# MOND (Modified Newtonian Dynamics) – Definition & Detailed Explanation – Cosmology Glossary

## I. What is MOND (Modified Newtonian Dynamics)?

Modified Newtonian Dynamics (MOND) is a theory proposed as an alternative to the standard model of gravity, Newtonian dynamics. It suggests that the laws of gravity may need to be modified at very low accelerations, typically below 10^(-10) m/s^2. This modification is proposed to explain the observed discrepancies between the predicted and observed motions of galaxies and galaxy clusters.

MOND was first introduced by physicist Mordehai Milgrom in the early 1980s as a way to account for the observed flat rotation curves of spiral galaxies without the need for dark matter. Instead of invoking the existence of unseen matter to explain the discrepancies, MOND proposes a modification to the laws of gravity that becomes significant at low accelerations.

## II. How does MOND differ from Newtonian dynamics?

The key difference between MOND and Newtonian dynamics lies in their predictions for the motion of objects at low accelerations. In Newtonian dynamics, the force of gravity between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. This relationship holds true for all accelerations, including very low ones.

In contrast, MOND proposes a modification to the law of gravity that becomes significant at low accelerations. At these low accelerations, the force of gravity is no longer proportional to the product of the masses but instead scales linearly with the acceleration. This modification allows MOND to explain the observed flat rotation curves of galaxies without the need for dark matter.

## III. What is the evidence for MOND?

One of the main pieces of evidence in support of MOND is the observed flat rotation curves of spiral galaxies. In Newtonian dynamics, the rotation curves of galaxies are expected to decline as one moves away from the center due to the decreasing gravitational force. However, observations have shown that the rotation curves remain flat, indicating that there is additional mass present that is not accounted for by visible matter.

Additionally, MOND has been successful in predicting the dynamics of galaxy clusters without the need for dark matter. By modifying the laws of gravity at low accelerations, MOND is able to reproduce the observed motions of galaxies within clusters without the need for additional unseen matter.

## IV. How does MOND impact our understanding of cosmology?

The implications of MOND for cosmology are significant. If MOND is able to explain the observed discrepancies in the motion of galaxies and galaxy clusters without the need for dark matter, it would challenge the current understanding of the composition of the universe. Dark matter is thought to make up a significant portion of the mass-energy content of the universe, and its existence is crucial for explaining the large-scale structure and dynamics of the cosmos.

If MOND is confirmed as a valid theory, it would require a reevaluation of our understanding of gravity and the laws of physics at low accelerations. It would also raise questions about the nature of dark matter and the need for alternative explanations for the observed phenomena that dark matter is currently invoked to explain.

## V. What are the criticisms of MOND?

Despite its successes in explaining certain observational data, MOND has faced criticism from the scientific community. One of the main criticisms is that MOND is an ad-hoc modification of the laws of gravity that lacks a theoretical foundation. Critics argue that MOND does not arise naturally from any underlying theory and is instead introduced to fit the observed data.

Additionally, MOND has been challenged by observations on larger scales, such as the cosmic microwave background radiation and the distribution of galaxies in the universe. These observations provide strong evidence for the existence of dark matter and are difficult to reconcile with the predictions of MOND.

## VI. How is MOND being tested and researched in the field of cosmology?

Despite the criticisms, MOND continues to be an active area of research in the field of cosmology. Researchers are conducting a variety of tests and experiments to further investigate the predictions of MOND and compare them to the predictions of dark matter.

One approach is to study the dynamics of galaxy clusters and other large-scale structures in the universe. By comparing the predictions of MOND to the observed motions of galaxies within clusters, researchers can test the validity of the theory on larger scales.

Another approach is to study the dynamics of individual galaxies and their rotation curves. By observing a larger sample of galaxies and comparing the predictions of MOND to the observed data, researchers can further test the theory and refine its predictions.

Overall, the study of MOND represents an important avenue for exploring the nature of gravity and the composition of the universe. While it may not provide a complete explanation for all observed phenomena, MOND offers a valuable alternative perspective that challenges our current understanding of cosmology.