Is dark matter's main rival theory dead? Dark Matter vs Cassini: Unveiling the Fate of MOND

Is Dark Matter's Arch Nemesis Defeated? Recent Cassini Spacecraft Discoveries and Beyond Challenge MOND's Reign

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, making it one of the biggest mysteries in astrophysics today that the forces in galaxies do not seem to add up.
The notion of dark matter emerged to counteract the tendency of galaxies to disperse, requiring an extra gravitational force. Despite its invisibility and the absence of corresponding particles in the well-established Standard Model of particle physics, dark matter remains a mysterious and exotic substance.
Our recent research shows that the rival idea attributing galactic discrepancies to a breakdown of Newton's laws, known as Milgromian dynamics or MOND, proposed by Israeli physicist Mordehai Milgrom in 1982, is in trouble.
MOND's main postulate is that gravity deviates from Newton's expectations when it weakens significantly, such as at the outer edges of galaxies. Although MOND successfully predicts galaxy rotation without the need for dark matter and has other notable successes, many of its achievements can also be accounted for by dark matter, which maintains adherence to Newton's laws.
The key to definitively testing Modified Newtonian Dynamics (MOND) lies in its unique effect on gravity, which only alters behavior at low accelerations rather than at a specific distance from an object. When situated on the outskirts of any celestial body—be it a planet, star, or galaxy—one experiences lower acceleration compared to being close to it. However, it's the magnitude of acceleration, not the distance, that determines where gravity should be stronger.
The effects of MOND typically manifest several thousand light years away from a galaxy, but on the scale of an individual star, they become highly significant within a tenth of a light year—just a few thousand times larger than the astronomical unit (AU), the distance between the Earth and the sun. Weaker MOND effects should also be detectable on even smaller scales, such as in the outer solar system.
The Cassini mission, which spanned from 2004 until its dramatic conclusion with a fiery crash into Saturn in 2017, orbits the planet within 10 AU of the sun. Saturn's orbit, however, is subject to a peculiar effect of Modified Newtonian Dynamics (MOND), whereby the gravitational influence from the remainder of our galaxy should induce a subtle deviation from the Newtonian predictions.

Newton's laws remained effective in understanding Saturn's orbit despite hopes that Cassini's radio pulse measurements might reveal anomalies consistent with Modified Newtonian Dynamics (MOND). The precision of these measurements, while aiding in tracking Saturn's orbit and determining the Earth-Saturn distance, did not uncover any such deviations.

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