It was a cosmic collision that stunned scientists: 17 billion light years away, two objects that scientists think were massive black holes merged into a single monstrous entity imbued with the never-before-seen mass of 142 Suns.
The union was so tumultuous that it produced ripples in spacetime, known as gravitational waves, which traversed the universe for eons until they eventually swept through Earth on May 21, 2019. This gravitational wave event, named GW190521, is the most distant and massive black hole collision scientists have ever captured.
Or was it?
A team of researchers now suggests that this epic crash may be even stranger than previously assumed. Instead of an unprecedented black hole merger, GW190521 could have been produced by the collision of two “boson stars,” bizarre hypothetical objects that scientists believe might be made of dark matter, according to a recent study in Physical Review Letters.
If this proposed origin for the gravitational wave signal is true, GW190521 would confirm, for the first time, that these exotic stars actually exist. What’s more, the discovery could shed light on one of the biggest mysteries in science: the nature of dark matter, a non-luminous substance that is about six times more common in the universe than the familiar matter that makes up planets, stars, and our bodies.
The new research was co-led by Juan Calderón Bustillo, who serves as La Caixa Junior Leader and Marie Curie Fellow at the Galician Institute of High Energy Physics (IGFAE), a joint center of the University of Santiago de Compostela and Xunta de Galicia, and Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and the Instituto Superior Técnico at the University of Lisbon.
In an email, Calderón Bustillo and Sanchis-Gual explained that they were initially inspired to pursue this hypothesis after puzzling over the short signal of GW190521, which was snagged by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors. The researchers tried to reproduce the signal with models of head-on collisions of black holes, but found that those simulated wave signals were even shorter than GW190521.
“We realized we needed to consider two objects that, after merging, produce an object that does not immediately collapse, but rather oscillates for a while, producing the missing part of the signal,” the pair said. “The most natural option, namely two neutron stars, was discarded because the maximum mass of a neutron star is two solar masses while GW190521 involved objects of tenths (even hundreds) of solar masses.”
Enter: Boson stars, also known as Proca stars. Predicted by Einstein’s theory of general relativity, these objects are thought to be made of ultralight bosons, which are particles that are billions of times lighter than electrons.
Unlike fermions (the particles that primarily make up normal stars), bosons have the special ability to occupy the same quantum state, meaning that countless bosons can co-exist in the same space. This property is the reason why photons, which are a type of boson, can form a laser that is made of photons in the same quantum state.
Boson stars exhibit an analogous phenomenon, in which ultralight boson particles are packed into a compact space similar to black holes, except that boson stars do not trap light, so there is no event horizon. They can be thought of as a massive Bose-Einstein condensate, which is a coalesced group of atoms or particles existing at the lowest quantum state that is sometimes called the fifth state of matter. Ultralight bosons are considered a promising candidate to explain dark matter.
“Theoretical and particle physicists could be really interested in the possibility of discovering a new particle with such a small mass,” Calderón Bustillo and Sanchis-Gual. “From an experimentalist point of view, depending on how 'dark' the particle is—this is, how weakly the particle interacts with ordinary matter—it could be possible to detect this new particle in the LHC at CERN, for instance.”
“From a theoretical perspective, it raises the question of the origin of such a particle [and] what its place is in the Standard Model of particles,” they added.
In other words, if boson star collisions produce gravitational waves, they could open an unheard-of observational window into dark matter. To pursue this possibility, Calderón Bustillo and Sanchis-Gual teamed up with collaborators who had simulated boson star collisions.
The team discovered that two boson stars with a mass of 115 solar masses each could produce a signal like GW190521. Despite their huge masses, these objects would be very dense, with an effective radius measuring about 1,000 kilometers (621 miles), which would encompass up to 99 percent of the star (the authors note that the actual radius of a boson star would be infinite).
“Boson stars are exotic theoretical objects, but from a gravitational point of view they are similar to neutron stars: they also have a maximum mass that depends on the particle that forms them,” Calderón Bustillo and Sanchis-Gual said. “We estimate that our stars can have a maximum mass around 175 solar masses for the ultra-light boson particle that we propose.”
A collision between these massive boson stars would have created the temporary oscillation the team was looking for, before the whole entity collapsed into a black hole about 250 times as massive as the Sun. If GW190521 was created by boson stars, the collision would have been much closer than 17 billion light years from Earth, which is the distance LIGO and Virgo reported for the black hole merger origin.
However, the team emphasized that this is all very speculative and it will take more research and observations to figure out if the recent gravitational wave was, indeed, the progeny of such weird and unconfirmed objects. Future gravitational wave events may corroborate the team’s conclusion that a boson star collision is a slightly better fit for events like GW190521, compared to a black hole merger.
“First of all, we will have to confirm—or rule out!—the existence of boson stars,” Calderón Bustillo and Sanchis-Gual noted. “To this, we need to produce more gravitational waveforms from mergers of boson stars in different scenarios.”
On a broader scale, the pair concluded that they think their work “could stimulate research on more exotic compact objects, not only boson stars, and cause a reconsideration of the black hole paradigm for some of the gravitational-waves detected by LIGO and Virgo.”