In a couple of my blogs I have referred to the ongoing debate in cosmology between proponents of the Dark Matter hypothesis and proponents of the MOND hypothesis. There are many facets and wrinkles to this issue, including those who say it is a matter of semantics. For example, What is modified gravity and how to differentiate it from particle dark matter?, Calmet and Kuntz, arXiv: 1702.03832 v.1 [gr-qc] 10 Feb 2017.
These authors argue that from a theory point of view both the Dark Matter (DM) hypothesis and the modified gravity hypotheses, such as MOND, can be treated as equivalent theories by redefining the energy-momentum tensor in general relativity.
To me it is more than just the flip side of the same coin. At issue is the following dilemma:
Dark Matter – the law of gravity is not obeyed at the outer fringes of galaxies (the evidence is accepted by just about everybody) if the mass of galaxies is determined by the observed matter in stars and gas. Accepting that the law of gravity holds (Newton’s plus Einstein’s) there must be unobserved matter to account for the evidence. Hence the term, Dark Matter.
MOND: The law of gravity is not obeyed at the outer fringes of galaxies if the mass of galaxies is determined by the observed matter in stars and gas. Therefore the law of gravity must be modified (Newton’s and Einstein’s) . Hence the term, Modified Gravity, including theory of Modified Newtonian Dynamics (MOND).
There is ample proof that there is a divide between physicists on how to approach this dilemma. For example, in response to one of my earlier blogs: Archeology and Evolution of ….Paradigms in Physics, Professor Tippet, of the University of British Columbia, wrote in Facebook taking issue with my support of MOND and citing the common arguments that are used to support the DM paradigm. His main point in that discussion was that, in holding to the MOND hypothesis, I was falling into the same trap that I was writing about in my blog i.e., of physicists continuing to support ideas even after ample evidence has been gathered to put the idea in question to rest.
Professor Tippet did summarize some of the evidence that exists, viewed from the point of those who argue that there must be DM. He made little or no reference to the evidence that can be raised against the DM paradigm. He pointed out the data that was starting to mount from “the late 90’s” (actually much before that ). As he pointed out:
“…this data dictates that dark matter must be distributed in spherical shape around the galaxy, extending farther out beyond it; and astronomical surveys suggest that there must be 10X more dark matter (by mass) than luminous matter. The current theory is that dark matter is a type of elementary particle that doesn’t interact with the fields and forces of regular matter (it’s called a Weakly Interacting Massive Particle WIMP, and we’ve never detected one, though we are looking).” (emphasis mine)
There is also a whole suite of data that can be used to cast doubt on the DM paradigm which was not referred to by Professor Tippet and often seems to be simply ignored by many DM cosmologists and particle physicists.
To begin with I usually make reference to ArXiv.org, a repository of physics papers which have not yet all been peered reviewed and which is managed by Cornell University. I use this as my primary source to keep up to date on topics that interest me because it is free, online, and very current.
In any day’s articles in the section on High Energy Physics there will normally be a number of proposals for Dark Matter particles. For example, in the replacements section for Friday, Feb.14, 2017 there is: Simplified models of dark matter with a long lived co-annihilation partner, Khoza, Placencia and Sakurai: arXiv:1702.00750 v.2.
On this particular day the number of DM suggestions was particularly light. Usually there are more papers. This is all well and good, except it highlights that speculative theories about DM and experimental searches for it have become an industry in Physics that probably absorbs millions of dollars in research time and detection experiments (including searches at the Large Hadron Collider). What strikes me is the comparative paucity of research related to the highly successful phenomenology of MOND and efforts to finding a fundamental theoretical basis for it.
The notion that the Dark Matter paradigm is almost an embedded belief system in present day cosmology and particle physics is reinforced quite extensively, provocatively and humorously in a blog of Stacey McGaugh: The MOND Pages.
Most recently McGaugh and collaborators have found from a study of a very large sample of galaxies that supports the notion of a single underlying rule for the rotation data without the need to introduce DM: One Law to Rule Them All: The Radial Acceleration Relation of Galaxies, Lelli, McGaugh, Schombert, and Pawlowski, arXiv:1610.0898 v.1
In this paper, McGaugh, et.al. find even more extensive support for a modification of the laws governing galactic physics and especially for new physics that can incorporate the ideas inspired by Milgrom’s MOND. Specifically they are able to write a single relation (Eqn. 11 of that paper) which, with only one free parameter is able to fit the observed rotation curves of a the very large number of galaxies studied. This is to be compared with the common DM paradigm which requires fitting of many parameters for each galaxy studied. There is no existing theory from which the formula (11) can be derived – but that is the point that motivates advocates of a search for a more fundamental theory.
Yet another physicist, Pavel Kroupa, has commented on the “dogma” aspect of DM in an article in AEON: Has dogma derailed the scientific search for dark matter?
In this article Kroupa points to some of the data that actually contradicts the DM hypothesis and also makes the case that, in words, “…suggest a deeper fundamental theory of space, time and matter which has not yet been formulated.”
What I find even more striking in Kroupa’s comments his claim that there is a prevailing bias against the search for an alternative to DM. He writes “…[b]ecause of the prevailing dark matter dogma, few scientists dare to build on Milgrom’s ideas. Young researchers risk not getting a job; senior researchers face losing out on grants.”
Such comments coming from a researcher on the front lines of astrophysics, in my opinion, point to the conflict of ideas that exists between the Dark Matter hypothesis and Modified Gravity approaches – ideas that are certainly more than a matter of semantics.