Dark matter or mond? Binary systems complicate the situation

Dark matter, a fundamental component of modern cosmology, helps explain many observations from the properties of the first radiation emitted in the universe (the cosmic microwave background) to the velocity curve of rotating galaxies, including the formation of the large structures that make up the cosmic web. However, its nature remains unknown, despite numerous experimental attempts to directly detect it. Could dark matter be an artifact, a convenient argument to explain observations but without physical reality? This question becomes more urgent, because on a galactic scale there is another approach that works just as well, or even better. This consists of modifying the laws of gravity for low acceleration. However, determining the correct scenario, between approaching dark matter or modified gravity, proved much more difficult than expected. A recent test relies on careful study of the behavior of binary systems in which the two stars are very far from each other.

This test was made possible thanks to the European Space Telescope Gaia, launched in 2013, which measures the position and speed of billions of stars in the Milky Way. Astrophysicists exploit catalogs Gaia To study different questions. Binary systems with large separation are of particular interest to specialists in dark matter or modified gravity models, especially those based on the Mound model. (modified Newtonian dynamics)Proposed by Israeli physicist Mordechai Milgrom in 1983.

In a tight binary system, the gravity of the two stars is strong and their motions are accompanied by an acceleration greater than the threshold below which Mound effects appear (this threshold is about 10^-10 meters per square second). In contrast, in binary systems with large separation, the acceleration is small and the Mond effect must be measurable. However, measurement is difficult because these systems are immersed in the galaxy's global gravitational field, creating additional acceleration. This is 1.8 times the Mound threshold at the distance from the center of the galaxy corresponding to the solar system. However, during the first studies in 2019, researchers were able to identify slight excess acceleration (“boosting”) in the data expected for some binary systems, undermining Newton's laws.

Since then, the number of works on this topic has increased significantly. But the situation is far from clear. Recently, Indranil Banik, from the University of St. Andrews in the United Kingdom, and his colleagues revealed, by analyzing data from… Gaia, Mond was statistically excluded. “We used a parametric model that cannot take into account all the physical complexity involved,” says Benoît Fami, of the Strasbourg Astronomical Observatory. But it is not easy to show how exotic physics can reverse this trend and favor the dynamics of the world. »

Javier Hernandez, from the Autonomous University of Mexico, and Kyu Hyun Chae, from Sejong University in South Korea, published several articles in 2023, proving the impact of the global push. For them, Indranil Panik's study is biased because it would include too many three-body systems (on which the data are based). Gaia does not always allow for certain identification). “This is, in my opinion, a valid criticism, which we make in our article,” comments Benoit Famy. In fact, if we assume that the model parameter controlling the number of ternary systems remains relatively low, the results will be bad in both the Newtonian and global cases… but Newtonianism is still much more favourable. »

Thus the question of binary systems with large separation remains open. To see things more clearly, Benoit Famy and his colleagues Harry Desmond and Aurelien Hess transcended constraints at different levels: large separated binary systems, constraints within the solar system, and galactic rotation curves. How do constraints on these different scales combine? Does it lead to contradictions?

In Mound-type gravitational modification theories, the galaxy's gravitational field causes a slight distortion of the solar system's gravitational field. To track this effect, researchers use, for example, the trajectory of the probe Cassini which explored Saturn until 2017. Studying the trajectory provides very strong constraints on Mound-type models. In particular, it is too restrictive for the “transition function” that describes the intermediate system that transitions from the Newtonian system to the global system. This function is constant and fairly well defined by data on galactic scales. Researchers have shown that it becomes difficult to meet all constraints. “If we consider a transition function beyond the constraints of the solar system, there can be no Mond effect for binary systems with large separation distances, and this is consistent with the results of our article with Indranil Panik,” says Benoît Famy.

But this result is far from ending the discussion. Benoit Famy himself remains very cautious. “The comprehensive gauge relationships predicted by Mound remain observationally very well respected in rotating galaxies, and Mound remains a much more natural interpretation of these gauge relationships than any current explanations based on the Standard Model (with dark matter). This does not mean at all that Mound Right, obviously, and it doesn't rule anything out of all its known problems at supragalactic scales and, as here, at subgalactic scales. But it does mean that we can continue to take Mond's hypothesis seriously, despite its failure. On certain levels.”

One possible conclusion is that simple modified gravity models like Mound (or some relativistic version, like TeVeS) are insufficient to describe the full complexity of different situations. Solar system constraints suggest richer models. For example, the model of Konstantinos Skourdis and Tom Zuznik, of the Czech Academy of Sciences in Prague: In 2021 they proposed a model of modified gravity capable of satisfying the constraints imposed by the cosmic microwave background, constraints that have always been very thorny for us. Modified Gravity Models Currently, this model is limited to the limitations of the solar system, but researchers already have ideas about ways to adapt it, at the cost of increased complexity.

So dark matter or modified gravity? The question remains burning!