"Neutron stars are the collapsed cores of stars and densest stable material objects in the universe, much denser and colder than conditions that particle colliders can even create," explained Yunes, who is also the founding director of the Illinois Center for Advanced Studies of the Universe. "The mere existence of neutron stars tells us that there are unseen properties related to astrophysics, gravitational physics, and nuclear physics that play a critical role in the inner workings of our universe."

The discovery of gravitational waves has allowed many of these previously hidden properties to be observed.

"The properties of neutron stars imprint onto the gravitational waves they emit. These waves then travel millions of light-years through space to detectors on Earth, like the advanced European Laser Interferometer Gravitational-Wave Observatory and the Virgo Collaboration," Yunes said. "By detecting and analyzing the waves, we can infer the properties of neutron stars and learn about their internal composition and the physics at play in their extreme environments."

Yunes, a gravitational physicist, has been exploring how gravitational waves encode information about the tidal forces that affect neutron stars. These tidal forces distort the stars' shapes and influence their orbital motion. This research also sheds light on the dynamic material properties of neutron stars, such as internal friction or viscosity, which may offer insight into energy transfer processes within these systems.

Using data from the gravitational wave event labeled GW170817, Yunes and his colleagues - Illinois researchers Justin Ripley, Abhishek Hegade, and Rohit Chandramouli - employed computer simulations, analytical models, and advanced data analysis to confirm that out-of-equilibrium tidal forces within binary neutron star systems are detectable through gravitational waves. While the GW170817 event did not provide a direct measurement of viscosity, Yunes' team successfully established the first observational limits on how significant viscosity can be inside neutron stars.

The findings of the study were published in 'Nature Astronomy'.

"This is an important advance, particularly for ICASU and the U. of I.," Yunes added. "In the '70s, '80s, and '90s, Illinois pioneered many of the leading theories behind nuclear physics, particularly those connected to neutron stars. This legacy can continue with access to data from the advanced LIGO and Virgo detectors, the collaborations made possible through ICASU, and the decades of nuclear physics expertise already in place here."

The study was supported by the University of Illinois Graduate College Dissertation Completion Fellowship and the National Science Foundation.