Oddly viscous stars could be impersonating black holes

The exotic viscous stars could reflect undulations of space-time, imitating the signals that we observe from black holes.

Since 2015, researchers have learned to see the content of the universe by following not only light waves but also gravitational waves: ripples in the fabric of the universe. Jaime Redondo – Yuste at the Niels Bohr Institute in Denmark and his colleagues have now shown that, like waves of light, gravitational waves can be reflected – but only strange stars with an unusually viscous texture.

The researchers started by wondering if a mirror for gravitational waves could even exist. Although some previous studies have suggested, they had trouble writing equations that would describe such a mirror without breaking the laws of physics. Then they realized that the reflective object should not be flat.

“We can have a spherical mirror, then we just need a star,” explains Redondo – Yuste. But this star should have an extremely high viscosity, such as the cosmic equivalent of a molasses ball. Researchers’ calculations have shown that such a star would reflect gravitational waves because it would be too rigid to oscillate when they crossed it.

Daniel Kennefick at the University of Arkansas says that this behavior would be very unusual because most materials are transparent to gravitational waves, as glass is transparent in light. “Even if we were very close to a very powerful source of gravitational waves, that would not make us the slightest evil, because the energy would pass through us,” he said.

Adding to its oddity, a viscous star to divert gravitational waves should also be very compact and very close to collapsing in a black hole. In fact, Redondo – Yuste says that the black holes themselves are incredibly viscous – so much so that other very viscous objects can resemble them when their gravitational waves signatures are recorded on earth. At the same time, there could be small differences in these signatures. For example, the collisions between viscous stars and collisions between black holes would produce slightly different gravitational waves, because the stars would have more tidal effect on each other, he says.

The researchers previously observed cosmic objects considered as a high viscosity, as very hot neutron stars which form through mergers other neutron stars. But if this could become viscous enough to correspond to the team mathematical model is not yet clear, says Paolo Pani at the University of Sapienza in Rome in Italy.

He says that future gravitational waves detectors could provide more detailed information on the viscosity of the objects that we already know how to detect – and help us seek new ones. “This is an example of trying to anticipate what we should look for in advance,” says Kennefick.

So far, no observation data has given researchers a strong reason to think that what they have identified as a black hole is actually an exotic star. And the three researchers say that the chances that viscous stars are observed are not high.

“But I think it’s our duty to continue doing these tests,” explains Redondo – Yuste. It is the only way to create a complete inventory of objects that fill our universe.