This article was originally published on The conversation. The publication contributed the article to Space.com Expert voices: opinion pieces and insights.
Indranil Banik is a postdoctoral researcher in astrophysics at the University of St Andrews.
Harry Desmond is a Senior Research Fellow in Cosmology at the University of Portsmouth.
One of the greatest mysteries in… astrophysics today 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, a few extra ones gravity is necessary. 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 Mouthproposed by Israeli physicist Mordehai Milgrom in 1982. But our recent research shows that this theory is in trouble.
Related: Dark matter slows down the rotation of our Milky Way Galaxy
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 this predicting the rotation of galaxies without any dark matter, and it has a few other successes. But many of these can also be explained with dark matter, preserving Newton’s laws.
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 normally occur several thousand light years away from a galaxy, if we look at an individual star, the effects would become very significant at a tenth of a second. light year. That’s only a few thousand times bigger than one astronomical unit (AU) – the distance between the Soil and the sun. But weaker Mond effects should also be observable on even smaller scales, such as on the outer scale Solar system.
This brings us to the Cassini missionthat turned around Saturn between 2004 and the last fiery crash on the planet in 2017. Saturn orbits Earth the sun at 10AU. 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 was orbiting Saturn, this helped measure the Earth-Saturn distance and allowed 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 to investigate the results further. 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 wide double stars – two stars several thousand AU apart orbiting around a shared center. Mouth predicted that so stars should revolve around each other 20% faster than expected with Newton’s laws. But one of us, Indranil Banik, recently led a very detailed investigation excludes 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 is too failed to explain small celestial bodies in the distant outer solar system. Come eat those coming from outside have a much narrower energy distribution than Mond predicts. These bodies also have orbits that are usually only slightly inclined to the plane that all the planets orbit close to. 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 takes care of too many gravity. If we instead assume that Newton’s gravity, with five times as much dark matter as normal matter, is a fits well to the data.
The standard dark matter model of cosmology however, it is not perfect. There are things it takes effort to explain itby the universe‘s rate of expansion into 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.
Original published at The Conversation.