Abstract

Our understanding of gravity started with analysis of motions of solar system bodies, as interpreted by Newton and Einstein. When we go further away from the solar system, gravity takes on a less familiar empirical relation, which is sometimes phrased as evidences either for physics dark matter particles or for Modified Newtonian gravity. Even with 100% knowledge of the present position and velocity of every star in the universe, we still cannot predict the acceleration of each star, ie, the position and velocity of each star next year. If gravity were modified, then it would take three universal parameters to describe the acceleration: G, c, and the cosmological constant Lambda. If the added gravity were due to dark particles, then a full description of acceleration would take 2+Infinity parameters: G, c, and a spatially fluctuating dark-to-luminous ratio everywhere.

Fortunately a distinction can be made about these two flavors of interpretations in the cases of interactions in binary or triple systems, e.g., the Milky Way -Andromeda system, the wide binary of stars in our Galaxy.