The universe is not only expanding; recent cosmological experiments confirm that its expansion is accelerating. Qualitatively, this is not surprising. What puzzles us is that the rate of acceleration is dozens of orders of magnitude smaller than predicted by otherwise well-tested theories such as the standard model of particle physics. This is known as the cosmological constant problem.
String theory predicts an enormous universe containing diverse regions, each larger than the observed universe. Some regions will have the right amount of the "dark energy" to account for the above observations. Thus, what we think of as physical laws and constants of nature may be only one among many possibilities realized in what is now called the "string theory landscape".
Predictions in this framework will be statistical. This has caused some panic, since in cosmology we don't get to repeat experiments. In fact it just means that we need to focus on predictions that can be made with probability close to one. Many predictions we make in physics, and all that we make in thermodynamics, already are of this type.
My own research is currently focused on how cosmological selection effects help allow us to make predictions and, ultimately, to test the landscape. In landscape models, the big bang is merely the most recent in a sequence of dramatic phase transitions. The primordial cosmological dynamics will introduce a bias---some regions of the landscape will be abundantly explored, others never visited.
We have recently demonstrated that effects of this type can lead to sharp predictions in toy models of the string landscape. The goal of further work is to extend these results, and to apply our methods to increasingly realistic models.
R. Bousso, R. Harnik, G. Kribs, and G. Perez: "Predicting the Cosmological Constant from the Causal Entropic Principle", e-Print: hep-th/0702115
R. Bousso, “Holographic Probabilities in Eternal Inflation,” Phys. Rev. Lett. 97, 191302 (2006). e-Print: hep-th/0605263
R. Bousso, “The Holographic Principle,” Rev.Mod.Phys.74:825-874 (2002). e-Print: hep-th/0203101.
R. Bousso and J. Polchinski, “Quantization Of Four Form Fluxes And Dynamical Neutralization Of The Cosmological Constant,” JHEP 0006:006 (2000). e-Print: hep-th/0004134.
R. Bousso, “A Covariant Entropy Conjecture,” JHEP 9907:004 (1999). e-Print: hep-th/9905177.