This Huxleyan vision of clean refutation buttresses one of our worst stereotypes about science. We tend to view science as a truth-seeking machine, driven by two forces that winnow error: the new discovery and the crucial experiment — prime generators of those nasty, ugly little facts. Science does, of course, seek truth, and even succeeds reasonably often, so far as we can tell. But science, like all of life, is filled with rich and complex ambiguity. The path to truth is rarely straight marked by a gate of entry that sorts applicants by such relatively simple criteria as age and height….
– Stephen Jay Gould, Eight Little Piggies
Dark matter is one of the most frustrating things in the Universe — at least for those of us who make it our life to study the Universe. Its presence is pervasive, it shapes galaxies, galaxy clusters, and the structure of the cosmos itself. Yet we don’t know what it is, thanks to the fact that it’s entirely invisible: light passes through it, and if it interacts with ordinary matter at all, that interaction is subtle at best. Our experiments to detect dark matter particles directly have either failed or produced ambiguous results.
So, it’s no wonder that some scientists have looked to alternative explanations for the same phenomena that dark matter produces. The OG of that movement is Mordehai Milgrom, who developed a relatively simple modification to Newtonian gravitational dynamics called (wait for it) MOdified Newtonian Dynamics, or MOND. Specifically, MOND was designed as a way to explain the rotation of spiral galaxies without the need for dark matter, a task it’s very good at. Proponents of MOND are a small but very vocal community in the astronomy and astrophysics community.
However, MOND has consistently failed to account for the other phenomena for which dark matter is the standard explanation.
- Galaxy clusters, despite their name, are not mostly made of galaxies: most of the mass is X-ray-emitting hot gas and … something else. That something else is what most astronomers call dark matter. MOND requires something like dark matter (extra heavy neutrinos, for example) to explain galaxy cluster dynamics, obviating the original motivation for the theory.
- Galaxies and galaxy clusters aren’t arranged randomly. Instead, they clump together, form along filaments, and make very large structures. This large-scale structure of the Universe is described well by a model containing dark matter, but not by MOND.
- As I’ve written about extensively, the strongest evidence for dark matter may be from the cosmic microwave background (CMB), the radiation left over from when the Universe became transparent. (Try these posts: “Our weird and wonderful Universe” and “C is for cosmic microwave background“.) The fluctuations in the CMB allow us to put precise numbers on the major components of the Universe, including the amount of stuff that doesn’t correspond to atoms and other ordinary matter. The image above is a breakdown of the CMB spectrum; the third peak of that spectrum tells us that about 80% of the mass of the Universe is dark matter. MOND is a non-relativistic theory of gravity, so it’s unable to cope with cosmology, which requires something akin to general relativity. That isn’t to say there can’t be a relativistic version of MOND, a point I’ll get back to momentarily.
Arguably, the rotation of spiral galaxies is the least compelling evidence for dark matter (even though historically it’s the first to gain traction in the astronomy community), simply because galaxies are messy places. We don’t understand all the dynamics of gas and stars within galaxies, so it’s hard to pinpoint exactly where the dark matter is and how it behaves. However, I don’t want to overstate that problem: many astrophysicists are working hard on galactic dynamics, and most of them haven’t thrown up their hands over dark matter.
Over the years, several attempts have been made to generalize MOND into a relativistic theory of gravity. The one that caught a lot of eyes when I was in graduate school was TeVeS (which stands for Tensor Vector Scalar, not one of the better theory names out there). TeVeS failed to reproduce the cosmic microwave background as it’s observed, and besides which is a baroque and difficult theory to work with — and I say this as someone who happily worked with baroque and difficult theories back in the day. Arguably, aesthetics are a bad way to judge a theory, but it would suggest an untoward maliciousness on the part of the designer of the Universe. (That’s a joke, for use by physicists only.)
The latest of the relativistic MOND analogs comes out of the University of Salford in England, and was published in Physical Review D. I haven’t had much time to spend with the paper: that sort of thing is important to do properly, and as a full-time writer who needs to eat, I’m not sure I have it in me. (Oh, for a graduate student!) However, my main concern is that the authors are only concerned in the paper with reproducing MOND, which as we’ve already seen isn’t adequate as a dark matter replacement. I’m also a little leery of the central premise, in which our physical Universe is embedded in an abstract higher-dimensional mathematical space, which I’d like to see motivated by something other than “we can get MOND out of it”.
That’s not to say we should dismiss the theory outright. Since it’s relativistic, this new model could conceivably overcome the problems in MOND; I’d like to see a galaxy cluster calculation and early-Universe models for the CMB, as others did for TeVeS. (I’d also like to see a full parameterized post-Newtonain [PPN] analysis.) I’m actually sympathetic to alternative gravity models, though my interests are more in trying to solve dark energy than for dark matter. It’s worth a look, not least since this theory is a lot simpler than TeVeS.
The paper vs. the PR
However — and this is a huge however — the theory’s authors’ piece on The Conversation uses extremely problematic language, from the title on. Reproducing MOND in a relativistic context says nothing about dark matter, either way, yet the piece says that this theory alone “casts doubt” on dark matter’s existence. That to me is a troubling misunderstanding of how science works. A theory, any theory, is only worth something if it tells us something about the Universe. The higher-dimensional relativistic MOND theory may do that, or it may not. This theory reproduces MOND, but so far that’s all it can claim.
Dark matter is frustrating, but the models containing it are extremely successful, from (yes) spiral galaxies up to the structure of the Universe. I will not say (until we discover a dark matter particle) that no modified gravity scheme will ever work. Truly elegant theoretical solutions to difficult problems have appeared in the past, but they’re rare. For any alternative model to “cast doubt” on dark matter’s existence, it would need to mimic dark matter across a wide array of problems in astrophysics and cosmology. Dark matter as a model, for all its challenges, has an advantage of explaining all those disparate phenomena, including those that were unknown when the model was first proposed.