Our recent article on physicist Mordehai Milgrom described how his novel theory of gravity, called MOND, might revolutionize cosmology by doing away with dark matter—hypothetical, unseen mass believed to sculpt the structure of the universe. Here, in a new forum, experts in the field weigh in on Milgrom's controversial ideas.
In spite of Mordehai Milgrom's ingenious idea and Jacob Bekenstein's impressive work on MOND, my money is still on dark matter. First, there are several very plausible candidates for dark matter, in the form of elementary particles whose existence has been predicted. Second, there is a plethora of self-consistency checks (for example, through gravitational lensing, clusters of galaxies, and the general cold, dark matter scenario). Finally, the landscape has significantly changed since Milgrom's original suggestion. We now know that the universe is accelerating, seemingly propelled by a dark energy that makes up 74 percent of the cosmic energy density. MOND and its latest version, TeVeS, have nothing to say about that. I find it dissatisfying intellectually to say that we need a new theory of gravity, junking all the evidence for dark matter, only to find that we need yet another dark component.
Mario Livio Senior Scientist Space Telescope Science Institute
The elegant TeVeS framework developed by Jacob Bekenstein has certainly breathed new life into MOND and for the first time enables direct tests in a cosmological context. Still, cold dark matter is remarkably successful in explaining observations on scales from galaxies to the cosmic microwave background; the burden remains with MOND to demonstrate that it can provide a viable alternative on all scales. An important start would be to reconcile MOND with existing direct evidence for dark matter. For instance, there is a supersonic galaxy cluster merger (known as the bullet cluster) in which the mass of the smaller cluster, as traced by gravitational lensing, is offset from the hot cluster gas. This offset is easy to explain with dark matter—the gas is slowed by drag as it moves through the "atmosphere" of the more massive cluster, while dark matter sails through unimpeded. How to explain the offset with MOND is less obvious.
Anthony Gonzalez Assistant Professor University of Florida
There used to be a folklore, propagated by dark-matter advocates, that because MOND lacks a relativistic version, it should be discarded as bad physics. TeVeS, my relativistic formulation of MOND, pulled the rug from under that claim. Livio complains that TeVeS does not explain dark energy. It actually was not designed to explain dark matter or dark energy. Yet two cosmology groups found that TeVeS gives an account of the fluctuations in the cosmic microwave background, a phenomenon trumpeted as proof of the existence of dark matter. We may not need dark matter after all. And TeVeS, or some of its variants being now developed, might surprise us again and explain the phenomenon that passes for dark energy.
Jacob D. Bekenstein Polak Professor of Theoretical Physics Hebrew University of Jerusalem
It is always good to have alternatives to explore when dealing with physics that is still speculative. However, cold dark matter has been a remarkably successful and simple idea that is natural within the context of particle physics. It is hard to find an extension beyond the standard model of particle physics that does not produce some viable dark-matter candidate. Moreover, as often happens with correct ideas, the predictions arising from the existence of cold dark matter about things like large-scale structure formation agree with observations, even though it once looked like preliminary data might rule out this idea. Finally, assuming cold dark matter exists seems less radical than assuming a change in gravity. It is easy to make things dark. Even the cosmic microwave background, the source of most photons in the universe, remained invisible until 1965. All you need is another background from the early universe with particles that interact more weakly, and you're there.
Lawrence Krauss Ambrose Swasey Professor of Physics and Astronomy Director, Center for Education and Research in Cosmology and Astrophysics Case Western Reserve University
Let us not lose sight of the main point: MOND has predicted with uncanny precision a vast amount of data for over a hundred galaxies. It is inconceivable that the dark-matter doctrine will ever achieve this, since its expectations for galaxy behavior are a result of the unpredictable physics of galaxy formation, evolution, and interaction. The MOND predictions are precise and unavoidable. Even among the few predictions of dark matter, there are several that conflict with observations of galaxies. Yes, MOND is not yet a perfect and complete theory, and yes, it still suffers from some (curable) childhood diseases. But on the whole, MOND outperforms dark matter. It is a cleaner, more economic idea and, most important, has a far higher predictive power in the realm of the galaxies. Mind you, incompleteness characterizes all the physical theories we use today, from quantum theory through the standard model of particle physics to general relativity; none is universally applicable, and all are awaiting extension and improvement.
Mordehai Milgrom Professor, Department of Condensed Matter Physics Weizmann Institute