Why Modified Newtonian Dynamics (MOND) Failed to Replace Dark Matter

Modified Newtonian Dynamics (MOND) was introduced by Mordehai Milgrom in the early 1980s as a potential alternative to dark matter. The theory aimed to explain the observed rotation curves of galaxies without invoking the existence of dark matter. While MOND has shown promising empirical success, it has faced several challenges and criticisms. This article explores the reasons why MOND has not succeeded as a comprehensive theory of gravity and cosmic structure.

Lack of a Fundamental Framework

One of the primary criticisms of MOND is the lack of a fundamental theoretical framework that aligns with established physics, particularly General Relativity. MOND modifies Newton's laws of motion at low accelerations but fails to provide a comprehensive understanding of gravity that can seamlessly fit within the established framework. This absence of a deeper theoretical basis makes MOND less appealing to many physicists who prefer a theory that integrates smoothly with other known physical laws.

Galaxy Clusters and Dark Matter

MOND has shown its ability to explain the rotation curves of individual galaxies, but it struggles when applied to galaxy clusters. Observations of galaxy clusters have revealed an excess of mass that cannot be accounted for by MOND. This discrepancy indicates that galaxy clusters contain more mass than predicted by MOND, suggesting the need for additional mass, which is often attributed to dark matter. This inconsistency makes it difficult to accept MOND as a complete theory of gravity.

The Cosmic Microwave Background (CMB)

The Cosmic Microwave Background (CMB) provides strong evidence for the existence of dark matter and aligns well with the standard cosmological model, ΛCDM. MOND, which does not include dark matter, fails to fit these observations comprehensively, particularly regarding the large-scale structure of the universe. The CMB observations support the presence of dark matter and provide a robust framework for understanding the early universe, making it challenging for MOND to gain broader acceptance.

Gravitational Lensing and Dark Matter

Gravitational lensing studies observe how light from distant objects is bent by the mass of intervening galaxies and clusters. The amount of bending often exceeds what MOND would predict, further pointing towards the necessity of dark matter. Gravitational lensing is a crucial observational tool that provides direct evidence of the presence of dark matter. MOND's inability to accurately predict the lensing effects observed in these studies adds to its limitations as a fundamental theory of gravity.

Empirical Success of Dark Matter

The ΛCDM model, which includes dark matter, has been highly successful in explaining a wide range of astronomical observations, from galaxy formation to large-scale structure. The empirical success of dark matter makes it challenging for alternative theories like MOND to gain traction. The ΛCDM model has been tested extensively and has shown consistent results across multiple independent observations, making it a robust and well-supported framework.

Inconsistencies with Observations

Although MOND can explain some phenomena, there are instances where it fails to match observations as closely as the dark matter framework. For example, the dynamics of satellite galaxies and the behavior of certain stellar systems can be problematic under MOND. These inconsistencies highlight the limitations of MOND and suggest that dark matter remains a necessary component in explaining the observed phenomena in the universe.

In conclusion, while Modified Newtonian Dynamics (MOND) has provided valuable insights and has been successful in certain contexts, its inability to account for a wide range of astrophysical phenomena and its lack of a robust theoretical framework have led to skepticism about its viability as a complete theory of gravity and cosmic structure. The empirical success of dark matter and the consistency it brings to a wide range of astronomical observations make it a compelling framework for understanding the universe, at least at present.