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I am aware of How do we know the stars orbiting Sgr A* are orbiting a supermassive black hole and not just the center of mass of the Milky Way galaxy?, which asks why the motions of stars near the Galactic centre suggest a 4 million solar mass black hole.

But are there any theoretical ideas to explain these observations, that avoid the conclusion of a black hole, that remain unfalsified?

And will subsequent observations (e.g. with the Event Horizon telescope) be able to falsify these alternatives or indeed the hypothesis of a supermassive black hole?

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Most mundane alternative explanations (like a very dense star cluster) have been ruled out by measuring the trajectories of nearby stars. These make clear that we are dealing with a compact object of roughly 4 million solar masses.

A black hole seems the least exotic of the objects that it could, as it requires no new physics. However, various exotic explanation such a boson star or a gravastar can be offered. However, these typically require the introduction of new physics, and even then no clear mechanisms for their formation might be available.

The Event Horizon telescope certainly will be able to further constrain hypothetically exotic alternatives. (Although it may still not rule all of them out completely.)

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