Is dark matter’s main competitor theory dead?

One of the biggest mysteries in astrophysics today is that the forces in galaxies don’t seem to add up. Galaxies rotate much faster than predicted if you apply Newton’s law of gravity to their visible matter, although these laws work well throughout the solar system.

To prevent galaxies from flying apart, some additional gravity is required. This is why the idea of ​​an invisible substance called dark matter was first proposed. But no one has ever seen the stuff. And in the highly successful Standard Model of particle physics, there are no particles that could be dark matter – it must be something quite exotic.

This has led to the competing idea that the galactic discrepancies are instead caused by a breakdown of Newton’s laws. The most successful idea of ​​this kind is known as Milgrom dynamics, or lunar, and was proposed by Israeli physicist Mordehai Milgrom in 1982. However, our recent research shows that this theory is in trouble.

Mond’s main postulate is that gravity begins to behave differently than Newton expected when it becomes very weak, for example at the edges of galaxies. Moon is quite successful at predicting the rotation of galaxies without dark matter, and there are some other successes as well. But many of them can also be explained by dark matter while respecting Newton’s laws.

So how do we put Moon to the test? We have been pursuing this for many years. The key is that Moon only changes the behavior of gravity at small accelerations, not at a specific distance from an object. At the edge of a celestial object – a planet, a star or a galaxy – you will feel less acceleration than if you are nearby. But it is the magnitude of acceleration, not distance, that predicts where gravity should be stronger.

This means that although lunar effects would typically occur several thousand light-years away from a galaxy, when looking at a single star the effects would be significant after just a tenth of a light-year. That’s just a few thousand times larger than an astronomical unit (AU) – the distance between the Earth and the Sun. But weaker lunar effects should also be detectable on even smaller scales, such as in the outer solar system.

This brings us to the Cassini mission, which orbited Saturn between 2004 and its final fiery crash onto the planet in 2017. Saturn orbits the Sun at 10 AU. Due to a lunar quirk, the gravity of the rest of our galaxy should cause Saturn’s orbit to deviate subtly from Newtonian expectation.

This can be tested by timing radio pulses between Earth and Cassini. Since Cassini orbited Saturn, this helped measure the distance between Earth and Saturn and allowed us to accurately track Saturn’s orbit. But Cassini didn’t find an anomaly of the expected kind on Moon. Newton still works well for Saturn.