by One of the greatest mysteries in astrophysics today is that the forces in galaxies do not seem to be increasing. Galaxies rotate much faster than predicted by applying Newton’s law of gravity to their visible matter, despite the fact that those laws work well everywhere in the Solar System.
To prevent galaxies from flying apart, some extra gravity is needed. That’s 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 Standard Model of particle physics that could be dark matter – it must be very exotic.
This led to the competing idea that the Galactic discrepancies are caused by a breakdown of Newton’s laws. The most successful such idea is called Milgromian or Mond dynamics, proposed by the Israeli physicist Mordehai Milgrom in 1982. But our recent research shows that this theory is in trouble.
Mond’s main position is that gravity begins to behave differently from what Newton expected when it becomes very weak, as at the edges of galaxies. Mond is doing well in predicting the rotation of galaxies without any dark matter, and has a few more successes. But many of these can also be explained by dark matter, preserving Newton’s laws.
Read more: Dark matter: our review suggests 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 chasing this for many years. The key is that Mond only changes the behavior of gravity at low accelerations, not at a specific distance from an object. You will feel a lower acceleration at the edge of any celestial object – planet, star or galaxy – than when you are close to it. But the magnitude of the acceleration, rather than the distance, predicts where gravity should be stronger.
This means that while Mond’s effects would typically kick in several thousand light years away from a galaxy, if we look at a single star, the effects would become very noticeable 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 be detectable on even smaller scales, for example 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. Because of the Mond’s eccentricity, the gravity from the rest of our galaxy should cause Saturn’s orbit to deviate from the Newtonian expectation in a subtle way.
This can be tested by timing radio pulses between Earth and Cassini. Since Cassini was orbiting Saturn, this helped measure the Earth-Saturn distance and allowed us to precisely track Saturn’s orbit. But Cassini did not find any anomaly of the kind expected in Mond. Newton still works well for Saturn.
One of us, Harry Desmond, recently published a study that investigated the results in more depth. Maybe Mond would fit the Cassini data if we tweaked how we calculate the mass of galaxies from their brightness? That would affect the amount of gravitational boost Mond must provide to fit models of galaxy rotation, and therefore what we should expect in Saturn’s orbit.
Another uncertainty is the gravity from surrounding galaxies, which has a small effect. But the study showed that, given how Mond would have to work to fit models for galaxy rotation, it also can’t fit Cassini’s radio tracking results—no matter how we tweak the calculations.
With the standard assumptions that astronomers consider most likely and allow for a wide range of uncertainties, the odds of Mond matching Cassini’s results are the same as a 59-consecutive spinner landing. This is more than twice the “5 sigma” gold standard for discovery in science, which corresponds to about 21 consecutive coin flips.
More bad news for Monday
That’s not the only bad news for Mond. Another test is provided by a wide binary star – two stars orbiting a shared center thousands of AU apart. Mond predicted that these stars should orbit each other 20% faster than Newton’s laws predicted. But one of us, Indranil Banik, recently led a very detailed study that confirms this prediction. The odds of Mond being correct taking these results into account is the equivalent of landing a fair coin up 190 times in a row.
Results from another team show that Mond fails to explain small bodies in the distant Solar System. Comets arriving from there have a much narrower energy distribution than Mond predicted. These bodies also have orbits that are usually only slightly inclined to the plane of the orbits of all the nearby planets. Mond would make the inclinations much greater.
Newtonian gravity was much better than the Moon on distance scales below a light year. But Mond also fails on scales larger than galaxies: it cannot explain the movements 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 requires more gravity to hold it together than the visible mass can provide.
Mond cannot provide sufficient gravity either, at least in the central regions of galaxy clusters. But at its edge, Mond provides too much gravity. Assuming instead that Newtonian gravity, with five times as much dark matter as normal matter, fits the data well.
The standard dark matter model of cosmology is not perfect, however. It struggles to explain everything from the rate of expansion of the universe to massive 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 from what the Standard Model suggests. Or gravity may be stronger than we think – but only on very large scales.
Ultimately, however, Mond, as currently constructed, can no longer be considered a viable alternative to dark matter. We may not like it, but the dark side still has influence.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Indranil Banik receives funding from the Science and Technology Facilities Council to test MOND using the dynamics of wide binary stars.
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 does not disclosed any relevant affiliations beyond their academic appointment.
