Meetings Small and Large

The end of classes here in Syracuse was last Tuesday, and I was able to assign grades for my Relativity and Cosmology course rather quickly, opening up the possibility of settling into my summer schedule a week earlier than I had planned. This master plan started off just fine, with progress on a paper that is about to appear (right Sean?), catching up with my graduate students, organizing space for the three undergraduates working with me for the summer, and dealing with overdue referee reports. On Monday, we had a very focused one-day workshop in collaboration with some of our colleagues from Cornell. The topic was Observational Tests of Modified Gravity, and the cosmologists here - myself, Cristian Armendariz-Picon, Levon Pogosian and students, were joined by Rachel Bean, Eanna Flanagan and students from Cornell. In addition, official FOS (Friend of Syracuse) Moshe Rozali, from the University of British Columbia, was around, and the day's seminar speaker, Phil Mannheim, from the University of Connecticut, also participated. As I've mentioned before, there are three broad classes of approaches to explaining the present-day acceleration of the universe. The first is that cosmic acceleration may be due to a pure cosmological constant. The second assumes that the true vacuum energy of the universe vanishes, and that the dynamics of some exotic matter component, such as a scalar field, might be driving acceleration. This approach is like having a version of cosmic inflation happen in the late universe, and goes by the name quintessence. The final approach is again to set the vacuum energy to zero, and then to have a long-range modification of General Relativity be responsible for cosmic acceleration. At various times I've worked on aspects of each of these approaches, but most recently the modified gravity one, with lots of collaborators, but originally with Sean, Vikram Duvvuri and Michael Turner. An interesting question about these approaches is the extent to which one might be able to perform cosmological observations to distinguish them from other proposed explanations, such as a cosmological constant or quintessence. This question was our primary concern on Monday. We had a highly productive meeting, with sessions led by a couple of people who would lay out the main issues of a given topic, and then a lively discussion of the outstanding issues and directions forward. We discussed various approaches to modified gravity, including those on which Sean and I have been working. Another idea we discussed in detail is that of Gia Dvali and collaborators, who have an extra-dimensional brane-world model, known as DGP gravity, in which gravity becomes five dimensional at very large distances. We talked about geometric tests and contrasted these with dynamical measures, coming from structure formation. In the middle of the day we stopped for Phil Mannheim's High Energy/Relativity/Cosmology seminar on Conformal Gravity, which fit into our general theme anyway, and them went right back to our workshop. Finally, we covered the Integrated Sachs-Wolfe effect, which is something I'm actively thinking about, while I still have our local expert - Levon - around, and mostly tried to at least start a discussion about what is generic and what is model-specific in modified gravity models. I thought the workshop was very successful, both in detail - I learned a lot - and for the fact that it keeps our existing and growing collaborative efforts with Cornell strong and healthy. For this I have Levon to thank, since the workshop was his idea and he put in all the organizational work. We will miss him greatly when he takes up a faculty job at Simon Fraser University in the Fall (what was I thinking writing him a good letter?). So things were going fine until yesterday. I was supposed to take an early flight to deliver one of those fun plenary talks I often give at particle physics conferences, in which I get to give an overview of particle physics connections to baryogenesis, dark matter and dark energy. However, unfortunately I ended up having to cancel due to a couple of things that I won't bother going into here (for fear of boring everyone). Right now I'm working from home and probably will do for the rest of the day, while I - glamorous jet-setter that I am - wait for a plumber. This weekend is our graduation ceremony, and I shall be marching at the Arts and Sciences ceremony and celebrating with our graduating physics majors back at the department. Then, assuming no more plumbing disasters, summer will really start.