Gravitational wave signals that seem to emanate from black hole collisions may actually come from the clashes of odd, exotic stars 鈥� which have been theorised but may or may not exist. If they do, then physicists will have to rethink their standard theories of gravity and particles.
For almost 60 years, researchers have been thinking up cosmic objects that may be possible if there is more to gravity than is suggested by Albert Einstein鈥檚 general theory of relativity. One such object is a hypothetical 鈥渂oson star鈥�, which would be made from some new, undiscovered particle similar to dark matter. Some of these stars would also be extremely compact, making them similar to dense bodies like black holes or neutron stars.
If two of these dense bodies merged, they would produce ripples in space-time that instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO) can measure. at the University of Cambridge and her colleagues wanted to know whether such detectors could also pick up 鈥� and pick out 鈥� clashes between boson stars.
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鈥淲e wanted to address whether we can detect binary boson stars, and the short answer is 鈥榶es鈥�,鈥� she says. 鈥淏ut whether we can actually differentiate boson stars from black holes and neutron stars is more complicated.鈥�
To answer these questions, the researchers simulated different boson star mergers on a computer. They focused on gravitational waves, or ripples in space-time, that would emanate from two boson stars crashing into each other and forming either another boson star or a black hole. Then they combined the results of these simulations with some of the data from LIGO and performed a mathematical test to see how the exotic signals compared with other readings made using the detector.
It was possible to distinguish the boson star merger when it resulted in a boson star. But when the two exotic stars produced a black hole, the test was less conclusive. Evstafyeva says that in some cases, signals from black hole mergers and mergers of boson stars looked so similar that current data analysis methods at LIGO would be unlikely to tell them apart.
鈥淚f a binary boson star merger occurred in the universe, current detection techniques would likely be unable to identify the detected gravitational wave signal as originating from such a system. Instead, it would be misclassified as originating from a binary black hole,鈥� says at Princeton University in New Jersey, who wasn鈥檛 involved in the research.
at Long Island University in New York says that the new analysis invites both an optimistic outlook, because boson stars could be detected, and a pessimistic one, because they could produce ambiguous signals that would be mistaken for something else. While it isn鈥檛 yet clear whether boson stars exist at all, he says, they are a proxy for new and exotic physics that now seems to be within reach of existing detectors.
Evstafyeva says that her team鈥檚 work is indeed the first concrete, high-precision example of what a gravitational wave signal that departs from standard theories of gravity would look like. Still, she isn鈥檛 expecting an object like a boson star to be found anytime soon. But she is optimistic for future LIGO runs since a recent upgrade made it significantly more sensitive.
鈥淭his is a very exciting prospect for potentially observing some gravitational wave signal that would make the whole 鈥榖eyond general relativity鈥� community go a little crazy,鈥� she says.
Physical Review Letters
