Dynamics of the $N$-body system in energy-momentum squared gravity: Equations of motion to the first post-Newtonian order

In the energy-momentum squared gravity (EMSG), the matter energy-momentum tensor is not conserved due to nonminimal interaction between the usual and modified matter fields. For this reason, the $N$-body acceleration may host the EMSG effects that can be probed at the solar scale by the perihelion shift of the planets and experimental tests of the strong equivalence principle (SEP). To shed light on it, in this paper, we present the $N$-body equations of motion in the weak-field limit of the EMSG theory. To do so, the post-Newtonian (PN) hydrodynamic equations, the viral identities, as well as the corresponding equilibrium conditions are introduced in this theory. Armed with these relations, we derive the dynamics of the $N$-body system and its PN interbody metric. It is shown that the EMSG theory is not ruled out by the classical test, the perihelion advance of Mercury, and the test of SEP. In other words, in the first PN order, it is not possible to constrain the free parameter of this theory and even distinguish it from general relativity (GR) using these local tests.