Is the Main Rival Theory to Dark Matter Obsolete? Recent Tests, including those from the Cassini Spacecraft, Suggest so One of the most intriguing puzzles in astrophysics today revolves around the discrepancy in forces observed within galaxies. Galaxies rotate at much higher speeds than expected based on Newton’s law of gravity applied to their visible matter alone, despite the successful application of these laws within the solar system.
To reconcile this, scientists proposed the existence of an invisible substance called dark matter. However, the lack of direct observation of dark matter particles poses a challenge, as there are no particles in the widely accepted Standard Model of particle physics that could account for it. This has led to the emergence of a competing idea: that the discrepancies in galaxies are due to a breakdown in Newton’s laws.
The most prominent alternative theory, known as Milgromian dynamics or MOND, suggested by physicist Mordehai Milgrom in 1982, posits that gravity behaves differently than expected at very weak levels, such as at the edges of galaxies. While MOND has had some success in predicting galaxy rotation without the need for dark matter, recent research casts doubt on its validity.
The cornerstone of MOND is its assertion that gravity deviates from Newton’s predictions at low accelerations, rather than at specific distances from objects. This distinction becomes significant when examining individual stars, as the effects of MOND would be pronounced even at relatively small scales. For instance, the Cassini mission, which orbited Saturn, provided an opportunity to test MOND’s predictions by measuring subtle deviations in Saturn’s orbit due to the gravity from the rest of the galaxy.
However, analyses of the Cassini data and other recent tests have failed to find the expected anomalies that would support MOND. Despite efforts to adjust galaxy mass calculations and consider various uncertainties, MOND’s predictions consistently fail to align with observational data.
Moreover, additional tests involving wide binary stars and small bodies in the outer solar system further challenge MOND’s validity. Observations of wide binary stars contradict MOND’s prediction of faster orbital speeds, while the distribution of small bodies in the outer solar system does not match MOND’s expectations.
While MOND provided a possible alternative to dark matter, its inability to explain various astronomical phenomena across different scales weakens its status as a viable theory. Instead, the evidence continues to support the existence of dark matter, albeit with some unanswered questions regarding its nature. Ultimately, the dark matter paradigm remains predominant, suggesting that alternative explanations like MOND may not provide a complete solution to the mysteries of the universe.
