Modified Newtonian Dynamics (MOND) and emergent gravity are two alternative theories that attempt to explain the observed galactic rotation curves without the need for dark matter. While they differ in their underlying principles and mechanisms, both theories propose modifications to the laws of gravity at low accelerations, where the deviations from standard Newtonian dynamics become significant.
- Modified Newtonian Dynamics (MOND): MOND was proposed by Mordehai Milgrom in the early 1980s. It suggests that the acceleration-dependent behavior of gravitational interactions causes a departure from the standard Newtonian dynamics at low accelerations. In MOND, the gravitational force deviates from the inverse square law at low accelerations and approaches a different functional form. Specifically, the acceleration-dependent modification introduces a new acceleration scale, known as the Milgrom constant (a₀), below which the gravitational force becomes stronger than predicted by Newton's laws.
By modifying the gravitational laws at low accelerations, MOND attempts to explain the observed galactic rotation curves without the need for dark matter. The theory proposes that the additional gravitational force provided by MOND at low accelerations can account for the anomalous rotation curves observed in galaxies.
- Emergent Gravity: Emergent gravity is a framework proposed by Erik Verlinde in 2010 that seeks to explain gravity as an emergent phenomenon from the statistical behavior of microscopic degrees of freedom. It suggests that gravity is not a fundamental force but instead arises from the collective behavior of other fundamental constituents.
In the context of galactic rotation curves, emergent gravity proposes that the observed discrepancies can be explained by modifications to the gravitational force laws on galactic scales. It suggests that gravity emerges from the statistical behavior of the underlying microscopic degrees of freedom, such as the entanglement of quantum fields.
Emergent gravity aims to provide an alternative explanation for the observed rotation curves without invoking dark matter. By considering gravity as an emergent phenomenon, it attempts to account for the gravitational effects in galaxies through modifications to the gravitational laws on large scales.
It's important to note that while MOND and emergent gravity offer alternative explanations for galactic rotation curves, they face challenges in providing a comprehensive explanation for various other observations at different scales, such as the dynamics of galaxy clusters and the observed large-scale structure of the universe. The standard dark matter paradigm, supported by a wealth of observational evidence, remains the prevailing explanation for these phenomena in the scientific community. However, ongoing research and exploration of alternative theories continue to contribute to our understanding of the fundamental nature of gravity and the composition of the universe.