Invisible “dark matter” stars may explain puzzling gravitational waves
Last year scientists detected gravitational waves from what appeared to be the most massive black hole collision ever recorded. But now an international team of astrophysicists has proposed an exotic alternative – the data may actually favor a collision between two boson stars, hypothetical objects that would be invisible, incredibly dense and could even help untangle the mystery of dark matter.
Gravitational waves are ripples in the very fabric of spacetime, produced in some of the most energetic cataclysms in the cosmos – most commonly, collisions between black holes and/or neutron stars. Incredibly precise instruments run by the LIGO/Virgo Collaboration (LVC) pick up these waves as they wash over Earth, and the signals can be analyzed to learn about the masses of the objects involved in the original merger.
Around 50 gravitational wave signals have been detected since the first discovery in 2015, but one particularly interesting event, known as GW190521, was described by LVC in September 2020. At 65 and 85 times the mass of the Sun, the two colliding objects were the most massive ever detected through this method, and the object created in the aftermath measured 142 solar masses, placing it in a rare class of intermediate-mass black holes (IMBHs).
With those masses, the original objects were presumed to be black holes, albeit unusually large ones. But now, a team of astrophysicists has put forward a new explanation that may fit the bill more neatly – a collision between two exotic objects called boson stars.
As the name suggests, boson stars (if they exist) would be mostly made up of bosons, one of two classes of elementary particles. Regular stars are mostly made up of the other class, known as fermions. Boson stars would theoretically function much like black holes, sucking in matter from their surroundings thanks to their strong gravitational pull, but with one major difference.
Famously, even light itself cannot escape from a black hole. But boson stars don’t have this “point of no return,” meaning these exotic objects wouldn’t be pitch black but transparent and largely invisible.
In the new study, the researchers simulated mergers of boson stars, and found that they would produce a signal consistent with last year’s detection of the GW190521 event. In fact, the team says that boson stars are an even neater explanation than black holes.
The problem with the colliding black holes explanation is that the mass of one of these black holes would fall outside the accepted categories. There are stellar mass black holes, formed from the collapse of stars and resulting in masses of between five and a few dozen Suns. Then there are the aforementioned intermediate-mass black holes, with a range between 100 and 10,000 solar masses. At 85 solar masses, the object fell square in that “forbidden” mass range, puzzling astronomers on its discovery.
The boson star hypothesis removes that obstacle, although it does so by introducing a brand new one. This revised hypothesis would also change a few other properties of the collision.
“First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole,” says Juan Calderón Bustillo, an author of the study. “Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LVC. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true.”
While the team says this could represent the first evidence of boson stars, it’s all still very speculative for now. Further study and improved simulations will be needed to investigate the possibility.
If the study holds up to scrutiny, the implications extend far beyond new celestial objects. They could help unravel one of the most confounding mysteries of cosmology – dark matter. For boson stars to exist, they’d need to be made up of a stable boson that’s “self-repulsive,” and a hypothetical particle called an axion fits the bill. Axions are also a leading candidate for dark matter particles.
The research was published in the journal Physical Review Letters.
Source: City University Hong Kong