Is dark matter’s main competitor theory dead? The Cassini spacecraft and other recent tests could invalidate MOND

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 MOND, 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. MOND is quite successful at predicting the rotation of galaxies without dark matter, and it has several 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 MOND to the test? We have been pursuing this for many years. The key is that MOND 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 feel less acceleration than when you are near it. But it is the magnitude of acceleration, not distance, that predicts where gravity should be stronger.

This means that although MOND effects would normally 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 MOND 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 into the planet in 2017. Saturn orbits the Sun at 10 AU. Due to a quirk of MOND, 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 at MOND. Newton still works well for Saturn.

One of us, Harry Desmond, recently published a study in the Monthly Notices of the Royal Astronomical Society which examines the results in more depth. Maybe MOND would match the Cassini data if we tweaked the way we calculate galaxy masses based on their brightness? This would affect how much MOND needs to boost gravity to fit models of galaxy rotation, and thus what we can expect for Saturn’s orbit.

Another uncertainty is the gravity of the surrounding galaxies, which has a minor influence. But the study showed that given the way MOND would have to work to match models for galaxy rotation, it cannot also match the Cassini radio-tracking results – no matter how we tweak the calculations.

Using the standard assumptions considered most likely by astronomers and taking into account a wide range of uncertainties, the chance that MOND will agree with the Cassini results is as great as a coin being flipped heads up 59 times in a row lands. That’s more than double the “5 sigma” gold standard for a scientific discovery, which is equivalent to about 21 coin tosses in a row.

More bad news for MOND

That’s not the only bad news for MOND. Another test is wide binary stars – two stars that orbit a common center several thousand AU apart. MOND predicted that such stars should orbit each other 20% faster than would be expected according to Newton’s laws. But one of us, Indranil Banik, recently conducted a very detailed study that rules out this prediction. The chance of MOND being right given these results is the same as a fair coin landing heads-up 190 times in a row.

Results from another team show that MOND cannot explain even small bodies in the distant outer solar system. Comets that come from there have a much narrower energy distribution than MOND predicts. These bodies also have orbits that are usually only slightly inclined to the plane near which all planets orbit. MOON would cause the slopes to be much larger.

At length scales less than about a light year, Newtonian gravity is strongly preferred to MOND gravity. But MOND also fails at scales beyond galaxies: it cannot explain the movements within galaxy clusters. Dark matter was first proposed by Fritz Zwicky in the 1930s to explain the random movements of galaxies within the Coma Cluster, which requires more gravity to hold it together than the visible mass can provide.

MOND also cannot provide enough gravity, at least in the central regions of galaxy clusters. But on its outskirts, MOND creates too much gravity. Assuming instead that Newtonian gravity contains five times as much dark matter as normal matter seems to provide a good fit with the data.

However, the standard model of dark matter in cosmology is not perfect. There are things it struggles to explain, from the expansion rate of the universe to vast cosmic structures. Therefore, we may not have the perfect model yet. It appears that dark matter persists, but its nature may be different from what the Standard Model suggests. Or gravity could actually be stronger than we think – but only on very large scales.

Ultimately, however, MOND in its current form can no longer be viewed as a viable alternative to dark matter. We may not like it, but the dark side still rules.