TAG: GS 3: SCIENCE AND TECHNOLOGY
THE CONTEXT: The debate between dark matter and Modified Newtonian Dynamics (MOND) has been ongoing in astrophysics.
EXPLANATION:
- Dark matter is a hypothetical form of matter that is believed to address the missing mass problem in galaxies, while MOND proposes that gravity behaves differently at low accelerations.
- The missing mass problem in galaxies was first identified by Oort in 1932.
- The discrepancy between the observed rotation curves of galaxies and the predicted curves based on Newton’s law of gravity led to the proposal of dark matter.
- However, the nature of dark matter remains unknown, and alternative theories like MOND have been proposed to explain the observed phenomena.
MOND Theory
- MOND has been proposed by Mordehai Milgrom in 1982.
- It suggests that gravity behaves differently at low accelerations.
- The theory is successful in predicting galaxy rotation without dark matter, but it has its limitations.
- MOND only changes the behavior of gravity at low accelerations, not at a specific distance from an object.
- This means that MOND effects would typically kick in several thousand light years away from a galaxy, making it difficult to detect at smaller scales.
Cassini Mission and MOND
- The Cassini mission, which orbited Saturn between 2004 and 2017, provided a crucial test for MOND.
- The mission aimed to measure the Earth-Saturn distance and track Saturn’s orbit to test the predictions of MOND.
- However, the results showed that Newton’s laws still work well for Saturn, and MOND failed to match the data.
- The study concluded that the chance of MOND matching the Cassini results is the same as a flipped coin landing heads up 59 times in a row, making it highly unlikely.
Wide Binary Stars and MOND
- Another test of MOND is provided by wide binary stars.
- MOND predicted that such stars should orbit around each other 20% faster than expected with Newton’s laws.
- However, a recent study led by Indranil Banik ruled out this prediction, showing that the chance of MOND being right given these results is the same as a fair coin landing heads up 190 times in a row.
Small Bodies in the Outer Solar System and MOND
- MOND also fails to explain the motions of small bodies in the distant outer Solar System.
- Comets coming in from out there have a much narrower distribution in energy than MOND predicts.
- These bodies also have orbits that are usually only slightly inclined to the plane that all the planets orbit close to.
- MOND would cause the inclinations to be much larger.
Galaxy Clusters and MOND
- MOND cannot provide enough gravity in the central regions of galaxy clusters.
- However, it provides too much gravity in their outskirts.
- Assuming Newtonian gravity with five times as much dark matter as normal matter seems to provide a good fit to the data.
Dark Matter
- Dark matter is a hypothetical form of matter that is believed to address the missing mass problem in galaxies.
- It is not directly observable, but its presence can be inferred through its gravitational effects.
- Dark matter is composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation.
- Dark matter is material that cannot be seen directly.
- Dark matter is a key component of the standard model of cosmology, which struggles to explain certain phenomena such as the universe’s expansion rate and giant cosmic structures.
Comparison of Dark Matter and MOND
- Several studies have compared the dark matter and MOND hypotheses using Bayesian statistics.
- The results suggest that both hypotheses are equally plausible in explaining the data, with no strong evidence in favor of either theory.
- However, the Bayesian Information Criterion (BIC) values strongly favor the dark matter hypothesis.