Neutron Star Theory
Neutron stars are compact objects with masses comparable to the Sun's, but with radii of roughly 10 kilometers. Their internal densities exceed that of nuclear matter and their gravitational fields are the strongest in the Universe, except for those of black holes. Their magnetic fields are extreme, maybe reaching 1016 Gauss in magnetars. With spin periods anywhere from milliseconds to seconds, these neutron stars emit radiation in the radio and sometimes in the X-ray band that reach Earth. Every time the radiation beams cross Earth's line of sight they can be detected with radio telescopes on Earth, or X-ray telescopes such as NICER. When neutron stars are in a binary system, they also emit gravitational waves, which can be detected by ground-based interferometers, such as LIGO, Virgo, KAGRA and LIGO-India. All of these radiation contain precise information about neutron stars, such as their spin period, their motion if in a binary system, and about the gravitational theory in play.
Professor Yunes' research group studies neutron stars, their internal composition, and their potential use as probes of extreme gravity. These object exhibit a great degree of approximate universality between different macroscopically observable quantities, such as their tidal deformability, their induced quadrupole moment and their moment of inertia. By exploring these and other approximately universal relations in gravitational wave data analysis, future gravitational wave observations of neutron star mergers with ground-based interferometers (such as LIGO, Virgo, LIGO-India and KAGRA) can be used to discern the equation of state of supra-nuclear matter. Moreover, the gravitational waves emitted in neutron star mergers and the radio emission produced in binary pulsars can be used to test General Relativity to extremely stringent levels, orders of magnitude more strongly than in the Solar System.