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Will Cassini deal the death blow to MOND, the alternative theory of gravity?

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The flat rotation curve of the galaxy M33. Credit: Stefania.deluca

After observations by Vera Rubin and others in the 1970s, we know that galaxies rotate faster than predicted. To explain this, the idea that galaxies contain more matter than visible, invisible or dark matter was put forward. The “flat rotation curve” of galaxies could be explained by this additive. But what if the theory on which that prediction was made is wrong, if Newton’s theory of gravity is wrong? Israeli physicist Mordehai Milgrom first came up with this in 1982. He came up with the theory Modified Newtonian dynamics, or SUU. According to Milgros, in strong gravitational fields gravity would behave just like Newton’s laws say, only in areas with weak gravity there would be a difference.

Cassini-Huygens probe on Saturn; ESA

MOND can be used to describe the gravity of galaxies well: flat rotation curves can be well described even without dark matter if you use MOND. But how do you know if what the MOND equations say is correct? To find out, Harry Desmond and his colleagues analyzed data collected by Cassin, the NASA space probe that observed Saturn up close for years (and eventually dived into it). As mentioned, MOND works mainly in regions of weak gravity, far from the mass. What is important is not so much the gravity as the low acceleration there. Far away from masses like the sun, you have low acceleration. In galaxies, you will find the low acceleration where MOND effects appear, thousands of light years from the center. In the case of the Sun, this can already be measured in about a tenth of a light year, which is a few thousand times the distance between the Earth and the Sun (AU). Saturn is 10 AU from the Sun, so too close for these effects. But one of the assumptions of MOND is that the mass of more distant objects such as the Milky Way galaxies and even other galaxies in the universe matters (ie the so-called galactic external field effect, EFE). If you include that mass as well, MOND effects should already be measurable on Saturn, as small deviations of the distance to the Sun compared to Newton’s predictions.

Between 2004 and 2017, Cassini orbited Saturn and it was possible to calculate very precisely how far Saturn was from us at the time the radio signal reached Earth. What Desmond and his team’s analysis shows: The MOND deviation is not measurable, everything Saturn did in terms of distance is exactly Newtonian. Desmond’s colleague Indranil Banik recently falsified MOND theory in another way by looking at the low accelerations of binary systems, which can be read here.

Alles on na te lezen in het vakartikel van Harry Desmond et al, On the tension between the radial Acceleration ratio and the solar quadrupole in modified Gravity MOND, Monthly Bulletins of the Royal Astronomical Society(2024).



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