Exotic viscous stars might emulate signals from black holes, mirroring the ripples in spacetime.

Since 2015, scientists have been uncovering the universe’s secrets by monitoring both light waves and gravitational waves, the ripples in the cosmos. Jaime Redondo-Yuste from the Neals Bohr Institute in Denmark and his team found that they can reflect gravitational waves, similar to light waves, but only from unusually viscous celestial objects.

The researchers began exploring the possibility of creating a gravitational wave mirror. While earlier studies hinted at its feasibility, developing equations that adhere to physical laws proved challenging. They eventually understood that reflectors don’t need to be flat.

“We can have a spherical mirror, and we need stars,” explains Redondo-Yuste. However, these stars must possess an extraordinarily high viscosity akin to molasses. Their calculations indicated that such stars could indeed reflect gravitational waves, as they are too rigid to be disturbed by passing waves.

Daniel Kennefick from the University of Arkansas highlights that this behavior is rare since most materials are transparent to gravitational waves, just as glass is to light. “Even when we are very near sources of powerful gravitational waves, they pass through us without any noticeable effect,” he remarks.

In addition to their strangeness, stars capable of deflecting gravitational waves must be compact and on the brink of collapsing into black holes. Redondo-Yuste notes that black holes themselves are very viscous. Therefore, when gravitational wave signals reach Earth, other highly viscous objects could be misidentified as black holes, with subtle differences in their signals. For instance, collisions between viscous stars and black holes would yield slightly distinct gravitational wave signatures due to tidal influences.

Researchers have previously detected celestial bodies believed to have heightened viscosity, such as extremely hot neutron stars formed from the merger of others. However, it’s still uncertain whether these stars possess sufficient viscosity to align with the team’s mathematical model, according to Paolopani from the University of Sapienza in Rome, Italy.

He suggests that forthcoming gravitational wave detectors will enhance our understanding of the viscosity of known objects and assist in discovering new ones. “This serves as a prelude to what we should be searching for,” Kennefick says.

To date, observational data hasn’t provided strong evidence for classifying what scientists identify as a black hole as an exotic star. All three researchers agree that the likelihood of observing these viscous stars has been minimal thus far.

“However, it’s our responsibility to continue these investigations,” insists Redondo-Yuste. “Only in this way can we compile a complete catalog of the entities populating our universe.”