Ratio of the relative accelerations that the galaxies have in the simulation, a_sim, to the acceleration calculated using the two-body-force formula (Eq. (12)), a_2BF, as a function of the galaxy separation, r₁₂. If the accelerations a_sim and a_2BF were equal, the points would lie on the horizontal dashed line. The points are colored with respect to simulation time, t. Only the period between the first and second pericenter is displayed.