Gravitational waves reveal new properties of neutron stars

A better understanding of the inner workings of neutron stars will shed light on the dynamics that underlie how the universe works and could also help guide future technologies, said Nicolas Yunes, a professor of physics at the University of Illinois at Urbana-Champaign. A new study led by Yunes details how new insights into how dissipative tidal forces operate within double or binary neutron star systems will inform our understanding of the universe.

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

However, many of these previously invisible properties became observable with the discovery of gravitational waves.

“The properties of neutron stars are reflected in the gravitational waves they emit. These waves then travel millions of light-years through space to detectors on Earth, such as the European Laser Interferometer Gravitational-Wave Observatory and the Virgo collaboration,” Yunes explained. “By detecting and analyzing the waves, we can infer the properties of neutron stars and learn more about their internal composition and the physics at play in their extreme environments.”

As a gravitational physicist, Yunes wanted to determine how gravitational waves encode information about the tidal forces that distort the shape of neutron stars and affect their orbital motion. This information could also tell us more about the stars’ dynamic material properties, such as internal friction or viscosity, “which could give us insight into the non-equilibrium physical processes that drive the net transfer of energy into or out of a system,” Yunes said.

Using data from the gravitational-wave event identified as GW170817, Yunes, along with Illinois researchers Justin Ripley, Abhishek Hegade and Rohit Chandramouli, used computer simulations, analytical models and sophisticated data-analysis algorithms to verify that non-equilibrium tidal forces within binary neutron star systems are detectable via gravitational waves. The GW170817 event was not strong enough to allow a direct measurement of viscosity, but Yunes’ team was able to place the first observational constraints on the magnitude of viscosity inside neutron stars.

The work is published in the journal Astronomy of nature.

“This is a significant step forward, particularly for ICASU and the University of Illinois,” Yunes said. “In the 1970s, ’80s and ’90s, Illinois pioneered many fundamental theories of nuclear physics, particularly those related to neutron stars. That 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.”