How Are We to Make Progress With w?

Oops, there I go again with the provocative "w" in my title. Once again, I feel compelled to tell you up front that this post is about dark energy, not "dubya". One of my primary research interests over the past six or seven years has been the question of what is driving the accelerated expansion of the universe. As I've mentioned before, cosmologists now have a clear and surprising accounting of the energy budget of the cosmos. Multiple techniques provide compelling evidence for (roughly) 5% baryonic matter (the stuff of which you are made, that makes up planets and stars, and which occupies much of the space between them), 25% dark matter (a mysterious, weakly interacting component that clumps together and provides the immense gravitational wells into which regular matter can form and combine with dark matter to make galaxies) and a whopping 70% dark energy, with negative pressure, sufficiently negative to cause the expansion of the universe to accelerate. The best-known evidence for this comes from two sources. The first is from observations of the light curves of Type Ia supernovae. These data are much better fit by a universe dominated by a some kind of dark energy than by a flat matter-dominated model. The second is from studies of the small anisotropies in the Cosmic Microwave Background Radiation (CMB), culminating in those made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. The statistical details of these fluctuations allow the extraction of a variety of cosmological parameters, including the relative densities of various cosmic energy components. As I described in a previous post, it is sometimes convenient to parameterize dark energy by what is called the equation of state parameter, w. This parameter tells us how, if we treat dark energy as a perfect fluid, the pressure is related to the energy density of the fluid. Current observational bounds place w in the range (very roughly) -0.8 to -1.2. Acceleration will occur for any w<-1/3. Last time I posted on dark energy, I described some of the theoretical challenges raised by the w<-1 region of the observationally allowed range. Here though, I would like to raise the issue of the experimental and observational challenges to making progress on the critical, but difficult question of what is responsible for cosmic acceleration. Theoretically, there are three broad classes of ideas about the nature of dark energy. 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. Which of these sets of ideas (if any) is correct can only be decided through increasingly accurate measurements of our universe and of the microphysical and macrophysical interactions that govern its behavior and evolution. And this is where the big question comes in. What are the best hopes for making significant observational and/or experimental progress in understanding cosmic acceleration? How do we best figure out if w is constant in space and time (a cosmological constant) or not (some kind of quintessence or modified gravity model)? If we manage this, how do we then make further progress towards narrowing down the possibilities and identifying the root of cosmic acceleration? The great news here is that we are not without ideas ("we" here is not meant to imply that I could build a modern experiment even if my life depended on it). On the "Outer Space" side of all this, there are plans for a number of space and ground based experiments to probe the dark energy through more accurate measurements of supernovae, through even better measurements of the CMB, and through detailed observations of the statistics of gravitational lensing. These are powerful techniques and hold great promise. On the "Inner Space" side, we will soon see the turn on of the Large Hadron Collider (LHC) at CERN, near Geneva. The world's biggest machine and particle smasher is expected to reveal new physics at previously inaccessible energies and, perhaps in conjunction with a future International Linear Collider (ILC), may reveal to us a new picture of how particle physics fits together with the physics of space and time. This connection may take the form of supersymmetry (SUSY) or extra spatial dimensions, but we won't know until we've done the experiments. It may be that the new physics discovered at these machines will provide us with a crucial new insight into dark energy. I've been deliberately sparse and sketchy in the last two paragraphs, mentioning only a few types of experiments and techniques. This is because I don't want to talk about too many experiments or their specifics, or my views of them right here. Rather, I would like to open up a discussion of the best ways of getting at the physics of cosmic acceleration. Given what we have established about the accelerating universe, what are the most promising ways of making progress? What are the pros and cons of existing and newly proposed ideas? Are there creative ways to get at the microphysical properties of dark energy? I'd love to hear some smart, thoughtful discussion of these issues, of the sort for which Cosmic Variance commenters are becoming known.