Black hole binaries in shift-symmetric Einstein-scalar-Gauss-Bonnet gravity
In shift-symmetric Einstein-scalar-Gauss-Bonnet gravity, stationary black holes have a non-vanishing scalar charge. During the inspiral, the phase evolution is modified by several effects,primarily an additional scalar dipole radiation, which enters at -1PN order. Including corrections up to 2PN, this effect accelerates the inspiral when compared to general relativity, but changes in the conservative dynamics may do the opposite. Using fully non-linear numerical simulations of quasi-circular, comparable mass binaries, we characterize the late stages of the orbital dynamics. We find that the overall effect is still an accelerated merger phase for the modified gravity case, but noticeably less than predicted by PN alone, which we relate to the conservative dynamics, showing that at the late inspiral stage more energy must be emitted in scalar-Gauss-Bonnet gravity to induce a given change in frequency. In longer signals, this may lead to a distinctive frequency evolution relative to general relativity as the binary approaches merger, but its effect close to merger is to suppress the difference with GR. This work suggests we may need to revisit existing constraints on the theory that are obtained assuming PN approximations apply up to merger, or based on order by order approximations that neglect backreaction effects on the metric, and shows the importance of including non-linear effects that modify the gravitational sector in the strong field regime.