Michael Turner, a cosmologist at the University of Chicago, sees the contrast more starkly. "MOND was a clever idea, but we are pretty far down the path in cosmology now, and the simpler solution is to assume that there are just particles out there that don't give off light." Turner says he will always keep an open mind to interesting new theories, but the successes of dark matter are just too hard to ignore. "It's like you're at a baseball game, and it's the ninth inning, and dark matter has a 14-run lead over MOND, which isn't even on base yet. The game may not be over, but people are heading for the parking lot."
Turner's analogy drives home the role that human resources play in developing a theory. For whatever reason, MOND had remained a wallflower at the astrophysics ball. It was hard to find dance partners, physicists willing to devote the long hours to figuring out the gritty mathematical details by which the theory would live or die. String theory, for example, is an exceedingly abstract piece of work, but because of its perceived promise, armies of talented young physicists are drawn to it. MOND had no such fan base.
To Turner, this makes perfect sense. "As a theorist you see that with a really good idea, you put a quarter in and you get 10 dollars out, and people should flock to your idea," he says. "I just have not seen that with MOND."
With only a few diehards working on MOND, progress was necessarily slow. Peebles applauds the existence of MOND as an alternative to dark matter, but he points out its recruitment problems. "With so much success happening with dark matter and cosmology, if you were a young scientist, would you bet your career on MOND?" In the face of such pressure, it was left to the old guard to find a way forward.
On March 25, 2004, a paper from Physical Review Letters D appeared on the Los Alamos preprint server, a Web site where physicists post their newest articles. The paper, titled "Relativistic Gravitation Theory for the MOND Paradigm," was written by Jacob Bekenstein, Milgrom's collaborator since the 1980s. Building on earlier attempts, Bekenstein was finally able to generate a MOND theory that Einstein might have loved. The new theory was called TeVeS, an acronym for tensor, vector, and scalar—mathematical terms that describe how matter and energy interact with space and time in general relativity.
"TeVeS does everything," says Mario Livio with enthusiasm. A self-described agnostic in the MOND debate, but one with an obvious love for the underdog, Livio says that Bekenstein's work is "a phenomenal paper." In TeVeS, he adds, all the right things happen. Its results mesh with what physicists know about gravity from Einstein, and when gravity is very weak, it reduces to the behavior Milgrom envisioned in his first MOND papers. "With Bekenstein's theory we should now be able to explore all aspects of relativistic behavior," Milgrom says, unable to hide his pride. "This includes the bending of light by gravity, and in principle, the new theory should be OK for galaxy formation."
TeVeS may not be the final word on making MOND jibe with relativity, but it is a critical first step. "Perhaps there are more beautiful and elegant forms of the theory out there," Livio says. What matters most is that now the contest between MOND and dark matter can really get started. "With MOND you could not do cosmology," he says, "but you can do it with TeVeS."
TeVeS has already passed a crucial first test, Milgrom says, because it can be used to explain the phenomenon of gravitational lensing, a cosmic optical illusion in which matter bends light. It's one of the stranger predictions of general relativity, and astronomers have confirmed it many times over. When observing a distant galaxy, for example, massive objects between Earth and the galaxy act like a giant lens and bend the galaxy's light, creating multiple images of the single galaxy. Dark-matter models have been used to explain the lensing of many distant galaxies. If Milgrom can convince his colleagues that TeVeS does it just as well, it's a whole new ball game, and maybe some of the fans will start returning to their seats.
Papers comparing TeVeS and dark matter are just beginning to appear, and it is far too soon to say how the new work will ultimately be judged. While TeVeS and all the other MOND approaches may fall into the trash bin of scientific history, the same fate may await dark matter. Expensive and elaborate searches for dark-matter particles have come up empty-handed. If those ongoing hunts continue to fail, and if the momentum for MOND-inspired theories continues to build, the tide may turn, and the entire elaborate cosmological scaffolding may be rocked from its foundations. It's a startling thought that dark matter, now such an integral if mysterious part of the modern picture of the universe, might simply vanish overnight. That wouldn't be heresy. That would be a revolution.