Dark matters

My first response to Tuesday’s announcement that this year’s Nobel Prize would be shared among scientists who measured the acceleration of the universe’s expansion, was surprise. It’s a young discovery, only made in 1998; usually when someone wins a Nobel Prize it’s for work done several decades previously. What was so surprising to me, was that the committee seemed to have leapfrogged over the discovery of dark matter to honor the discovery of dark energy.

In a nutshell, Perlmutter, Schmidt and Riess won for discovering that the universe is accelerating while it’s expanding, totally contrary to expectations that held that because of gravity, the universe’s expansion should be slowing down. No question that this is Nobel Prize worthy research, as it totally shook the foundations of cosmology when it was announced. Soon after the discovery the term “dark energy” was coined, and it was calculated to make up three quarters of the known universe.

But what about this other “dark” stuff that’s out there; dark matter, the idea that most of the matter in the universe is invisible? Why hasn’t anyone who’s worked on that won a Nobel Prize? It’s been a well accepted theory for decades, much longer than dark energy which was only announced in the late 1990s.

The person most associated with laying the groundwork for the theory is Vera Rubin at the Carnegie Institution of Washington. In the 1960s and 1970s, Rubin and her collaborator Kent Ford analyzed the rotation of galaxies and found some startling anomalies. The outer edges of galaxies should be rotating around at much slower speeds than the central region, but Rubin and Ford found that the galaxies were rotating around at more or less the same speed at all points.

Needless to say this was really weird. Predicting how a galaxy should rotate should have been easy; one just needs to estimate how much mass is in the galaxy based on the visible stars, plot the distances of stars from the center of gravity and plug in those factors into orbital equations that have been around since the time of Johannes Kepler. However Rubin and Ford’s observations defied all of the models about galaxy rotation. Clearly something else was going on, but what? They dubbed it the galaxy rotation problem.

Several theories have come forward to explain the galaxy rotation problem. By far the most popular is that there’s a kind of matter, dubbed “dark matter” out in the universe that simply does not emit or reflect light or any other kind of electromagnetic radiation. This matter is often referred to as Weakly Interacting Massive Particles or WIMPs. So far dark matter has corresponded with just about all of the observational results, not just aligning with the rotation of galaxies, but also explaining how galaxies cluster together and how light bends in parts of space where there doesn’t seem to be anything there.

Scientists seem tantalizingly close to actually directly detecting these exotic particles, if they are in fact there. Multiple detectors around the world are looking for any sign of WIMPs. The premise for most of these detectors is that if there is dark matter out there, it’s also got to be here on Earth too. If a particle of dark matter smacks into a particle of normal matter, the normal matter should recoil ever so slightly, but enough we can see it. So far, there’s been the occasional blip that looks promising, but not at a consistent rate that can really be called a discovery.

So maybe when a WIMP is pinned down for sure and a new particle of matter is discovered then Rubin will get her prize? Well maybe. This year’s laureates didn’t win for discovering “dark energy” and certainly not for understanding what it is (we’re way far away from that); just the observation that the universe is accelerating. I’m betting that when someone can figure out exactly why it’s doing that, that’s another Nobel Prize waiting in the wings.

This in a way is why I find this reason for the Prize Committee passing over Rubin unsatisfying. It seems a bit inconsistent to insist on waiting to understand the “why” of dark matter but not waiting to understand the “why” of dark energy. On the face of it, she made observations of the movements of galaxies and found huge inconsistencies in existing theory long before Perlmutter, Schmidt and Riess made their, except they have a Nobel Prize and she doesn’t.

There is another possible reason. While the majority of physicists are pretty convinced that these wimpy WIMPs are out there somewhere, not all are totally sold on the idea of some new exotic kind of matter. Of those that think something else might be the cause most think that the answer lays in readjusting the equations that govern gravity over great distances. Those pursuing this line of inquiry often call their hypothesis Modified Newtonian Dynamics or MoND. However this line of inquiry is pretty controversial, there are a lot of people doing real serious work on it, but the field also attracts a lot of cranks. More often than not, if you’re talking to someone inside the physics community if you say you’re working on MoND, eyes start to roll and glaze over.

And that may fundamentally be the reason that Vera Rubin has been passed over for the Nobel Prize so many times. She doesn’t seem to be a fan of the theory of dark matter. In a 2005 article in New Scientist she said “If I could have my pick, I would like to learn that Newton’s laws must be modified in order to correctly describe gravitational interactions at large distances… That’s more appealing than a universe filled with a new kind of sub-nuclear particle.”

So only the Nobel Committee really knows why Vera Rubin has been passed over for so long. Maybe they’re waiting on the detection of a WIMP, or maybe it’s because she doesn’t think they’re actually out there. Maybe the committee doesn’t want to award two prizes, one for the theory and one for the detection, or maybe the committee just plain doesn’t think her work is important enough to recognize with the most prestigious award in science. Who really knows what goes on in the mind of the committee…

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