Tuesday marked our second "blogiversary" here at the Cocktail Party, and we pretty much missed it, because we were far too busy putting in our first full day as Journalist in Residence at the Kavli Institute of Theoretical Physics in Santa Barbara. It's an utterly gorgeous campus, with picture-perfect weather, prompting Jen-Luc Piquant to wonder how anyone ever gets any physics done whatsoever in such a setting, although the Spousal Unit assures us that yes, indeed, much excellent physics gets done here all the time. I've been excited about coming here for many months, for lots of reasons, not least of which is to finally figure out just what it is that the Spousal Unit does all day as a theoretical physicist. The job description appears to be something like, "Think really hard about big, difficult, questions involving complicated equations and bizarre counter-intuitive notions, then argue about your conclusions with your colleagues." Every now and then, someone writes it all down and publishes a paper summarizing the arguments to date. And everyone argues about that for awhile, more papers get published, and slowly some sort of theoretical consensus emerges.
If my first couple of days are any indication, this pretty much turns out to be the model at work at KITP, except there are cookies and fresh coffee every afternoon in the Common Room. I did not have the most auspicious start, thanks to a nasty bout with the flu over the weekend that hadn't quite worked its way out of my system by the time Tuesday morning rolled around. It's tough to be gregarious when your sinuses are so clogged, you feel like one of those cartoon giant-balloon-heads in the old Sudafed commercials. Nonetheless, I gamely tried to keep pace with that morning's speaker, Christof Wetterich, who had some thoughts about why the cosmological constant goes to zero.
Now, I have a passing familiarity with the history of the cosmological constant, a.k.a., lambda, a.k.a., "Einstein's Biggest Blunder." Quite probably most reasonably scientifically literate people have heard of it, especially since physicists discovered that the expansion of the universe is actually accelerating instead of, say, slowing down. It's one of the reasons the Spousal Unit (among many others) considers this to be a kind of golden age for cosmology -- or at least the equivalent of a "full employment act" for cosmologists. Heck, there was time not so long ago when we didn't even realize the universe was expanding; scientists assumed the cosmos was static and unchanging, and would pretty much run forever.
That was the state of things in 1917 when Albert Einstein put the finishing touches on general relativity. But there was a niggling problem: his calculations indicated that the universe should be expanding. To get the theory to fit in with prevailing scientific consensus, he introduced a mathematical "fudge factor": the cosmological constant. It implied the existence of a repulsive force pervading spacetime that counteracts the gravitational attraction holding the galaxies together. With the "push" and "pull" essentially balanced out, the universe would indeed be static.
Einstein probably should have trusted his mathematical instincts, because 12 years later Edwin Hubble was studying distant galaxies, noticed a pronounced "shift" to the red end of the electromagnetic spectrum, and reasoned that this cosmic Doppler effect could only occur if the light were traveling across space that was still expanding. That's why Einstein denounced lambda as his "greatest blunder." Except maybe it wasn't such a big blunder after all. Our universe still holds a few surprises. In 1998, two separate teams of physicists measured the change in the universe's expansion rate, using distant supernovae as mileposts. And they found that the universe's expansion is actually speeding up, thanks to the repulsive gravity of a mysterious thing called dark energy.
While it's not the only theoretical game in town, lambda is arguably the leading candidate for the dark energy. It's not a perfect explanation (it appears to be way too big, for starters), but quite a lot of very smart people are curious about the peculiar nature of this cosmological constant. I don't know why it's so critical to figure out why lambda goes to zero; honestly, I was under the impression it kinda fluctuated around zero. Or 1. Basically some tiny amount that is not quite zero, because that's the vacuum energy. Quantum physics holds that even the emptiest vacuum is still teeming with virtual particles that wink in and out of existence in an intricate quantum dance, so even a completely empty universe would still have some residual energy in these quantum fluctuations.
Wetterich is one of the folks who's been worrying away at the problem for quite some time now, and had some novel (to me, anyway) thoughts on the topic. At the risk of over-simplifying things, there's two basic approaches to the problem of dark energy: one that assumes it is, indeed, constant, i.e., static, and has been set since the birth of our universe, and another type of model ("quintessence") that assumes it is dynamical and changes over time. Most observations to date seem to support a static (or mostly static) dark energy.
Wetterich seems to uncomfortably straddle both schools of thought: namely, his model seems to assume that lambda used to be dynamical, and now it's mostly static. He's noticed an intriguing connection between dark matter density and neutrino properties, i.e., the present dark matter density is given by neutrino mass. As best I could figure out, in early cosmology, neutrino mass wasn't all that important (says Wetterich, per my scribbled notes, who claims it was still "relativistic" back then), but then neutrino mass began to grow, eventually getting so large that it became "non-relativistic," thereby constituting a kind of transitional "trigger event" marking the end of the "matter dominated period" and the beginning of accelerated expansion. And so we get the almost static dark energy that scientists believe permeates the cosmos today.
I think that's the gist of what he said. Things quickly went over my head, technically, with talk of cosmon couplings, (SU)3 symmetries, warped branes, even mention of a solution to Einstein-Maxwell theory in six dimensions, just for giggles. So I cannot say much about the merit of Wetterich's argument, except to say that many of the other physicists present were having none of it, and they weren't at all shy about saying so, which made for a hugely entertaining discussion, from my perspective. Allusions to "quantum fluctuations" got batted about as a major objection -- quantum fluctuations have a lot to answer for, if you ask me -- along with accusations that this was nothing more than a "self-tuning universe" argument in sheep's clothing.
I kept waiting for Wetterich to throw down the proverbial gauntlet and an outright brawl to ensue, perhaps to be resolved with pistols at sunrise the next day (there is a very pretty beach just outside and plenty of other scientists around to serve as seconds). But in the end, physicists are civilized sorts, and kept themselves to verbal fisticuffs. Nobody seems to have taken things personally. By the time cookies were served in the Common Room, everyone was best of friends again. Cookies and caffeine can go a long way to smoothing over ruffled feathers, I guess.
It's not all pitched academic battles over the finer points of modern cosmology, of course. There are forms to be filled out, parking permits to be grappled with, wireless connections to be made, and the handing out of the ceremonial coffee mug. But the heart of the institute is exactly the kinds of scientific exchanges that took place that first Tuesday morning -- maybe the debates aren't all quite as heated, but any decent theory needs to be able to withstand a rigorous pounding now and then. That's how science progresses. So it was kind of nice to be able to have a ringside seat and watch that progress in action -- a privilege I can relish for the next 12 weeks.