As promised, I'm ready to provide a fuller report of the science at the meeting I was at last week - From Quantum to Cosmos: Fundamental Physics Research in Space, held by NASA at the Airlie Center in Warrenton, Virginia. I mentioned last time that I had seen some exciting talks by people involved in cold atom physics, which is quite far away from what I do, but which fascinated me. This time I wanted to report, albeit idiosyncratically, on discussions that were more closely related to my fields of expertise. In the first session of the conference, there were a number of talks on cosmology. The meeting was kicked off by a nice overview of the dark energy and dark matter problems by Orfeu Bertolami, from Lisbon, which led into my presentation on Cosmic Acceleration and Modified Gravity. You've heard me go on about this topic before, so let me move on to other people's talks. Right after me came someone I've known for a long time and who it was really nice to see again here - Joao Magueijo, from Imperial College. Joao is perhaps best known for his popular science book - Faster Than the Speed of Light, but here he gave an excellent presentation focused on how one might be able to perform a definitive test of MoND with a space-based solar-system experiment. Modified Newtonian Dynamics (MoND) is an innovative attempt, proposed in its original form by Mordechai Milgrom, to modify gravity to explain the phenomena usually attributed to dark matter. It is very difficult to do this by merely modifying the Newtonian force equation because, although one might manage to describe galactic rotation curves, the gravitational lensing signal will differ from that in dark matter models. However, recent years have seen a covariant version of MoND, proposed by Jacob Bekenstein and nicknamed TeVeS (Tensor, Vector, Scalar theory), in which lensing should work just fine. Although this model is not yet seen as an equal competitor for dark matter, it would be great to come up with a clear test through which it could be ruled out. Joao's claim (with Milgrom) is that if a form of MoND is correct, then one may be able to measure the behavior inside the solar system. As described in the abstract to their paper
... in particular near saddle points of the total gravitational potential. Whereas in Newtonian theory tidal stresses are finite at saddle points, they are expected to diverge in MOND, and to remain distinctly large inside a sizeable oblate ellipsoid around the saddle point. ... Space missions, such as the LISA Pathfinder, equipped with sensitive accelerometers, may be able to explore the larger perturbative region.
Certainly this seems like the kind of science that NASA should be doing. After Joao was HongSheng Zhao, from St. Andrews, who discussed Constraining Modified Gravity and then came my good friend Tanmay Vachaspati, under whom I was a postdoc at Case Western Reserve University, and with whom I split a rental car from Dulles. Tanmay's talk was on Light Superconducting Strings in Our Galaxy. I've mentioned topological defects before, so if you need a primer, I hope you'll look back and that it is helpful. Tanmay is interested in a particular variety - cosmic strings - line-like solutions that develop in certain field theories and which may have a variety of important cosmological consequences. In particular, in this talk, Tanmay was proposing that particle emission from cosmic strings at the TeV scale might provide an explanation for the 511keV photons observed in our galaxy. What is particularly interesting about this suggestion is that if this were the correct physics, then it would imply new physics at the TeV scale - for example an extended gauge group, such as a new U(1) symmetry - which might be accessible at the next generation of particle colliders (the LHC and the ILC). Since I'm focusing on cosmology in this post, I'll skip past great talks by Alan Kostelecky and Tom Weiler and jump right to the Monday evening session in which David Spergel gave a tight and crystal clear review of the WMAP3 results, and then Sean spoke on Cosmological Constraints on Modified Gravity. I will be deliberately sketchy about Sean's talk because most of it described work which will appear in a couple of papers, one of which we are writing together with our graduate students, within the next week. But a broader part of the talk concerned how one might differentiate between dark energy and modified gravity as explanations for cosmic acceleration. Sean discussed how, in modified gravity, Newtonian potentials can grow differently than in General Relativity (GR) (even sometimes at different rates on different scales), and how this might be used not only to distinguish it from dark energy, but to distinguish between different suggestions for modifying gravity. Tuesday was a big cosmology day, starting with thought-provoking talks from Alexander Kusenko (UCLA) on Detecting Dark Matter in Space, David Cline (also from UCLA) on The Future Search for Warm and Cold Dark Matter in Space, and my friend Katie Freese (University of Michigan) on Inflation after WMAP After other talks including Dark Energy Task Force presentations from Bob Cahn (LBNL), and Andy Albrecht (UC Davis), and a nice talk by Bruce Winstein (University of Chicago) on Scientific Targets for "CMBPOL", came the session I chaired, on Modified Gravity and Experiment First up was Gia Dvali from NYU, whose talk was titled Large Distance Modification of Gravity as Dark Energy Alternative. Gia, along with Gregory Gabadadze and Massimo Porrati, has developed an interesting extra dimensional model which has become known as "the DGP Braneworld". In this model, matter fields are confined to a four-dimensional submanifold and gravity is described by two different terms; one five-dimensional gravitational action and another four-dimensional action. As a result, gravitational physics looks four-dimensional at short distances and five-dimensional at large distances. The model has interesting cosmological consequences, including a way to explain the accelerated expansion (essentially through a modified four-dimensional Friedmann equation), as were described by our next speaker, Cedric Deffayet, from the IAP in Paris. These two intensely theoretical talks were then followed by two experimental talks. In the first, Eric Adelberger from the University of Washington discussed his precision tests of the inverse square law of gravity, and in the second, Ho Jung Paik from the University of Maryland discussed complementary measurements in space. Well, I've gone on long enough about the cosmology aspects of the meeting. I hope you agree that, on the scientific front, it was a pretty exciting and varied meeting. I'm hoping to find time to discuss the political parts a little later.
