MOND versus Dark Matter

Dr. Mordehai Milgrom is a well-known physicist at the Weizmann Institute of Science, Department of Physics and Astrophysics in Israel. In Astrophysics he is best known for originating MOND theory (Modified Newtonian Dynamics) in 1983. His discovery is that Newton’s Law of Gravity and hence, General Relativity also, must be modified at low accelerations, a0 ≤ 10 -8 cm/s2.

The reason for introducing this modification is that at the outer limits of most galaxies studied to date (particularly spiral and disc galaxies) the velocities of stars are too great to be bound by the galaxies, assuming the total mass of any galaxy is given by the “observed” matter in the galaxy. The observed matter in galaxies is deduced from the light emitted from the galaxies and is generally dubbed “baryonic matter”. Baryon is the generic name for particles like neutrons, protons and their excited states seen in nuclear reactions. Matter that we observe in our own immediate environment (our solar system, for example) is baryonic.

This led to the idea that there must be missing mass that we do not see in most galaxies, in order to bind these distant stars to any given galaxy. This “missing mass” has been known to be a problem for quite a long time. For example, observations by Zwicky, 1933 provided early evidence of anomalous rotation velocities, and coined the term Dark Matter.

The evidence for “missing mass” to account for these orbital velocities of stars in galaxies has grown and is accepted as fact. The result has led to the search for what has been dubbed “Dark Matter” (a misnomer – Invisible Matter would make more sense). Dark Matter, because it has not been observed in any other way other than this gravitational effect in galaxies and clusters of galaxies, is taken to be different from ordinary or baryonic matter that we are familiar with.

Milgrom’s MOND provides an alternative way of explaining and dealing with the velocity rotation curves observed in most galaxies, without invoking the need for Dark Matter.

It should be noted that there are also a wide variety of theories of Modified Gravity (MOG), which seek to deal with the “Dark Matter” problem as well as the “Dark Energy” problem. E.g. Brownstein, Moffat, arxiv:astro-ph/0702146

Milgrom’s idea is striking in it’s simplicity and the remarkable success it has had in predicting not only the details of observed galaxy rotation curves, but also for predicting other observed galaxy properties, such as the Tulley- Fisher relation.

By contrast, to explain galaxy rotation curves using Dark Matter (DM) requires a lot of detailed fitting of parameters related to the distribution and hypothetical properties and behavior of DM – and there are numerous inconsistencies in the predictions from models using DM, such as the number and distribution of satellite galaxies around major galaxies.

Milgrom’s MOND involves modifying gravitation theory, which, among other things involves modifying General Relativity, and with that probably modifying some of our theoretical notions of space-time.

There are a number of theorists who have approached MOND with the goal of providing a general theoretical background for MOND to make it “fit” better into the Standard Model. E.g. Blanchet, LeTiec, arxiv:0807.1200v.1

Notwithstanding these efforts, it is still remarkable that Milgrom’s approach and its successes are all too often simply ignored by other physicists, particularly Particle Physicists, who operate only on the assumption that there is Dark Matter and that an extensive search for this “missing matter” is warranted.

The search for Dark Matter has consumed a lot of time and energy of physicists involved in Astrophysics and Particle Physics. For example, research at the Large Hadron Collider (LHC) at CERN (Geneva, Switzerland) includes searches for Dark Matter. E.g. http://www.Iss.fnal.gov

As another example, in a brief paper (F. Wiliczek, Theory Vision, LHCP 2016), Dr. Wiliczek offers his vision for the future of particle physics. He makes reference to a number of Dark Matter searches being conducted in many different centers, including, of course, reference to his own Dark Matter candidate – the axion. He makes no reference at all to the alternative models for DM including no mention at all of the most successful alternative explanation for DM, namely MOND.

It seems to me it should be a major task of theorists in astrophysics and particle physics to come up with theories to explain the significance or reason for Milgrom’s “a0 “ as well as a number of other numerical relationships (coincidences?) that have been pointed out by Milgrom.

McGaugh versus Milgrom???

In the midst of all this background about MOND I was quite surprised to read two recent papers that suggest a conflict between two of the most consistent supporters of MOND.

One is: McGaugh, Lelli and Schombert, The Radial Acceleration Relation in Rotationally Supported Galaxies, (arXiv:1609.05917 v.1) and the other is: Milgrom, MOND impact on and of the recently updated mass-discrepancy-acceleration relation, (arXiv:1609.06642v.3)

In past works McGaugh and collaborators have written papers analyzing galaxy data and have continually found strong support for Milgrom’s MOND. In this most recent paper they examine an extensive sample of galaxies from which they derive what they claim is a new and evidently universal relation to describe radial acceleration in galaxies. The only reference to MOND is a single reference to Milgrom’s original paper (Milgrom, Astrophys.J. 270 , 371 (1983))

Milgrom’s paper is in response to that of McGaugh et.al. And criticizes their paper as not having any new or original theory and that the relation they claim to have discovered was predicted and can be derived from the original MOND. He is also critical of what he calls a passing reference to MOND in their paper. He states: “(McGaugh, et.al.) gives only vague lip-service, in passing, to the influence of, and relevance of all this to MOND.“

As well, comments made by Milgrom in his Summary and Discussion implies a rift between him and McGaugh. I find this published display of antipathy unusual to say the least. This, especially so, since most of the work by McGaugh and other collaborators has been strongly supportive of MOND. As well in any of the papers I have read, including conference proceedings McGaugh has been strongly supportive of Milgrom.

I will be most interested in finding out what underlies this apparent dissension.

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