One of the greatest mysteries in contemporary astrophysics is that the forces in galaxies don’t seem to add up. Galaxies rotate much faster than predicted by applying Newton’s law of gravity to their visible matter, even though these laws work well everywhere in the solar system.
To prevent galaxies from flying apart, some extra gravity is needed. This is why the idea of an invisible substance called dark matter was first proposed. But no one has ever seen the stuff. And there are no particles in the hugely successful Standard Model of particle physics that could be dark matter – it must be something very exotic.
This has led to the rival idea that the galactic discrepancies are instead caused by a failure of Newton’s laws. The most successful idea is known as Milgromian dynamics or Mond, proposed by Israeli physicist Mordehai Milgrom in 1982. But our recent research shows that this theory is in trouble.
Mond’s most important postulate is that gravity will behave differently than Newton expected when it becomes very weak, such as at the edges of galaxies. Mond is quite successful in predicting the rotation of galaxies without any dark matter, and has a few other successes. But many of these can also be explained with dark matter, preserving Newton’s laws.
Read more: Dark matter: Our review suggests it’s time to abandon it in favor of a new theory of gravity
So how do we put Mond to a definitive test? We have been pursuing this for many years. The key is that Mond changes the behavior of gravity only at low accelerations, and not at a specific distance from an object. You feel a lower acceleration at the edge of a celestial body (a planet, star or galaxy) than when you are close to it. But it’s the amount of acceleration, not distance, that predicts where gravity should be stronger.
This means that although Mond effects are typically several thousand light years away from a galaxy, when we look at an individual star the effects would become very significant at a tenth of a light year. That’s only 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 observable 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 last fiery crash on the planet in 2017. Saturn orbits the Sun at 10 AU. By a whim of Mond, the gravity of the rest of our Galaxy should cause Saturn’s orbit to subtly deviate from the Newtonian expectation.
This can be tested by timing radio pulses between Earth and Cassini. Because Cassini orbited Saturn, this helped measure the distance between Earth and Saturn, allowing us to accurately track Saturn’s orbit. But Cassini did not find any anomaly of the kind expected at Mond. Newton still works well for Saturn.
One of us, Harry Desmond, recently published a study that explored the results in more depth. Maybe if we adjusted how we calculate the mass of galaxies based on their brightness, Mond would fit the Cassini data? That would affect how much Mond needs to boost gravity to fit into models of galaxy rotation, and thus what we can expect for Saturn’s orbit.
Another uncertainty is the gravity of surrounding galaxies, which has a small effect. But the study showed that given the way Mond would have to work to fit galaxy rotation models, it cannot also fit the Cassini radio tracking results – no matter how we adjust the calculations.
Using the standard assumptions that astronomers consider most likely and taking into account a wide range of uncertainties, the probability of Mond matching Cassini’s results is the same as a coin landing heads up 59 times in a row. This is more than twice the “5 sigma” gold standard for a discovery in science, equivalent to about 21 coin changes in a row.
More bad news for Mond
That’s not the only bad news for Mond. Another test is provided by broad binaries – two stars several thousand AU apart orbiting a shared center. Mond predicted that such stars should orbit each other 20% faster than expected based on Newton’s laws. But one of us, Indranil Banik, recently led a very detailed study that rules out this prediction. The probability that Mond is right, given these results, is the same as a fair coin landing heads-up 190 times in a row.
Results from yet another team show that Mond also fails to explain small celestial bodies in the far 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 relative to the plane that all the planets orbit near. Mouth would cause the tendencies to be much greater.
Newtonian gravity is strongly favored over Mond on length scales less than about one light year. But Mond also fails on scales larger than galaxies: it cannot explain the motions within galaxy clusters. Dark matter was first proposed by Fritz Zwicky in the 1930s to account for the random motions of galaxies within the Coma Cluster, which require more gravity to hold them together than visible mass can provide.
Mond also cannot provide enough gravity, at least not in the central regions of galaxy clusters. But in their suburbs, Mond creates too much gravity. By instead assuming that Newton’s gravity, with five times as much dark matter as normal matter, fits the data well.
However, the standard dark matter model of cosmology is not perfect. There are things it struggles to explain, from the expansion rate of the universe to gigantic cosmic structures. So we may not have the perfect model yet. It appears that dark matter is here to stay, but its nature may be different than what the Standard Model suggests. Or gravity may indeed be stronger than we think – but only on very large scales.
But ultimately Mond, as currently formulated, can no longer be considered a viable alternative to dark matter. We may not like it, but the dark side still prevails.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Indranil Banik receives funding from the Science and Technologies Facilities Council to test MOND using the dynamics of wide binaries.
Harry Desmond does not work for, consult with, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.