This afternoon, I attended a talk at Texas Tech University by John W. Moffat, author of the 2008 book Reinventing Gravity. Moffat, a retired University of Toronto professor who more recently has been associated with the Perimeter Institute in Waterloo, Ontario, has proffered a modified theory of gravity (MOG) that, he claims, can explain a great deal of astrophysical data without having to resort to dark matter. I had seen Reinventing Gravity in the bookstores over the summer, but was unsure if I wanted to read it. Then, when the list of this semester's physics colloquium speakers came out during the first week of classes, I noticed Moffat's name and decided to get the book.
Before I discuss the book and the talk, some background. Dark matter has never been detected directly; it was proposed only via inference from astronomical observations. The rationale for invoking dark matter is conveyed in the following passage from the American Museum of Natural History on the 1970s-era observations of Vera Rubin and Kent Ford:
The stars far from the centers of galaxies, in the sparsely populated outer regions, were moving just as fast as those closer in. This was odd, because the visible mass of a galaxy does not have enough gravity to hold such rapidly moving stars in orbit. It followed that there had to be a tremendous amount of unseen matter in the outer regions of galaxies where the visible stars are relatively few.
As Moffat notes in his book, "To save Einstein's and Newton's theories [of gravity], many physicists and astronomers have postulated that there must exist a large amount of 'dark matter' in galaxies and also clusters of galaxies that could strengthen the pull of gravity and lead to an agreement of the theories with the data" (p. x). In Newton's law of gravity, of course, gravity's attractive force is proportional to the masses of the objects in question.
During today's talk, Moffat was soft-spoken, though spirited. He described himself as a "particle-buster," one who does not like to see the proliferation of new types of matter. Hence, as noted above, he has been developing his theory over the years, so that by modifying conventional theories of gravity, he can account for the kind of astrophysical data described above without resorting to dark matter. Moffat cautioned during the talk that, "I'm not saying dark matter doesn't exist. I'm someone who is intellectually curious..." and thus wants to see if alternative conceptualizations are possible.
Modified gravity appears to have two main revisions to conventional (i.e., Newtonian and Einstein's general relativity) gravitational theories. Moffat adds a fifth force of nature to the existing ones (gravitational, electromagnetic, weak nuclear, and strong nuclear), plus allows for variation in Newton's gravitational constant, G. "Together with the fifth force, this varying element strenthens the pull of gravity in faraway galaxies and in clusters of galaxies" (p. xiii).
Both in the book and in the talk, Moffat emphasized the considerable difficulty in successfully establishing a modified gravity theory. On pages 144-145 of the book, he lists nine major astrophysical findings with which any new theory must be consistent. With the older gravity theories, he noted in the talk, there were far fewer established findings to account for.
Any new theory also, of course, must make testable predictions that are different from the predictions that could be derived from other theories. Both in the book and in the talk, Moffat proposed two tests for MOG. One involves data from the Sloan Digital Sky Survey (SDSS), which currently appears to suffer from a sparseness of observations. In the figure caption on page 198, it says, "When more data points are available, either MOG or cold dark matter will be verified." The other possible test entails an experiment on board a space station (p. 191).
In the few years I've been attending physics colloquia at Texas Tech, this is the first time I've seen the room filled to capacity (probably around 60 people). Further, the question-and-answer part of the talk lasted longer than I recall ever seeing. The professor who introduced Moffat described him as an "unorthodox" thinker. In spite of this -- or perhaps because of it -- the audience seemed receptive to hearing Moffat's ideas, although it would also be fair to say that, during the Q&A, some skepticism toward them was also in evidence.
I definitely would recommend Reinventing Gravity. For readers interested in further background on dark matter (and dark energy, which Moffat also addresses in the book), I would in addition recommend Dan Hooper's 2006 book Dark Cosmos, which I previously reviewed here.