Last year, the astronomical community achieved an absolute wonder. For the very first time, the world collectively laid eyes on an actual image of the shadow of a black hole. It was the culmination of years of work, a magnificent achievement in both human collaboration and technical ingenuity.
And, like the best scientific breakthroughs, it opened a whole new world of enquiry. For a team led by astrophysicist Hector Olivares from Radboud University in the Netherlands and Goethe University in Germany, that enquiry was: how do we know M87* is a black hole?
"While the image is consistent with our expectations on what a black hole would look like, it is important to be sure that what we are seeing is really what we think," Olivares told ScienceAlert.
"Similarly to black holes, boson stars are predicted by general relativity and are able to grow to millions of solar masses and reach a very high compactness. The fact that they share these features with supermassive black holes led some authors to propose that some of the supermassive compact objects located at the center of galaxies could actually be boson stars."
So, in a new paper, Olivares and his team have calculated what a boson star might look like to one of our telescopes, and how that would differ from a direct image of an accreting black hole.
Boson stars are among the strangest theoretical objects out there. They're not much like conventional stars, except that they're a glob of matter. But where stars are primarily made up of particles called fermions - protons, neutrons, electrons, the stuff that forms more substantial parts of our Universe - boson stars would be made up entirely of... bosons.
Fermions are subject to the Pauli exclusion principle, which means you can't have two identical particles occupying the same space. Bosons, however, can be superimposed; when they come together, they act like one big particle or matter wave. We know this, because it's been done in a lab, producing what we call a Bose-Einstein condensate.
In the case of boson stars, the particles can be squeezed into a space which can be described with distinct values, or points on a scale. Given the right kind of bosons in the right arrangements, this 'scalar field' could fall into a relatively stable arrangement.
That's the theory, at least. Not that anybody has seen one in action. Bosons with the mass required to form such a structure, let alone one with the mass of a supermassive black hole, are yet to be spotted.
If we could identify a boson star, we would have effectively located this elusive particle.
"In order to form a structure as large as the SMBH candidates, the mass of the boson needs to be extremely small (less than 10-17 electronvolts)," Olivares said.
"Spin-0 bosons with similar or smaller masses appear in several cosmological models and string theories, and have been proposed as dark matter candidates under different names (scalar field dark matter, ultra-light axions, fuzzy dark matter, quantum wave dark matter). Such hypothetical particles would be extremely difficult to detect, but the observation of an object looking like a boson star would point to their existence."
Boson stars do not fuse nuclei, and they would not emit any radiation. They'd just sit there in space, being invisible. Much like black holes.
Unlike black holes, however, boson stars would be transparent - they lack an absorbing surface that would stop photons, nor do they have an event horizon. Photons can escape boson stars, although their path may be bent a little by the gravity.
But some boson stars may be surrounded by a rotating ring of plasma - a lot like the accretion disc that surrounds a black hole. And it would look fairly similar, like a glowing doughnut with a dark region inside.
So, Olivares and his team performed simulations of the dynamics of these plasma rings, and compared them to what we might expect to see of a black hole.
"The plasma configuration that we use is not set up 'by hand' (under reasonable assumptions), but results from a simulation of plasma dynamics. This allows the plasma to evolve in time and to form structures as it would in nature," Olivares explained.
"In this way we could relate the size of the dark region in the boson star images (which mimics a black hole shadow) to the radius where a plasma instability stops operating. In turn, this means that the size of the dark region is not arbitrary - it will depend on the properties of the boson star space-time - and also allows us to predict its size for other boson stars that we have not simulated."
They found that the boson star's shadow would be significantly smaller than the shadow of a black hole of similar mass. Thus, M87* could be ruled out as a boson star - at least as modelled by the team.
"The mass of [M87*] inferred from stellar dynamics is consistent with the expectations on the size of its shadow for the case of a black hole, so the dark region is too big to correspond to a non-rotating boson star similar to those we studied," Olivares told ScienceAlert.
But the team also took into account the technical capabilities and limitations of the Event Horizon Telescope which delivered that first black hole image; they deliberately set about visualising their results as they thought boson stars might look as imaged by the EHT.
This means their results can be compared to future EHT observations, to determine if what we're looking at is indeed a supermassive black hole.
If it were not, that would be a very big deal. It wouldn't mean that supermassive black holes don't exist - the range of masses for black holes is way too broad for boson stars. But it would hint that boson stars are real, and in turn that would have huge implications, for everything from the inflation of the early Universe to the search for dark matter.
"It would mean that cosmological scalar fields exist and play an important role in the formation of structures in the Universe," Olivares told ScienceAlert.
"The growth of supermassive black holes is still not understood very well, and if it turns out that at least some of the candidates are actually boson stars, we would need to think of different formation mechanisms involving scalar fields."
The research was published in July in the Monthly Notices of the Royal Astronomical Society.