Binary-star study favours modified gravity over dark matter

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.

Binary tests

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.

Elliptical orbits

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.

Furthermore, Chae says that when one takes into account the external field effect – an integral part of MOND that describes how a much larger gravitational field, in this case that of the Milky Way galaxy as a whole, can affect a smaller gravitational system such as a binary star – the gravitational acceleration at wide separations is boosted by a factor of 1.4. This matches a prediction of a specific model of MOND called AQUAL, which was developed by Milgrom and the late physicist Jacob Bekenstein of the Hebrew University of Jerusalem.

�It is truly remarkable that when the external field effect is taken into account, the boost factor is about 1.4,” says Chae. �To me, this cannot be a coincidence.”

Mixed reactions

Reaction to Chae’s results has been mixed. Stacy McGaugh of Case Western Reserve University in the US, who is one of the world’s leading proponents of MOND, tells Physics World that he thinks, �we should be excited about the possibility of binary stars to provide a fresh and potentially decisive test. Chae shows a very clean result that is formally highly significant, so we should take it seriously.”

Nevertheless, McGaugh also urges caution about drawing conclusions so soon, citing other researchers who have made similar measurements similar to Chae’s but found no evidence for MOND. One of those is Indranil Banik of the UK’s University of St Andrews, who earlier this year led a team that analysed wide binaries from Gaia DR3 in search of MOND’s influence, but did not find any evidence for it.

�I disagree very much with the results of Chae,” Banik told Physics World.

Data analysis

Banik’s objections are related to the way the data have been analysed by Chae, and the lack of a strict cut-off on the uncertainty in the relative velocity of the stars in a binary system. In particular, Banik says that in his own work he quantified uncertainties in the measurements of the velocities of the stars and he only considered binary systems where those velocities are accurately known. �The failure to do so is the main problem with what Chae did,” says Banik. �So I am afraid there is no MOND signal in local wide binaries.”

Confirmation one way or the other could potentially come from follow-up work using different instruments. While Gaia DR3 provides the best quality data for a statistical analysis of many thousands of binaries, large observatories could follow-up on individual binary systems and measure their line-of-sight velocities and take deep images to confirm their separation.

Chae, for his part, is unperturbed by the criticism, and thinks that dark matter’s days are numbered.

�The evidence is already conclusive,” he argues. �There is no longer a need for a large amount of dark matter.”