Global InformationModified Newtonian dynamics information
Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's second law to account for observed properties of galaxies. It is supported by a minority of astrophysicists as an alternative to the more widely accepted hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics.[1]
Created in 1982 and first published in 1983 by Israeli physicist Mordehai Milgrom,[2] the hypothesis' original motivation was to explain why the velocities of stars in galaxies were observed to be larger than expected based on Newtonian mechanics. Milgrom noted that this discrepancy could be resolved if the gravitational force experienced by a star in the outer regions of a galaxy was proportional to the square of its centripetal acceleration (as opposed to the centripetal acceleration itself, as in Newton's second law) or alternatively, if gravitational force varied inversely with the radius directly (as opposed to with the inverse square of the radius, as in Newton's law of gravity). MOND departs from Newton's laws at extremely small accelerations (for instance, far from any sources of gravitational field). Such low accelerations are characteristic of the outer regions of galaxies, or the inter-galaxy forces within galaxy clusters, but are far below anything encountered in the Solar System or on Earth.[3][dubious ]
- 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?
MOND is an example of a class of theories known as modified gravity, and is an alternative to the hypothesis that the dynamics of galaxies are determined by massive, invisible dark matter halos. Since Milgrom's original proposal, proponents of MOND have claimed to successfully predict a variety of galactic phenomena that they state are difficult to understand as consequences of dark matter.[4][5]
Though MOND explains the anomalously great rotational velocities of galaxies at their perimeters, it does not fully explain the velocity dispersions of individual galaxies within galaxy clusters. MOND reduces the discrepancy between the velocity dispersions and clusters' observed missing baryonic mass from a factor of around 10 to a factor of about 2. However, the residual discrepancy cannot be accounted for by MOND, requiring that other explanations close the gap such as the presence of as-yet undetected missing baryonic matter.[6]
The accurate measurement of the speed of gravitational waves compared to the speed of light in 2017 ruled out a certain class of modified gravity theories but concluded that other MOND theories that dispense with the need for dark matter remained viable.[7] Two years later, theories put forth by Constantinos Skordis and Tom Zlosnik were consistent with gravitational waves that always travel at the speed of light. Later still in 2021, Skordis and Zlosnik developed a subclass of their theory called "RMOND", for "relativistic MOND", which had "been shown to reproduce in great detail the main observations in cosmology, including the cosmic-microwave-background power spectrum, and the matter structure power spectrum."[6][8]
- ^ Hossenfelder, Sabine; McGaugh, Stacy S. (2018-07-17). "Is Dark Matter Real?". Scientific American. 319 (2): 36–43. Bibcode:2018SciAm.319b..36H. doi:10.1038/scientificamerican0818-36. ISSN 0036-8733. PMID 30020902.
- ^ Milgrom, M. (1983). "A modification of the Newtonian dynamics as a possible 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..
- ^ 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.
- ^ 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.
- ^ 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.
- ^ a b Mordehai, M. (2014) "The MOND paradigm of modified dynamics". Scholarpedia, 9(6):31410.
- ^ Oran, Sibel; Desai, Santana; Kaya, Emre; Woodard, Richard (2018). "GW170817 Falsifies Dark Matter Emulators". Physical Review D. 97 (4): 041501. arXiv:1710.06168. Bibcode:2018PhRvD..97d1501B. doi:10.1103/PhysRevD.97.041501. S2CID 119468128.
- ^ Constantinos Skordis & Tom Złośnik (2021). "New Relativistic Theory for Modified Newtonian Dynamics". Physical Review Letters. 127 (16): 161302. arXiv:2007.00082. Bibcode:2021PhRvL.127p1302S. doi:10.1103/PhysRevLett.127.161302. PMID 34723619. S2CID 220281053.