The phenomenology of the external field effect in cold dark matter models

In general relativity (GR), the internal dynamics of a self-gravitating system under free-fall in an external gravitational field should not depend on the external field strength. Recent work has claimed a statistical detection of an 'external field effect' (EFE) using galaxy rotation curve data. We show that large uncertainties in rotation curve analyses and inaccuracies in published simulation-based external field estimates compromise the significance of the claimed EFE detection. We further show analytically that a qualitatively similar statistical signal is, in fact, expected in a ?-cold dark matter (?CDM) universe without any violation of the strong equivalence principle. Rather, such a signal arises simply because of the inherent correlations between galaxy clustering strength and intrinsic galaxy properties. We explicitly demonstrate the effect in a baryonified mock catalogue of a ?CDM universe. Although the detection of an EFE-like signal is not, by itself, evidence for physics beyond GR, our work shows that the sign of the EFE-like correlation between the external field strength and the shape of the radial acceleration relation can be used to probe new physics: e.g. in MOND, the predicted sign is opposite to that in our ?CDM mocks.

Automated summary

This study questions the claimed statistical detection of an 'external field effect' using rotation curve data, highlighting uncertainties in analysis and inaccuracies in external field estimates. It demonstrates that a similar signal is expected in a ΛCDM universe without violating the strong equivalence principle, and suggests that the sign of the correlation between external field strength and radial acceleration can be used to probe new physics.