An observational test of the Vainshtein mechanism

TL;DR

This paper tests the Vainshtein mechanism in modified gravity theories by examining offsets between black holes and stars in galaxies, finding that observed offsets challenge the theory unless the scalar force is much stronger than gravity.

Contribution

It provides an observational test of the Vainshtein mechanism by analyzing galaxy offsets, offering constraints on the strength of scalar forces in modified gravity theories.

Findings

Observed offsets in M87 cannot be explained unless scalar force exceeds gravity

Offsets depend on galaxy density and can reach up to 0.1 kpc

Systematic offsets from galactic plane are also considered as signatures

Abstract

Modified gravity theories capable of genuine self-acceleration typically invoke a galileon scalar which mediates a long range force, but is screened by the Vainshtein mechanism on small scales. In such theories, non-relativistic stars carry the full scalar charge (proportional to their mass), while black holes carry none. Thus, for a galaxy free-falling in some external gravitational field, its central massive black hole is expected to lag behind the stars. To look for this effect, and to distinguish it from other astrophysical effects, one can correlate the gravitational pull from the surrounding structure with the offset between the stellar center and the black hole. The expected offset depends on the central density of the galaxy, and ranges up to ~0.1 kpc for small galaxies. The observed offset in M87 cannot be explained by this effect unless the scalar force is significantly stronger than gravity. We also discuss the systematic offset of compact objects from the galactic plane as another possible signature.