Wide Binaries, Retardation and the External Field Effect

Abstract

A low acceleration gravitational anomaly was reported for wide binaries: two binary stars which are separated by more than five kilo astronomical units (kau). The increase in gravitational force was reported to be about $1.37$ greater than Newtonian gravity. At the same time binaries separated less than one kau were shown to obey standard Newtonian gravity. A possible explanation for this was given in the framework of MOND correction to gravity which is applicable for low acceleration. However, it was noticed that the explanation is only adequate in the framework of Milgromian AQUAL theory which considers the "external field effect" that is the effect of the gravitational field of the rest of the galaxy on the binary system. Recently it was shown that many "anomalous gravity" effects can be explained in the framework of general relativity and its weak field approximation. It was shown that anomalous galaxy rotation curves, anomalous gravitational lensing and the Tully-Fisher relations can be understood in terms of retarded gravity. Moreover, the anomalous mass of galaxy clusters derived from the virial theorem was also shown to be a retarded gravity effect. On the other hand, it was shown that retarded effects are not important in the solar system and the correction to the anomalous shift of Mercury's perihelion is connected with the motion of the solar center of mass with respect to the sun and not to retardation. It is desirable to investigate if the wide binary acceleration gravitational anomaly can be explained in the framework of Einstein-Newton general relativity. We show that the scale of five kau separation arises naturally within such a theory if the effect of the galaxy on the wide binary is considered (a Newtonian external field effect) suggesting the anomaly found can be explained without the need to invoke theoretical modifications of the accepted theory.