Parametrized post-Newtonian theory of reference frames, multipolar expansions and equations of motion in the N-body problem

Abstract Post-Newtonian relativistic theory of astronomical reference frames based on Einstein's general theory of relativity was adopted by General Assembly of the International Astronomical Union in 2000. This theory is extended in the present paper by taking into account all relativistic effects caused by the presumable existence of a scalar field and parametrized by two parameters, β and γ , of the parametrized post-Newtonian (PPN) formalism. We use a general class of the scalar-tensor (Brans-Dicke type) theories of gravitation to work out PPN concepts of global and local reference frames for an astronomical N-body system. The global reference frame is a standard PPN coordinate system. A local reference frame is constructed in the vicinity of a weakly self-gravitating body (a sub-system of the bodies) that is a member of the astronomical N-body system. Such local inertial frame is required for unambiguous derivation of the equations of motion of the body in the field of other members of the N-body system and for construction of adequate algorithms for data analysis of various gravitational experiments conducted in ground-based laboratories and/or on board of spacecrafts in the solar system. We assume that the bodies comprising the N-body system have weak gravitational field and move slowly. At the same time we do not impose any specific limitations on the distribution of density, velocity and the equation of state of the body's matter. Scalar–tensor equations of the gravitational field are solved by making use of the post-Newtonian approximations so that the metric tensor and the scalar field are obtained as functions of the global and local coordinates. A correspondence between the local and global coordinate frames is found by making use of asymptotic expansion matching technique. This technique allows us to find a class of the post-Newtonian coordinate transformations between the frames as well as equations of translational motion of the origin of the local frame along with the law of relativistic precession of its spatial axes. These transformations depend on the PPN parameters β and γ , generalize general relativistic transformations of the IAU 2000 resolutions, and should be used in the data processing of the solar system gravitational experiments aimed to detect the presence of the scalar field. These PPN transformations are also applicable in the precise time-keeping metrology, celestial mechanics, astrometry, geodesy and navigation. We consider a multipolar post-Newtonian expansion of the gravitational and scalar fields and construct a set of internal and external gravitational multipoles depending on the parameters β and γ . These PPN multipoles generalize the Thorne-Blanchet-Damour multipoles defined in harmonic coordinates of general theory of relativity. The PPN multipoles of the scalar-tensor theory of gravity are split in three classes—active, conformal, and scalar multipoles. Only two of them are algebraically independent and we chose to work with the conformal and active multipoles. We derive the laws of conservations of the multipole moments and show that they must be formulated in terms of the conformal multipoles. We focus then on the law of conservation of body's linear momentum which is defined as a time derivative of the conformal dipole moment of the body in the local coordinates. We prove that the local force violating the law of conservation of the body's linear momentum depends exclusively on the active multipole moments of the body along with a few other terms which depend on the internal structure of the body and are responsible for the violation of the strong principle of equivalence (the Nordtvedt effect). The PPN translational equations of motion of extended bodies in the global coordinate frame and with all gravitational multipoles taken into account are derived from the law of conservation of the body's linear momentum supplemented by the law of motion of the origin of the local frame derived from the matching procedure. We use these equations to analyze translational motion of shperically symmetric and rigidly rotating bodies having finite size. Spherical symmetry is defined in the local frame of each body through a set of conditions imposed on the shape of the body and the distribution of its internal density, pressure and velocity field. We prove that our formalism brings about the parametrized post-Newtonian EIH equations of motion of the bodies if the finite-size effects are neglected. Analysis of the finite-size effects reveal that they are proportional to the parameter β coupled with the second and higher-order rotational moments of inertia of the bodies. The finite-size effects in the translational equations of motion can be appreciably large at the latest stage of coalescence of binary neutron stars and can be important in calculations of gravitational waveform templates for the gravitational-wave interferometers. The PPN rotational equations of motion for each extended body possessing an arbitrary multipolar structure of its gravitational field, have been derived in body's local coordinates. Spin of the body is defined phenomenologically in accordance with the post-Newtonian law of conservation of angular momentum of an isolated system. Torque consists of a general relativistic part and the PPN contribution due to the presence of the scalar field. The PPN scalar-field-dependent part is proportional to the difference between active and conformal dipole moments of the body which disappears in general relativity. Finite-size effects in rotational equations of motion can be a matter of interest for calculating gravitational wave radiation from coalescing binaries.