Modified Newtonian dynamics

"MOND" redirects here. For other uses, see Mond.

Modified Newtonian dynamics (MOND) is a theory that proposes a modification of Newton's second law to account for observed properties of galaxies. Its primary motivation is to explain galaxy rotation curves without invoking dark matter, and is one of the most well-known theories of this class. However, it has not gained widespread acceptance, with the majority of astrophysicists supporting the Lambda-CDM model as providing the better fit to observations.[1][2]

MOND was developed in 1982 and presented in 1983 by Israeli physicist Mordehai Milgrom.[3] Milgrom noted that galaxy rotation curve data, which seemed to show that galaxies contain more matter than is observed, could also be explained if the gravitational force experienced by a star in the outer regions of a galaxy decays more slowly than predicted by Newton's law of gravity. MOND modifies Newton's laws for extremely small accelerations (characteristic of the outer regions of galaxies, or the inter-galaxy forces within galaxy clusters), fitting the galaxy rotation curve data.[4] In addition, the theory predicts that the mass of the Galactic Center should even affect the orbits of Kuiper Belt objects.[5]

Unsolved problem in physics:
  • What is the nature of dark matter? Is it a particle, or do the phenomena attributed to dark matter actually require a modification of the laws of gravity?
(more unsolved problems in physics)

Since Milgrom's original proposal, MOND has seen scattered successes. It is capable of explaining several observations in galaxy dynamics,[6][7] some of which can be difficult for Lambda-CDM to explain.[8] However, MOND struggles to explain a range of other observations, such as the acoustic peaks of the cosmic microwave background and the Bullet cluster; furthermore, because MOND is not a relativistic theory, it struggles to explain relativistic effects such as gravitational lensing and gravitational waves. Finally, a major weakness of MOND is that galaxy clusters show a residual mass discrepancy even when analyzed using MOND.[6][9][10]

A minority of astrophysicists continue to work on the theory. Jacob Bekenstein developed a relativistic generalization of MOND in 2004, TeVeS, which however had its own set of problems. Another notable attempt was by Constantinos Skordis [d] and Tom Złośnik [d] in 2021, which proposed a relativistic model of MOND compatible with cosmic microwave background observations.[1]

  1. ^ a b Keith Cooper (6 February 2024). "Cosmic combat: delving into the battle between dark matter and modified gravity". physicsworld.
  2. ^ Ethan Siegel (19 October 2022). "Why modifying gravity doesn't add up".
  3. ^ Milgrom, M. (1983). "A modification of the Newtonian dynamics as an alternative to the hidden mass hypothesis". Astrophysical Journal. 270: 365–370. Bibcode:1983ApJ...270..365M. doi:10.1086/161130.. Milgrom, M. (1983). "A modification of the Newtonian dynamics - Implications for galaxies". Astrophysical Journal. 270: 371–383. Bibcode:1983ApJ...270..371M. doi:10.1086/161131.. Milgrom, M. (1983). "A modification of the Newtonian dynamics - Implications for galaxy systems". Astrophysical Journal. 270: 384. Bibcode:1983ApJ...270..384M. doi:10.1086/161132..
  4. ^ Milgrom, Mordehai (February 2015). "MOND theory". Canadian Journal of Physics. 93 (2): 107–118. arXiv:1404.7661. Bibcode:2015CaJPh..93..107M. doi:10.1139/cjp-2014-0211. ISSN 0008-4204.
  5. ^ Katherine Brown; Harsh Mathur (22 September 2023). "Modified Newtonian Dynamics as an Alternative to the Planet Nine Hypothesis". The Astronomical Journal. 166 (4): 168 ff. arXiv:2304.00576. Bibcode:2023AJ....166..168B. doi:10.3847/1538-3881/acef1e.
  6. ^ a b McGaugh, S. (2015). "A Tale of Two Paradigms: the Mutual Incommensurability of LCDM and MOND". Canadian Journal of Physics. 93 (2): 250–259. arXiv:1404.7525. Bibcode:2015CaJPh..93..250M. doi:10.1139/cjp-2014-0203. S2CID 51822163.
  7. ^ Kroupa, P.; Pawlowski, M.; Milgrom, M. (2012). "The failures of the standard model of cosmology require a new paradigm". International Journal of Modern Physics. 21 (14): 1230003. arXiv:1301.3907. Bibcode:2012IJMPD..2130003K. doi:10.1142/S0218271812300030. S2CID 118461811.
  8. ^ See for example [1]
  9. ^ Mordehai, M. (2014) "The MOND paradigm of modified dynamics". Scholarpedia, 9(6):31410.
  10. ^ Hodson, A.O.; Zhao, H. (2017). "Generalizing MOND to explain the missing mass in galaxy clusters". Astronomy & Astrophysics. 598 (A127): 127. arXiv:1701.03369. Bibcode:2017A&A...598A.127H. doi:10.1051/0004-6361/201629358.