On the MOND External Field Effect in the Solar System

TL;DR

This paper investigates the potential orbital effects of the External Field Effect predicted by MOND in the solar system, finding that such effects could be significantly larger than current observational bounds, thus testing MOND's violation of the Strong Equivalence Principle.

Contribution

It provides a phenomenological analysis of how the MOND External Field Effect could influence planetary orbits, deriving upper bounds on the external field components based on orbital precession data.

Findings

Inner planets' orbital effects are 4-6 orders of magnitude larger than current bounds.

Upper limits on external field components are E_x <= 10^-15 m/s^2, E_y <= 2x10^-16 m/s^2, E_z <= 3x10^-14 m/s^2.

Results support MOND's violation of the Strong Equivalence Principle.

Abstract

In the framework of the MOdified Newtonian Dynamics (MOND), the internal dynamics of a gravitating system s embedded in a larger one S is affected by the external background field E of S even if it is constant and uniform, thus implying a violation of the Strong Equivalence Principle: it is the so-called External Field Effect (EFE). In the case of the solar system, E would be A_cen\approx 10^-10 m s^-2 because of its motion through the Milky Way: it is orders of magnitude smaller than the main Newtonian monopole terms for the planets. We address here the following questions in a purely phenomenological manner: are the Sun's planets affected by an EFE as large as 10^-10 m s^-2? Can it be assumed that its effect is negligible for them because of its relatively small size? Does $\bds E$ induce vanishing net orbital effects because of its constancy over typical solar system's planetary orbital periods? It turns out that a constant and uniform acceleration, treated perturbatively, does induce non-vanishing long-period orbital effects on the longitude of the pericenter of a test particle. In the case of the inner planets of the solar system and with E\approx 10^-10 m s^-2, they are 4-6 orders of magnitude larger than the present-day upper bounds on the non-standard perihelion precessions recently obtained with by E.V. Pitjeva with the EPM ephemerides in the Solar System Barycentric frame. The upper limits on the components of E are E_x <= 1 x 10^-15 m s^-2, E_y <= 2 x 10^-16 m s^-2, E_z <= 3 x 10^-14 m s^-2. This result is in agreement with the violation of the Strong Equivalence Principle by MOND.