Kerr black hole model, which is completely specified by the black hole mass and angular momentum (spin).

Although theoretically supported, the claim that astrophysical black holes abide by this canonical model is an hypothesis in need of observational confirmation.

The first image of a black hole released in 2019 by the EHT collaboration provides new observational data to test the Kerr hypothesis. Einstein’s general relativity, and the Kerr model are consistent with the data. But, within the present accuracy, how much can these data distinguish alternative black hole models that may be, theoretically, also viable?

In this letter Cunha, Herdeiro and Radu show how a spin selection effect may mask non-Kerrness. In a class of alternative models, which are dynamically viable, only small spin black holes deviate significantly from the canonical model, whereas high spin black holes are observationally indistinguishable, or exactly equal, to the Kerr model.  This yields a concrete realization on a richer landscape of black holes, which may exhibit different properties only for certain ranges of mass or spin.