Does NGC 720 provide evidence for MOND?
An alternative theory to dark matter has successfully predicted the rotational properties of two elliptical galaxies. The work was done in Israel by Mordehai Milgrom using the modified Newtonian dynamics (MOND) theory that he first developed nearly 30 years ago. By showing that MOND can be used to explain the properties of complicated elliptical galaxies – as well the much simpler spiral galaxies – Milgrom argues that MOND offers a viable alternative to dark matter when it comes to explaining the bizarre properties of galaxies.
Dark matter was proposed in 1933 to explain why galaxies in certain clusters move faster than would be possible if they contained only the “baryonic” matter that we can see. A few decades later, similar behaviour was detected in individual galaxies, whereby the rotational velocity of the outermost stars was found not to “drop off” as a function of distance but instead remain flat. These observations directly contradicted Newtonian gravity, which should hold true in extragalactic regions just as it does on Earth and in the solar system. But by assuming there are “haloes” of invisible matter in and around galactic structures, Newton’s familiar inverse square law is restored.
Since it was first invoked to explain these galactic irregularities, physicists have tried to make direct measurements on dark matter to try to work out exactly what it is – with very little success. As a result, there are some researchers who do not believe that dark matter exists and have proposed alternative explanations for the strange behaviour of galaxies.
Spectacular success - Now a new analysis suggests that one alternative theory called MOND describes the properties of two elliptical galaxies just as well as dark matter. MOND was originally formulated to describe spiral galaxies and has had spectacular success in predicting certain properties of these structures. Its extension to cover elliptical galaxies could strengthen the arguments in favour of this alternative theory. This is because elliptical galaxies are predicted to have formed by a different process from spiral galaxies and their properties are much more difficult to calculate. More here Dark-matter alternative tackles elliptical galaxies