Do Elliptical Galaxies Reveal an Alternative to Dark Matter?




An alternative theory to dark matter developed in 1983 by the astrophysicist Mordehai Milgrom of the Weizman institute in Israel, called the modified Newtonian dynamics (MOND) has successfully predicted the rotational properties of two elliptical galaxies formed by a different process from spiral galaxies. According to the new analysis, MOND describes the properties of two elliptical galaxies as well as dark matter. MOND was originally formulated to describe spiral galaxies and has had singular success in predicting certain properties of these structures.

In the new research, Milgrom analyses the rotation curves of two elliptical galaxies and shows that the predictions of MOND –that under certain circumstances the gravitational force between two bodies decays more gently than the inverse square of the distance between them–are equally valid. This is important, Milgrom argues, because elliptical galaxies are thought to have evolved in a completely different way from spiral galaxies and other disc galaxies – they are thought to be formed by the collision and merging of two other galaxies. MOND's success, he argues, means that its predictive accuracy cannot simply be a coincidence and that it must hint at a deeper underlying truth.

Milgrom extends his alternative theory to significantly undermining the dark-matter hypothesis. "In the dark-matter picture" he says, "the galaxies we see today are the end result of very complicated and very haphazard formation processes. You start with small galaxies – they merge, they collide – there are explosions in the galaxies and so on and so forth. During this stormy evolution the dark matter and the normal matter are subject to these processes in very different ways and so you really do not expect to see any real correlations between the dark matter and the normal matter. This is a very weak point of the dark-matter picture."

"I have found it to be the case for quite some time now that MOND does a very good job of explaining the dynamics of galaxies," says particle astrophysicist and dark-matter expert Dan Hooper of Fermilab. "And this paper is yet another example of where MOND succeeds at the galactic scale. Where MOND fails is on larger scales such as in clusters of galaxies and on even larger cosmological scales.

The image at the top of the page shows the center of an elliptical galaxies that has been found to emit unexpectedly high amounts of blue and ultraviolet light. Most blue light from spiral galaxies originates from massive young hot stars, in contrast to the red light from the old cool stars thought to compose ellipticals.

In this false-color photograph by the Hubble Space Telescope, the center of nearby dwarf elliptical M32 has actually been resolved and does indeed show thousands of bright blue stars. The answer is probably that these blue stars are also old and glow blue, reaching relatively high temperatures by the advanced process of fusing helium, rather than hydrogen, in their cores. M32 appears in many pictures as the companion galaxy to the massive Andromeda Galaxy (M31).

The Daily Galaxy via and Physical Review Letters.

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