One of the biggest mysteries in astrophysics today is that the sum of the forces in galaxies is not equal. Galaxies are spinning much faster than predicted by applying Newton’s law of gravity to visible matter, despite the laws working well everywhere in the Solar System.
Some additional gravity is needed to keep the galaxies from falling apart. For this reason, the idea of an invisible substance called dark matter was first put forward. But no one has ever seen these things. 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 led to the rival idea that galactic differences resulted from a breakdown of Newton’s laws. The most successful idea of this kind is known as Milgrom dynamics, or Mond, proposed by Israeli physicist Mordehai Milgrom in 1982. But our latest research shows that this theory is in trouble.
Mond’s main assumption is that when gravity becomes very weak, such as at the edges of galaxies, it begins to behave differently than Newton expected. Mond is pretty good at predicting the rotation of the galaxy without any dark matter, and he has a few other successes to his credit. But most of these can also be explained by dark matter, preserving Newton’s laws.
Read more: Dark matter: our review shows it’s time to ditch it in favor of a new theory of gravity
So how do we put Mond to the definitive test? We have been pursuing this issue for many years. The important thing is that Mond’s gravity changes its behavior only at low accelerations and not at a certain distance from the object. You will feel less acceleration on the outskirts of any celestial object (planet, star, or galaxy) than when you are closer to it. But it is the amount of acceleration, rather than distance, that predicts where gravity should be stronger.
This means that although Mond effects typically begin a few thousand light-years away from a galaxy, when we look at a single star the effects become extremely significant within a tenth of a light-year. This is only a few thousand times larger than the astronomical unit (AU), which is the distance between the Earth and the Sun. However, weaker Mond effects are also expected to be detected on smaller scales, such as the outer Solar System.
This brings us to the Cassini mission, which orbited Saturn in 2004 until its final violent impact with the planet in 2017. Saturn orbits the Sun at 10 AU. Due to a peculiar property of Mond, gravity from the rest of our galaxy should cause Saturn’s orbit to deviate slightly from Newton’s expectation.
This can be tested by timing radio pulses between Earth and Cassini. Having Cassini orbiting Saturn helped us measure the Earth-Saturn distance and allowed us to track Saturn’s orbit precisely. However, Cassini did not find the expected type of anomaly in Mond. Newton still works well for Saturn.
One of us, Harry Desmond, recently published a study that looks deeper into the results. If we changed the way we calculated the mass of the galaxy from their brightness, maybe Mond would fit the Cassini data? This will affect how much support Mond must provide to gravity to accommodate galaxy rotation patterns, and therefore what we should expect for Saturn’s orbit.
Another uncertainty is gravity from surrounding galaxies, which also has a small effect. But the study showed that, given how Mond must work to accommodate galaxy rotation patterns, it doesn’t fit Cassini radio tracking results either, no matter how we fine-tune the calculations.
Given the standard assumptions most likely taken into account by astronomers, which allow for a wide range of uncertainties, Mond’s chance of matching the Cassini results is about the same as flipping the coin 59 times in a row. This is more than twice the “5 sigma” gold standard for a discovery in science; This corresponds to approximately 21 consecutive coin tosses.
More bad news for Mond
That’s not the only bad news for Mond. Another test is provided by large binary stars (two stars orbiting a common center several thousand AU apart). 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 disproves this prediction. Given these results, Mond’s chances of being right are the same as a fair coin coming up heads 190 times in a row.
Another team’s results show that Mond also fails to explain small objects in the distant outer Solar System. The energy distribution of comets coming from there is much narrower than Mond predicted. These bodies also have orbits that are usually only slightly inclined to the plane in which all the planets orbit close. Mond will cause the slopes to be much larger.
On length scales below about one light year, Newtonian gravity is strongly preferred over Mond. But Mond also fails at scales larger than galaxies: It cannot explain motions within galaxy clusters. Dark matter was first proposed by Fritz Zwicky in the 1930s to explain the random motions of galaxies within the Coma Cluster; this requires more gravity to hold it together than visible mass can provide.
Mond also cannot provide sufficient gravity, at least in the central regions of galaxy clusters. But at their foothills the Mond provides too much gravity. Instead, assuming Newtonian gravity, with five times more dark matter than normal matter, seems to provide a good fit to the data.
But the standard dark matter model of cosmology is not perfect. There are things that he finds difficult to explain, from the expansion rate of the universe to giant cosmic structures. So we may not have the perfect model yet. It looks like dark matter will persist, but its nature may differ from what the Standard Model suggests. Or gravity may actually be stronger than we think; but only on very large scales.
Ultimately, however, Mond as currently formulated can no longer be considered a viable alternative to dark matter. We may not like it, but the dark side is still in effect.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Indranil Banik is receiving funding from the Science and Technology Facilities Council to test MOND using the dynamics of large binary stars.
Harry Desmond does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic duties.