jcapJournal of Cosmology and Astroparticle PhysicsJ. Cosmol. Astropart. Phys.1475-7516IOP PublishingJCAP_108P_072110.1088/1475-7516/2021/10/075JCAP_108P_0721paperSpin-orbit effects for compact binaries in scalar-tensor gravityBraxPhilippe1philippe.brax@ipht.frDavisAnne-Christine2a.c.davis@damtp.cam.ac.ukMelvilleScott2scott.melville@damtp.cam.ac.ukWongLeong Khim1leong-khim.wong@ipht.fr Université Paris-Saclay, CNRS, CEA, Institut de Physique Théorique, 91191, Gif-sur-Yvette, France DAMTP, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, U.K. 1020212610202126102021202110075307202129920213092021© 2021 The Author(s)2021 Published by IOP Publishing Ltd on behalf of Sissa Medialab. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Abstract

Gravitational waves provide us with a new window into our Universe, and have already been used to place strong constrains on the existence of light scalar fields, which are a common feature in many alternative theories of gravity. However, spin effects are still relatively unexplored in this context. In this work, we construct an effective point-particle action for a generic spinning body that can couple both conformally and disformally to a real scalar field, and we show that requiring the existence of a self-consistent solution automatically implies that if a scalar couples to the mass of a body, then it must also couple to its spin. We then use well-established effective field theory techniques to conduct a comprehensive study of spin-orbit effects in binary systems to leading order in the post-Newtonian (PN) expansion. Focusing on quasicircular nonprecessing binaries for simplicity, we systematically compute all key quantities, including the conservative potential, the orbital binding energy, the radiated power, and the gravitational-wave phase. We show that depending on how strongly each member of the binary couples to the scalar, the spin-orbit effects that are due to a conformal coupling first enter into the phase at either 0.5 PN or 1.5 PN order, while those that arise from a disformal coupling start at either 3.5 PN or 4.5 PN order. This suppression by additional PN orders notwithstanding, we find that the disformal spin-orbit terms can actually dominate over their conformal counterparts due to an enhancement by a large prefactor. Accordingly, our results suggest that upcoming gravitational-wave detectors could be sensitive to disformal spin-orbit effects in double neutron star binaries if at least one of the two bodies is sufficiently scalarised.

Gravitational waves in GR and beyond : theorydark energy theorymodified gravityarxivppt2107.10841