Researchers at the Illinois Center for Advanced Studies of the Universe (ICASU) have developed a novel theory-agnostic, equation-of-state-insensitive test of general relativity (GR). This multimessenger test is the first to combine data from the Neutron star Interior Composition Explorer (NICER), an x-ray telescope located on the International Space Station, with data from the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo interferometer, two ground-based gravitational wave detectors.
In a paper published today in Physical Review Letters, a team of researchers led by Illinois Physics Professor Nicolás Yunes present two conclusions. First, using NICER’s observation datamand universal relations among various properties of neutron stars, the authors infer the moment of inertia; the tidal Love number; the quadrupole moment; and the surface eccentricity of neutron star PSR J0030+045. Next, they use their inferences to propose and implement a novel test of GR.
The moment of inertia tells us how fast a neutron star is spinning. The Love number tells us how easy it would be to deform a neutron star. The quadrupole moment and surface eccentricity tell us how far from spherical the neutron star is.
Yunes says, “Because of universal relations, if we have one piece of information, we can infer a great deal about a neutron star. This result represents the first time these four values have been inferred for a single object. When combined, they offer essential information about the spin, deformability, and shape of a neutron star, and thus provide insights into the spacetime around it.”
Neutron stars pose a particular challenge for scientists, because the matter inside these extremely compact objects is so dense and hot that it cannot be measured or replicated on Earth. While the nature of the matter at a neutron star’s core remains unknown, scientists can use universal relations to understand a neutron star’s gravitational interaction with its environment.
Illinois Physics graduate student Alejandro Cárdenas-Avendaño explains, “These relations are equation-of-state insensitive, meaning that we do not need to understand the matter inside neutron stars to make inferences about some of their properties.”
In 2013, Yunes and University of Virginia Professor of Physics Kent Yagi found one such set of universal relations: the I-Love-Q relations between the moment of inertia (I), the tidal Love number (Love), and the quadrupole moment (Q). This new work relies on the I-Love-Q relations—as well as on another relation based on the compactness of the star—to make inferences about the characteristics of neutron star PSR J0030+045.
The I-Love-Q relations are insensitive to the matter content of a neutron star, but they depend on the theory of gravity in play around it. To test GR, the authors use the moment of inertia inferred from the NICER data and the tidal Love number from the LIGO/Virgo data. Using these independent inferences from different instruments, the authors test whether the I-Love relation is satisfied.
Yunes says, “If gravity were not accurately described by GR, then the I-Love relation would have predicted a value of the Love number that conflicts with the LIGO/Virgo data. The Love number inferred by LIGO is consistent with that predicted by the GR I-Love relation, which shows that Einstein’s theory holds true.
“This is the first time that NICER data and LIGO/Virgo data are used together to test GR in a model-independent way, using these nifty universal relations we found years ago. Such a test has the potential to constrain modifications from general relativity around neutron stars, when gravity is much, much stronger than it is in the solar system.”
Cárdenas-Avendaño concludes, “Given that GR has passed every test, it is our best description of the gravitational phenomena that take place in our universe. However, there are still open questions in theoretical physics, which is why GR might not be the final word.”