Can the supermassive objects at the centers of galaxies be traversable wormholes? The first test of strong gravity for mm/sub-mm VLBI facilities

The near future mm/sub-mm VLBI experiments are ambitious projects aiming at imaging the "shadow" of the supermassive black hole candidate at the center of the Milky Way and of the ones in nearby galaxies. An accurate observation of the shape of the shadow can potentially test the nature of these objects and verify if they are Kerr black holes, as predicted by general relativity. However, previous work on the subject has shown that the shadows produced in other spacetimes are very similar to the one of the Kerr background, suggesting that tests of strong gravity are not really possible with these facilities in the near future. In this work, I instead point out that it will be relatively easy to distinguish black holes from wormholes, topologically non-trivial structures of the spacetime that might have been formed in the early Universe and might connect our Universe with other universes.