Physics on the Perimeter

Extremely early this morning, I returned from a visit to the Perimeter Institute, the theoretical physics center in Waterloo, Canada, founded by Mike Lazaridis, Co-CEO of Research In Motion (RIM) - makers of your Blackberry. I had spent the last day at PI, delivering the institute colloquium yesterday afternoon, with a talk titled "Challenges of Explaining Cosmic Acceleration through Modified Gravity". Perimeter is a wonderful place, with an increasing number of researchers devoted to physics issues of "foundational" importance. These topics include Quantum Gravity (with members working on causal sets, loop quantum gravity and string theory); Quantum Information Theory, Cosmology, Quantum Foundations and Particle Theory. These people are housed in a beautiful new building that, as well as being aesthetically pleasing (to me, at least), is also a tremendous intellectual playground, with collaborative spaces around every corner, equipped with espresso machines. Arriving in Waterloo the night before, I graded exams over dinner and a couple of beers before getting an early night so that I could get into PI reasonably early in the morning. This gave me plenty of time to get administrative details out of the way, settle into my office, and then meet up with postdocs Claudia de Rham, Andrew Tolley and Mark Wyman for coffee and a lengthy physics chat. We talked about what each of us was up to and then spent some time discussing the ghost states and strong coupling regimes of various modified gravity models. These are interesting questions, to do with some of the pathological problems that often arise when General Relativity is altered to try to explain cosmic acceleration. My host, Cliff Burgess, grabbed me for lunch at noon, and we spent an hour or so talking about string theory, quantum gravity, the sociology of these fields, and generally chatting about life at PI. This gave me a half hour to look over my talk before the colloquium at 2pm. Colloquium at the Perimeter Institute is a lively event. Certainly the faculty, at least, are happy to press and probe with many questions. My talk consisted of a broad introduction to the problem of cosmic acceleration, followed by an outline of the general issues presented by modifying gravity to account for this phenomenon. I used some specific examples to show how solar system constraints constrain certain models, and the appearance of ghost states renders other ones unworkable. I also talked about how we may exploit some loopholes in these constraints to arrive at viable modified gravity models. In the last part of the colloquium, I moved on to the question of the types of observations that might help distinguish modified gravity models from dark energy, or a cosmological constant, as competing explanations for cosmic acceleration - discussing comparisons between, for example, Cosmic Microwave Background measurements and large scale structure observations. Even with the many great questions, I managed to finish roughly on time, tired, but having had a thoroughly enjoyable time during the talk. I always find speaking to an audience exhausting, and this was no exception. But here Perimeter's ubiquitous espresso came in particularly useful. With the drug fresh in my system, Cliff and I spend a half hour in his office talking about the approaches he and his collaborators have to dark Energy in some string inspired models, before I met up with Lee Smolin to talk about approaches to related questions arising from loop quantum gravity. Later in the afternoon I got together with my former colleague and friend Rafael Sorkin. Rafael is a remarkable guy - an experienced relativist with unique ideas about the right way to construct a quantum theory of gravity from causal sets. Some questions about my talk, an update on his progress in teasing out the physics of causal sets, and a sketch of Rafael's other interests in the foundations of quantum theory took me all way up to when I had to leave and meet up with my hosts in the Black Hole Bistro (yes - really). There my visit wound up with dinner, where I got to spend time with, among others, someone I've known for a long time - and even wrote a paper with once - the theoretical physicist Slava Mukhanov. Slava is visiting PI for a few months - he's currently giving a cosmology mini-course there - and as well as being an accomplished physicists, is a hilarious storyteller - which made dinner wonderful fun. I had spent an extremely long and full day at PI, punctuated with fascinating meeting after fascinating meeting. My drive home passed quickly partly because I found myself mulling over a number of the ideas people had mentioned to me during the day. This is one of the great things about traveling as an academic - the exposure to different ideas in an informal context (not like reading papers) in which you can get lots of insight and instant feedback to your questions. Nice to be home though!