Black Holes Could Be Something Else Entirely. And We Can Test It

There is something at the center of the Milky Way. Whatever it is, it has 4.1 million times the mass of the Sun. It is very dense, so dense that stars are orbiting around it like comets going around the Sun. The center of the Milky Way orbits around this region in a way that is very peculiar and requires a very dense massive object there. Astronomers have gathered together a worldwide network of telescopes and produced an image of the shadow of the event horizon around this thing, and the traditional belief is that this is a supermassive black hole. But now, a team of researchers are proposing that it's not a black hole at all, but a blob of dark matter. Or specifically, a supermassive compact object composed of self-gravitating fermionic dark matter. Okay, so what is fermionic matter? Fermions are light subatomic particles, like quarks, leptons, they make up most of the matter in the universe. Baryons, atoms, atomic nuclei, protons, neutrons, regular stuff is made of fermions. And fermions fall under the Pauli exclusion principle. And we sort of mostly see this with neutron stars, that if you jam fermions close enough together, they refuse to be pushed together any further. So that's fermion. So what are dark matter fermions? Well, we know that 85% of the mass of the universe is something that we can't see. We can detect it through its mass, but it doesn't produce any electromagnetic radiation. And so there have been various proposed particles that would explain this, and one is a type of fermion, just not visible. And so if you've got these fermions that don't interact with electromagnetic radiation, then you can compress them very closely together. And they could form a sphere that would have the same kind of characteristics as a black hole, but not actually have an event horizon and be a black hole. And this would give you a dense enough object that would explain the observations around the supermassive black hole. So stars would still orbit around this thing in the same way that you would still get this, the movement of the inner Milky Way in the same way as if there was a supermassive black hole. And so the orbits of those stars would be almost exactly the same as what would be going around a black hole, off by about 1%, which is a really tricky observation to make with our current level of telescopes. But there's something that you see with a black hole that you wouldn't see with a fermionic blob of dark matter, and that is the photon ring. Just outside the event horizon, they can orbit around the black hole, and this will create this sort of shell of light around the black hole. But because the fermionic dark matter isn't as compact as the black hole and that its mass is a little bit more diffused, then you wouldn't get the photon ring. And so we're right at the limit of being able to detect the photon ring around a black hole. Hasn't been done yet. People have claimed that they've detected the photon ring, but they haven't seen it so far. And so when a next generation of bigger event horizon telescope comes online and has the resolving power to actually be able to detect the photon ring around a black hole, that could tell us whether this thing is a black hole or whether it's actually a compressed mass of dark matter. And if it is dark matter, then that will help us explain what dark matter is in general, that the characteristics of this would also be the characteristics of a lot of the dark matter that's out there across the universe. So, you know, it's an out there idea, but it's testable if you can detect the photon ring and could help us explain a lot of ideas in cosmology and astrophysics. Now, if you found this interesting, this is just a small example of the space news coverage that we do every week. We do a weekly show called Space Bites that collects together many more stories like this. I'll put a link in the show notes to this week's episode of Space Bites if you want much more space news and you can check it out and subscribe.