Future cosmology Nobels

I was asked the other day whether Alan Guth should expect to win the Nobel Prize for inflation, now that WMAP has found tentative evidence for a slight “tilt” in the primordial perturbations, just as we might expect from inflation. At the moment I’m leaning toward “not yet,” but it started me thinking about which cosmology discoveries have yet to be honored by Nobels but should be at some point. (After the 2004 prize for asymptotic freedom, there aren’t really any completely obvious particle-physics prizes lurking out there, although prizes for color, spontaneous symmetry breaking, and CP violation would be quite warranted.)

There are two discoveries that are obviously Nobel-worthy: the temperature anisotropies in the cosmic microwave background, and the acceleration of the universe. One that is a bit less obvious, but still extremely strong, is dark matter. In each case, however, it is not precisely clear which people should actually get the prize, given the constraints: (1) laureates should be individuals, not collaborations, (2) prizes are giving to living people, not posthumously, and (3) at most three people can share one prize.

The 1992 observation of CMB anisotropies by NASA’s COBE satellite was the first step in a revolution in how cosmology is done, one that has come to dominate a lot of current research. Subsequent measurements by other experiments have obviously led to great improvements in precision, and most importantly extended our understanding of the anisotropies to smaller length scales, but I think the initial finding deserves the Nobel. So to whom should the prize be awarded? On purely scientific grounds, it seems to me that there was an obvious three-way prize that should have been given a while ago, to David Wilkinson, John Mather, and George Smoot. Wilkinson was the grandfather of the project, and was the leading CMB experimentalist for decades. Mather was the Project Manager for the satellite itself (as well as the Principal Investigator for the FIRAS instrument that measured the blackbody spectrum), while Smoot was the PI for the DMR instrument that actually measured the anisotropies. Unfortunately, Wilkinson passed away in 2002. Another complicating factor is that there were various intra-collaboration squabbles, leading to books by both Smoot and Mather that weren’t always completely complimentary toward each other. Still, background noise like that shouldn’t get in the way of great science, and these guys definitely deserve the Nobel.

The first direct evidence for the accelerating universe came from two groups: the Supernova Cosmology Project and the High-Z Supernova Team. The issues of priority are a bit complicated, but both groups certainly deserve substantial credit in discovering this surprising and enormously influential result. The SCP is an easier case: they started first, and were clearly led by Saul Perlmutter, who is a shoo-in for the Nobel. The High-Z team was a bit more democratic, and started second but actually went on record first with the claim that the universe was accelerating. Their PI was Brian Schmidt (full disclosure: my old grad-school officemate); the first author on the discovery paper was Adam Riess; and their spiritual leader was Robert Kirshner (most of the team members were either students or postdocs of Bob’s at one point or another). Hard to construct a sensible prize from that mess, but if I were in charge of the universe I might give 50% of the prize to Perlmutter and 25% each to Schmidt and Riess, and feel really bad about not including Kirshner. But the discovery is clearly worthy of a Nobel, and I likely won’t complain with whatever way they choose to divvy up the award.

Then we get into murkier waters, I think. The idea of dark matter is one of the most influential and important in modern cosmology, and a Nobel would be perfectly appropriate. You might complain that we haven’t actually discovered dark matter yet, which is certainly true and relevant; but one way or another, something is going on with the dynamics of galaxies and clusters that is above and beyond what our current theories predict, and that empirical fact is hugely important. It was first pointed out by the late Fritz Zwicky in the 1930’s, comparing the velocities of galaxies in the Coma cluster to their total mass. But the field matured immensely with Vera Rubin‘s measurements of the rotation curves of spiral galaxies, giving direct evidence that the gravitational force fell off more slowly than the distribution of visible matter could account for. Rubin absolutely deserves the prize, in my opinion. Then there is the more specific cold dark matter idea, which is a specific model for the nature of dark matter and its role in galaxy formation; credit for that is more diffuse (although the paper by Blumenthal, Faber, Primack and Rees was obviously influential), and we’re less sure that the basic idea is right, so I don’t see any need for a prize there quite yet. I think it would be great to give a joint prize to Rubin and someone else, perhaps Wendy Freedman for measuring the Hubble constant, or Jim Peebles for developing physical cosmology.

Then we get to inflation, which is a sticky issue in various ways. There is absolutely no question that inflation has been one of the most, arguably the most, influential idea in cosmology in the last several decades. There is a great deal of discussion about who gets credit for it, since a number of papers discussed very similar-sounding ideas; but it was clearly Alan Guth‘s 1981 paper that put the story together in the right way. However, Guth’s model (“old inflation”) didn’t quite work, and the two follow-up papers by Andrei Linde and by Andreas Albrecht and Paul Steinhardt (“new inflation”) actually showed that the idea was plausible. That’s a total of four people, you’ll notice. Because Andy Albrecht was a graduate student at the time of his inflation paper with Steinhardt, and because both Linde and Steinhardt have gone on to write many more influential papers about inflation, credit is sometimes informally given to “Guth, Linde, Steinhardt and others…”, which is a little unfair.

But more importantly, we don’t know whether inflation is right. There is no question that it has made a number of predictions that have been dramatically verified: the universe is spatially flat, there is a spectrum of adiabatic and Gaussian primordial density perturbations, and that spectrum is nearly scale-free although not necessarily precisely so. And these predictions were by no means guaranteed in advance; models in which the perturbations were generated by cosmic strings, for example, were quite viable in the 1980’s, but have now been ruled out by CMB anisotropy observations.

Still, the idea that some non-inflationary mechanism set the initial conditions for the Big Bang still seems plausible to me, even if I don’t know what that mechanism would be. The predictions from inflation have been sharp, but they have not been the kinds of things that we couldn’t imagine getting from any other model. If we were to find evidence for gravitational-wave perturbations in the polarization of the CMB, of the type inflation could easily explain, then I might be convinced; but it’s quite possible that the gravity wave are really there but at a level too tiny to ever be observed.

So I’m somewhat torn. Inflation is a compelling and ingenious and influential idea, and it should be recognized. But the Nobel committee doesn’t like to give out prizes unless they’re completely sure that the discovery/theory to which they’re being given has no chance of being wrong. I’m not sure how to elevate inflation from the status of “probably on the right track” to the status of “correct beyond a reasonable doubt.” In the meantime, if the Nobel committee decides to take a risk and give Alan Guth the prize, you won’t hear any complaints from me.