Neutron stars (NSs) exhibit the most extreme gravitational, electromagnetic, and nuclear physics environments that stably exist in the universe. They are born when the cores of massive stars undergo gravitational collapse, driving off the star's outer layers as a supernova explosion. Their mass of around a solar mass of matter is squeezed by their tremendous gravity to a sphere merely twenty kilometers in diameter, with their central densities reaching up to several times nuclear density. The nature and composition of such ultra-dense matter has remained a longstanding science question since the extrapolations required to infer its composition based on known nuclear physics are too far to reach firm predictions. Various possibilities predict the emergence of new phases of matter, condensates of particles such as hyperons, or even conglomerates of deconfined quarks.
Learning about dense matter from neutron stars is challenging for several reasons: the microphysical many-body problem with unknown strong interactions and components is computationally intractable; laboratory measurements of dense matter parameters such as the nuclear charge radii of heavy nuclei elucidate properties mainly at the lower densities in NSs; and electromagnetic signals from NSs are emitted by poorly understood processes in their atmospheres and pass through the interstellar medium before reaching the telescopes, making the inferences from observations prone to large systematic errors.
Tidal effects in neutron star binary inspirals A major scientific prospect for LIGO is to resolve the longstanding question about the nature of ultra-dense NS matter with measurements of the gravitational waves (GWs) emitted during the coalescence of two NSs or a NS and a black hole. When the two objects are widely separated, they behave essentially as point masses. As their orbital separation decreases, the companion’s differential gravitational force across the NS matter causes it to deform and develop a quadrupole moment, similar to the tidal bulges raised on the earth due to the moon.
The dominant influence of the NS matter on the dynamics and inspiral signal is characterized by a tidal deformability parameter, the ratio of the induced quadrupole moment to the perturbing tidal field, that is related to the NS's quadrupolar tidal Love number. A NS’s tidal Love numbers are akin to those used in planetary science, e.g. to infer the subsurface composition and possible presence of a liquid layer of Saturn’s moon Titan from Cassini flyby data (see this news article from ESA) . The only difference is that Love numbers computed in Newtonian physics are completely inadequate for describing the strong-gravity environment in NSs.
>> Read a short paper about our first study where we identify the tidal deformability parameter and demonstrate LIGO's potential to measure it here and about the first fully relativistic computation of the tidal Love numbers of neutron stars in the initial paper using simple model equations of state.
>> Read the more detailed paper on the tidal deformability for realistic neutron star models and its signature in binary inspirals, where we limited our analysis to the regime where systematic uncertainties in our model were small, and an improved phasing model including relativistic corrections to the tidal effects in this paper post-Newtonian tidal effects.
Resonant shattering of neutron star crusts The resonant excitation of neutron star internal modes of oscillation (similar to Earth's oscillations excited by earthquakes and studied in seismology) by tides could be a potential source of short gamma-ray burst precursors. The driving of a crust-core interface mode during a binary inspiral, where the companion's tidal field varies periodically due to the orbital motion, could lead to shattering of the NS crust, liberating a large amount of energy seconds before the merger and potentially giving rise to a possible precursor flare associated with a short gamma-ray burst. In this scenario, future precursor detections could be used alongside coincident gravitational wave detections of the inspiral by LIGO to probe the neutron star structure.