Alternatives To Dark Matter
There are a number of attempts to solve the problem of galaxy rotation curves without invoking dark matter.
One of the most discussed alternatives is MOND (Modified Newtonian Dynamics), originally proposed by Mordehai Milgrom as a phenomenological explanation back in 1983 but which has been seen to have predictive power in accounting for galaxy rotation curves. This posits that the physics of gravity changes at large scale but, until recently, was not a relativistic theory. However, this changed with the development by Jacob Bekenstein of the tensor–vector–scalar gravity (TeVeS) theory, enabling gravitational lensing to be covered by the theory.
Another, similar, alternative is the relativistic modified gravity (MOG) theory, also called scalar–tensor–vector gravity (STVG), of John Moffat. Brownstein and Moffat applied MOG and MOND to the question of galaxy rotation curves, and demonstrated excellent fits to a large sample of over 100 low surface brightness (LSB), high surface brightness (HSB) and dwarf galaxies. Each galaxy rotation curve fit was made without dark matter, using only the available photometric data (stellar matter and visible gas) and a two-parameter mass distribution model which made no assumption regarding the mass to light ratio. The MOG results were compared to MOND and were nearly indistinguishable right out to the edge of the rotation curve data, where MOND predicts a forever flat rotation curve, but MOG predicts an eventual return to the familiar inverse-square gravitational force law.
Although these alternatives are not considered by the astronomical community to be as convincing as the dark matter model, gravitational lensing studies have been proposed as the means to separate the predictions of the different theories. Indeed, gravitational lensing by the Bullet Cluster was reported to provide the best current evidence for the nature of dark matter and to provide "evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)" as applied to large galactic clusters. In retort, Milgrom, the original proposer of MOND, posted an rejoinder online that claims MOND correctly accounts for the dynamics of galaxies outside of galaxy clusters, and removes the need for most dark matter in clusters, leaving twice as much matter as is visible, which Milgrom expects to be simply unseen ordinary matter rather than exotic cold dark matter.
Some Quantum Gravity theories also give alternative explanations, see alternative theories to dark matter.
Read more about this topic: Galaxy Rotation Curve
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