Hai-Qing Zhang

At the center of the Milky Way lurks something invisible but extremely massive — about four million times heavier than the Sun. It is usually called a black hole and designated Sagittarius A*. But astronomers are not letting their guard down: what if it is not a hole but something more exotic, like a giant ball of ultralight particles — a boson star?

The cosmos is full of mimicry. The clearwing moth wears a wasp's mask, a harmless fly buzzes like a bee. And in the world of gravity, the role of impersonator is perhaps played by boson stars — colossal clumps of scalar field, indistinguishable from black holes.