Confronting gravitational-wave observations with modern nuclear physics constraints from chiral effective field theory*

The correlation of the tidal polarizabilities Λ₁-Λ₂ for GW170817 is predicted by combining dense-matter equations of state (EOS) that satisfy nuclear physics constraints with the chirp mass and mass asymmetry for this event. Our models are constrained by calculations of the neutron-matter EOS using chiral effective field theory (EFT) Hamiltonians with reliable error estimates up to once or twice the nuclear saturation density. Chiral EFT is a systematic theory for nuclear forces that allows to develop consistent two- and three-nucleon interactions and enables calculations with controlled theoretical uncertainties. When using chiral EFT input up to twice saturation density, we find that GW170817 does not improve our understanding of the EOS.

We contrast two distinct extrapolations to higher density, a minimal model (MM) which assumes that the EOS is a smooth function of density described by a Taylor expansion, and a more general model parameterized by the speed of sound that admits phase transitions. This allows us to identify regions in the Λ₁-Λ₂ plots that could hint at the existence of new phases of matter inside neutron stars (NSs).

We predict the combined tidal polarizability of the NSs in GW170817 to be 80 ≤ Λ̃ ≤ 580 (280 ≤ Λ̃ ≤ 480 for the MM), which is smaller than the range suggested by the LIGO-Virgo data analysis. Our analysis also shows that GW170817 requires a NS with M =1.4M_⊙ to have a radius 9.01.4<13.6 km (11.31.4<13.6 km for the MM).