(New) physics around the supermassive black hole

Abstract

Horizon-scale images from the Event Horizon Telescope (EHT) now reveal two stable features of M87⋆: a dark inner shadow and a bright, polarized ring. These two features provide clean and robust standing points for new-physics searches. In the shadow region, any model that adds microwave emission would partially illuminate it, so the observed darkness becomes a background-free intensity test that yields strong bounds, which we showcased with dark matter annihilation. On the ring, the differential linear polarization angle between observing days, predicted and observed to be nearly zero, cancels accretion flow systematics; its stability leads to leading limits on the axion–photon coupling in the ultralight mass range, assuming a superradiant origin. I will also highlight recent progress on controlling observational uncertainties, strengthening which parts of these conclusions can be regarded as robust.