(Image: NIST)

“You will be assimilated.” In Star Trek, members of the strange and sinister race known as the Borg would utter this threat as if one. Their behaviour could be echoed in space if dark matter exists in a particular form: if so, it could create Borg-like stars in which every particle is in the same state at the same time.

Dark matter accounts for 80 percent of the matter in the universe, but we can’t observe it directly and its constituents are a mystery.

One theory is that dark matter could be made of particles called axions. Unlike protons, neutrons and electrons that make up ordinary matter, axions can share the same quantum energy state. They also attract each other gravitationally, so they clump together.


Together, those two properties mean that the clumps would exist as a Bose-Einstein condensate (BEC) – a state of matter in which all the particles occupy the same quantum state, according to calculations by Chanda Prescod-Weinstein at the Massachusetts Institute of Technology and her colleagues.

“They act like one super-atom together,” says Prescod-Weinstein. But those clumps are prone to fracturing, she adds. “The configuration the axions ‘want’ to settle into is not one giant BEC.” Rather, they break apart into smaller clumps, which the team calls Bose stars.

Asteroid-sized

These would have formed when the universe was a mere 47,000 years old and should survive to this day, she says. Such stars would be totally dark and relatively tiny, the size of the asteroid Ceres and about 20 times as dense.

Dark matter is hard to study because it does not interact much with ordinary matter, but axion dark matter could theoretically be observed in the form of Bose stars, if they are orbiting a pulsar. Under the right conditions, the interaction between the pulsar and the axions could produce radiation we can pick up, says Prescod-Weinstein.

This would be like a naturally occurring, space-based version of the Axion Dark Matter Experiment at the University of Washington in Seattle, which uses a large superconducting magnet to search for axions.

“I’m sure that experimentalists would express some scepticism about that,” she says. “But I tend to be optimistic that the universe is weirder than we think it is.”

“It’s a great paper, and we agree with their conclusions,” says Rohana Wijewardhana at the University of Cincinnati, Ohio, whose team has done similar calculations.

Wijewardhana adds that if a Bose star crashed into Earth, we might be able to observe its effects. It’s not something we need worry about, either: because a Bose star would interact weakly with matter, we would only see small gravitational effects even if the entire thing passed right through Earth.

Journal reference: Physical Review D, 10.1103/PhysRevD.92.103513