http://arxiv.org/abs/1402.0488
Neutron stars are celebrated to be excellent laboratories for determining the equation of state (EoS) of cold dense matter. Their strong gravity suggests they can also be used to constrain gravity models. Mass and radius (M-R) relations depend both on the choice of EoS and relativistic gravity, meaning that neutron stars can not be simultaneously good laboratories for both questions. An M-R measurement would constrain the less-well-known physics input. We calculate the radial profile of compactness and curvature within a neutron star and determine the domain not probed by the solar system tests of GR. We find that, except for a tiny sphere of radius less then a millimetre at the centre, the curvature is several orders of magnitude above the values present in solar system tests. Compactness is beyond the solar surface value for r>10 m increasing 5 orders of magnitude towards the surface. With density being only an order of magnitude higher than that probed by nuclear scattering experiments, our results suggests that employment of GR as the theory of gravity is a much remarkable extrapolation from a regime of tested validity than that of EoS models. Our larger ignorance of gravity within neutron stars suggests that an M-R measurement constrains gravity rather than EoS, and given that EoS is yet to be determined by nucleon scattering experiments, M-R measurements can not tightly constrain the gravity models either. Near the surface the curvature and compactness attain their largest values while EoS in this region is fairly well known. This renders the crust as the best site to look for deviations from GR.[abriged]
Tue, 4 Feb 14
60/69
You must be logged in to post a comment.