Oddly viscous stars could be imperials black holes

Exotic viscous stars could reflect ripples of space time and mimic the signals we observe from black holes.

Sales 2015 have learned to see the content of the universe by tracking not only light waves but also gravitational waves: ripples in the universe’s production. Jaime Redondo – Yuste at the Niels Bohr Institute in Denmark and Hans Colleugues has now shown that as waves of light, gravitational waves can be reflected – but only from odd stars with an unusual viscous structure.

The researchers started by questioning where a mirror for gravitational waves could even exist. Although some previous studies suggest it could, they struggled to write down 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, and then we just need a star,” says Redondo – Yuste. But this star should have extremely high viscosity, just as the cosmic equivalent of a melass ball. The researchers’ calculation showed that such a star would reflect gravitational waves because it would be too stiff to wing asy through it.

Daniel Kennefick at the University of Arkansas says this behavior would be very unusual because most of the case is transparent to gravitational waves, just as glass is transparent to light. “Even if we were very close to a very powerful source of gravitational waves, it would do us the slightest damage because the energy would go straight through us,” he says.

Adding to its subcontracting, a star viscous enough to divert gravitational waves would also be very compact and very close to collapsing in a black hole. In fact, Redundo says Yuste says that black holes themselves are incredibly viscous – so much so that other very viscous objects may look like them when the gravitational wave signatures are recorded on the ground. At the same time, there may be small different in these signatures. For example, collisions between viscous stars and collisions between black holes would provide slightly different gravitational wave signals because the stars would have more a tidal effect on ECH other, he says.

Researchers have previously observed cosmic objects assumed to have raised viscosity, such as very hot neutron stars formed through mergers of other neutron stars. But what these could be viscous enough to match the team’s mathematical model is yet to be clear, says Paolo Pani at Sapienza University of Rome in Italy.

He says that future gravitational waving detectors could provide more detailed information about the viscosity of objects we already know how to register – and help us look for new ones. “This is an example of trying to predict before what we need to look for,” says Kennefick.

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

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