This article was originally published on The conversation. The publication contributed the article to Space.com Expert Voices: Op-Ed & Insights.
Indranil Banik is a Postdoctoral Research Fellow 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 biggest mysteries in astrophysics it’s today that the forces in the 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 the galaxies from flying apart, some extra gravity 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 highly successful Standard model of the particle physics that could be dark matter – must be something quite exotic.
This led to the rival idea that galactic inconsistencies are instead caused by a violation of Newton’s laws. The most successful such idea is known as Milgrom’s dynamics or Mondproposed by Israeli physicist Mordechai Milgrom in 1982. But our recent research shows that this theory is in trouble.
Connected: Dark matter stops the rotation of our Milky Way galaxy
Mond’s main postulate is that gravity begins to behave differently than Newton expected when it becomes very weak, as at the edges of galaxies. Mond is quite successful in predicting galaxy rotation no dark matter and has several other successes. But many of them can also be explained by dark matter, keeping Newton’s laws.
So how do we put Mond to the ultimate test? We have been striving for this for many years. The key is that Mond changes the behavior of gravity only at low accelerations, not at a certain distance from an object. You will feel less acceleration at the outskirts of any celestial object – planet, star or galaxy – than when you are close to it. But the amount of acceleration, not the distance, is what predicts where gravity should be stronger.
This means that although Mond effects usually occur a few thousand light-years from a galaxy, if we look at an individual star, the effects will become very significant at one-tenth of light year. This is only a few thousand times larger than a astronomical unit (AU) – the distance between The Earth and the Sun. But weaker Mond effects are also to be found on even smaller scales, such as the external Solar system.
This brings us to Cassini missionwho was going around Saturn between 2004 and its last fiery impact to the planet in 2017, Saturn orbited the sun in 10 AU. Due to a Mond oddity, gravity from the rest of our galaxy should cause Saturn’s orbit to deviate from Newtonian expectations in subtle ways.
This can be tested by synchronizing radio pulses between Earth and Cassini. As Cassini orbited Saturn, it helped measure the Earth-Saturn distance and allowed us to precisely track Saturn’s orbit. But Cassini didn’t find any anomaly of the kind expected on Mond. Newton still works well for Saturn.
One of us, Harry Desmond, recently published a study exploring the results in more depth. Maybe Monde will fit the Cassini data if we change how we calculate the masses of galaxies from their brightness? This would affect how much gravity amplification the Mond must provide to match the galaxy’s rotation patterns, and thus what we should expect for Saturn’s orbit.
Another uncertainty is gravity from surrounding galaxies, which has a negligible effect. But the study showed that given how Mond would have to work to fit the galaxy’s rotation models, it couldn’t fit Cassini’s radio tracking results either—no matter how we tweaked the calculations.
With the standard assumptions considered most likely by astronomers and allowing for a wide range of uncertainties, the chance of Mond matching Cassini’s results is the same as a flipped coin landing heads up 59 times in a row. This is more than twice the “5 sigma” gold standard for discovery in science, which corresponds to about 21 consecutive coin tosses.
More bad news for Mond
That’s not the only bad news for Mond. Another test is provided by a wide double stars – two stars orbiting a shared center several thousand AU apart. Mond provided for that such stars they should orbit each other 20% faster than expected by Newton’s laws. But one of us, Indranil Banik, recently conducted a very detailed study which turns off this prediction. The chance of Mond being right, given these results, is the same as fair landing heads 190 times in a row.
Results from yet another team show that Mond does too fails to explain the small bodies in the far outer part of the solar system. Comets coming from outside have a much narrower energy distribution than Mond predicted. These bodies also have orbits that are usually only slightly inclined to the plane around which all the planets orbit. Mond would cause the inclinations to be much greater.
Newtonian gravity is strongly favored over Mond on length scales below about a light year. But Mond also fails at scales larger than galaxies: it cannot explain motions in galaxy clusters. Dark matter was first proposed by Fritz Zwicky in the 1930s to explain the random motions of galaxies in the Coma cluster, which requires more gravity to hold it together than the visible mass can provide.
Mond also cannot provide sufficient gravity, at least in the central regions of galaxy clusters. But on their outskirts, Mond provides too much gravity. Assuming instead Newtonian gravity, with five times as much dark matter as normal matter, appears to provide a good measure to the data.
The standard model of dark matter cosmology it’s not perfect though. There are things it’s hard to explainfrom The universethe rate of expansion to giant space structures. So we may not have the perfect model yet. Dark matter appears to be here to stay, but its nature may be different from what the Standard Model suggests. Or gravity might really 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 still rules.
Original published in the Conversation.
