A new study of data from the European Space Agency’s Gaia space mission claims to have found evidence of gravity acting contrary to the predictions of Newton and Einstein, but not everyone agrees that this is the smoking gun for a theory of modified gravity.

Observations of galaxies and galaxy clusters show that the gravitational forces binding these structures together are greater than those expected from the matter they contain. This has led physicists to predict the existence of dark matter, which is a hypothetical material that is invisible but interacts with normal matter via gravity. While dark matter has never been observed directly, it is included in the standard model of cosmology.

Modified Newtonian dynamics, or MOND for short, was developed in 1983 by Mordehai Milgrom of the Weizmann Institute in Israel as an alternative to models of dark matter. MOND seeks to explain the apparent extra gravity binding together galaxies and galaxy clusters by modifying the physics of gravity. In particular, MOND describes how gravity operates differently at very low accelerations.

Despite occasional successes for MOND, such as describing the motion of stars around galaxies, the evolution of open star clusters, and the survival of dwarf galaxies, MOND is not seen as a mainstream model. However, supporters of MOND point out that dark matter has yet to be observed directly, and that dark-matter models are everchanging as their predictions are excluded by experiments and observations. Proponents of dark matter, on the other hand, argue that MOND cannot yet explain many of the things that dark matter models can.

Stars separated by large distances in binary systems have long been considered objects that could put MOND to the test. This is because accelerations in such “wide binaries” are small enough that MOND is relevant, but dark matter is not expected to affect such systems.

Now, with Data Release 3 (DR3) from the Gaia astrometric mission, astronomers finally have the data to put MOND to the test in these binary systems. In a new paper published in The Astrophysical Journal, Kyu-Hyun Chae of Sejong University in South Korea has used statistics to analyse data describing the orbital motions of 26,500 wide-binary star systems, all located within 650­­­­ light– years of Earth. In particular, Chae calculated the gravitational accelerations of the stars around one another.

“A binary system is gravitationally bound, so it always experiences an internal gravitational acceleration in its orbit,” Chae tells Physics World.

If the stars had perfectly circular orbits around one another, their gravitational acceleration would remain constant. In reality, they have elliptical orbits, meaning that their separation from one another and hence their gravitational acceleration changes. The binary stars in the study can range in separation from between 200 AU to 30,000 AU – where 1 AU is the distance from the Earth to the Sun.

The measured gravitational acceleration is exceptionally tiny. At separations of less than 1000 AU, the gravitational acceleration is greater than 10 nm/s2 and gravity is observed to act as predicted by Newtonian physics. However, at separations of more than 2000 AU, where the gravitational acceleration is on the order of 1 nm/s2, Chae says that his analysis reveals discrepancies in the acceleration, with its value being greater than what the models of Newton and Einstein predict. At more than 5000 AU, where the gravitational acceleration is less than 0.1 nm/s2, the difference is clearly seen.