"One of the biggest mysteries in astrophysics today is that the forces in galaxies do not seem to add up," write two U.K. researchers in the Conversation:
Galaxies rotate much faster than predicted by applying Newton's law of gravity to their visible matter, despite those laws working well everywhere in the Solar System. To prevent galaxies from flying apart, some additional gravity is needed. This is why the idea of an invisible substance called dark matter was first proposed. But nobody has ever seen the stuff. And there are no particles in the hugely successful Standard Model of particle physics that could be the dark matter — it must be something quite exotic.
This has led to the rival idea that the galactic discrepancies are caused instead by a breakdown of Newton's laws. The most successful such idea is known as Milgromian dynamics or Mond [also known as modified Newtonian dynamics], proposed by Israeli physicist Mordehai Milgrom in 1982. But our recent research shows this theory is in trouble...
Due to a quirk of Mond, 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 to 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... Another test is provided by wide binary stars — two stars that orbit a shared centre several thousand AU apart. Mond predicted that such stars should orbit around each other 20% faster than expected with Newton's laws. But one of us, Indranil Banik, recently led a very detailed study that rules out this prediction. The chance of Mond being 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 bodies in the distant outer Solar System...
The standard dark matter model of cosmology isn't perfect, however. There are things it struggles to explain, from the universe's expansion rate to giant cosmic structures. So we may not yet have the perfect model. It seems dark matter is here to stay, but its nature may be different to what the Standard Model suggests. Or gravity may indeed be stronger than we think — but on very large scales only.
"Ultimately though, Mond, as presently formulated, cannot be considered a viable alternative to dark matter any more," the researchers conclude. "We may not like it, but the dark side still holds sway."
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