I’m back from an extraordinarily hectic yet unusually rewarding April Meeting of the American Physical Society in Dallas. The APS has two big meetings each year, the April meetings for very large- and small-scale types (particle physics, nuclear physics, gravitation, astrophysics), and the March meeting for medium-scale types (condensed matter, atomic physics, biophysics). The March meeting is a crucially important event for its constituency, while the April meeting suffers from too much competition and far less customer loyalty, and is correspondingly a much smaller conference (perhaps 1,000-1,500 attendees, as opposed to 6,000 at a typical March meeting). That’s a subject for another post, for those of you out there with an unhealthy interest in APS politics.

(For other reports from the meeting, see Jennifer Ouellette’s Cocktail Party Physics or the mysterious and anonymous Charm &c. Common refrain: “It’s 2006! Why isn’t there decent wireless in this hotel??!!”)

There’s a rule to the effect that any person can give no more than one invited talk at an APS meeting, but such rules are made to be broken and I sneaked in there with two talks. One was a general overview of the accelerating universe and its associated problems, at a special session on Research Talks Aimed at Undergraduates. Having a session devoted to undergrads was a splendid idea, although I suspect that the median age of attendees at my talk was something like 45. That’s because, when asked to pitch a talk to an audience of level of expertise x, most physicists will end up pitching it at a level of expertise x+3. So various people with Ph.D.’s concluded that their best chance of understanding a talk outside their specialty was to attend a session for undergraduates. Perhaps they were right. Before my talk they got to hear nice presentations by Florencia Canelli on particle physics and the top quark, and Paul Chaikin on packing ellipsoids. (Okay, “packing ellipsoids” doesn’t sound like the sexiest topic, but it was filled with fascinating tidbits of information. Did you know that both prolate and oblate ellipsoids pack more efficiently than spheres? That ordered crystalline packings are generally found to be more efficient than random packings, but nobody can prove it in general? That M&M’s are extremely reliable ellipsoids, to better than 0.1%? That the method by which the Mars Corporation makes their M&M’s so regular is a closely guarded secret?)

My other talk was at a joint double session on the past, present, and future of cosmology, co-sponsored by the Division of Astrophysics and the Forum for the History of Physics. Six talks naturally needed to be given: one each on the past/present/future of observational/theoretical cosmology, and organizer Virginia Trimble invited me to speak on The Future of Theoretical Cosmology. The observational session conflicted with my talk to the “undergrads,” but I got to hear the talks on the past and present of theory by Helge Kragh and David Spergel, respectively.

Of course nobody has any idea what the future of theoretical cosmology will be like, given that we know neither what future experiments will tell us, nor what ideas future theorists will come up with. So I defined “the future” to be “100 years from now,” by which time I figured (1) I won’t be around, or (2) if I am around it will be because we will all be living in pods and communicating via the Matrix, and nobody will be all that interested in what I said about the future of cosmology a century earlier.

With those caveats in mind, I did try to make some prognostications about how we will be thinking about three kinds of cosmological issues: composition questions, origins questions, and evolution questions. You can peek at my slides in html or pdf, although I confess that many were cannibalized from other talks. The abbreviated version:

• Composition Questions. We have an inventory of the universe consisting of approximately 4% ordinary matter, 22% dark matter, and 74% dark energy. But each of these components is mysterious: we don’t know what the dark matter or dark energy really are, nor why there is more matter than antimatter. My claim was that we will have completely understood these questions in 100 years. In each case, there is an active experimental program aimed at providing us with clues, so I’m optimistic that the matter will be closed long before then.
• Origins Questions. Where did the universe come from, and why do we find it in this particular configuration? Inflation, which received an important boost from the recent WMAP results, is a crucial ingredient in our current picture, but I stressed that there is a lot that we don’t yet understand. In particular, we need to understand the pre-inflationary universe to know whether inflation really provides a robust theory of initial conditions. Thinking about inflation naturally leads us to the multiverse, and I argued that untestable predictions of a theory are perfectly legitimate science, so long as the theory makes other testable predictions. We don’t yet have a theory of quantum gravity that does that, and I prevaricated about whether one hundred years would be sufficient time to establish one. (Naive extrapolation predicts that we won’t be doing Planck-scale experiments until two hundred years from now.)
• Evolution Questions. Given the initial conditions, we already understand the evolution of small perturbations up to the point where they become large (“nonlinear”). That’s when numerical simulations become crucial, and here I was a little more bold. The very idea of a computer simulation is only about 50 years old, so there’s every reason to expect that the way in which computers are used will look completely different 100 years from now. Quantum computers will be commonplace, and enable parallel processing of enormous power. More interestingly, the types of computation that we’ll be doing will be dramatically different; I suggested that the computers will not only be running simulations to test theories against observations, but will be coming up with theories themselves. Such a prospect is a natural outgrowth of the idea of genetic algorithms, so I don’t think it’s as crazy as it sounds.

The next day I managed to catch no fewer than three sessions filled with provocative talks — one on ultra-high-energy cosmic rays, one on cosmology and gravitational physics, and one on precision cosmology. And I would tell you all about them if I hadn’t lost the keys to my special time-stretching machine that allows me to put aside my day job for arbitrarily long periods so that I can blog at leisure. Probably the most intriguing suggestions were those by Shamit Kachru from SLAC, who argued that considerations from string theory (and in particular the constraint that scalar fields cannot evolve by amounts greater than the Planck scale) imply that gravitational waves produced by inflation will never be strong enough to be observable in the CMB, and those by David Saltzberg from UCLA, who listed an amazing variety of upcoming experiments to detect high-energy astrophysical neutrinos, including listening for sound waves (!) produced when a neutrino interacts with ocean water off the Bahamas. If I decide to become an experimentalist, that’s the one I’m joining.