Binary Love Relations
  When in a tight binary, the mutual tidal deformations of neutron stars
imprint onto observables, encoding information about their internal structure
at supranuclear densities and gravity in the extreme-gravity regime.
Gravitational wave observations of their late binary inspiral may serve as a
tool to extract the individual tidal deformabilities, but this is made
difficult by degeneracies between them in the gravitational wave model. We here
resolve this problem by discovering approximately universal relations between
dimensionless combinations of the individual tidal deformabilities. We show
that these relations break degeneracies in the gravitational wave model,
allowing for the accurate extraction of both deformabilities. Such measurements
can be used to better differentiate between equation-of-state models, and
improve tests of General Relativity and cosmology.
