Tidal Deformability in Neutron Stars from an Ab Initio Point of View

We present results for the tidal deformability of neutron stars, the tidal Love number k2, and the effective deformability of a binary system. The equation of state for cold β-stable neutron matter is based upon high-precision two-neutron forces and includes the chiral three-neutron forces required at each order. Although we show results at both the third (N2LO) and fourth (N3LO) orders of the chiral expansion, we emphasize that only the results at N2LO can be considered truly ab initio. This is because of standing problems with regularizing three-nucleon forces at N3LO and higher orders and maintaining chiral symmetry. Thus, the results at N3LO are for illustration purposes only. We review and motivate our choices for the high-density continuation of the microscopic equation of state. We discuss our predictions and observe that they are well within multimessenger constraints. In contrast, stiff equations of state that yield radii larger than about 13 km are ruled out by GW170817 constraints. One of the main contributions from this work is the finding that tidal properties are insensitive to the high-density continuation of the equation of state, suggesting that future tidal measurements can provide robust constraints on the equation of state in the medium-density region, where the predictions from chiral effective field theory are most reliable.