Fundamental Physics and Large Scale Structure (2010)

Instead of adding another dark component to the energy budget of the Universe, one can ask whether the observed accelerated expansion might in fact be due to the behavior of gravity itself on the largest scales. In this talk I will focus on two popular modified gravity theories which realize this scenario: f(R) gravity and the DGP model. While these models yield an accelerated expansion, they also affect the formation of structure on much smaller scales. We have studied this with cosmological N-body simulations which consistently solve for the modified gravitational force. I will discuss the effects of modified gravity on dark matter halo properties as well as cosmological observables. For f(R) gravity, our first simulation-calibrated constraints from the observed abundance of massive clusters improve on previous constraints from the CMB and ISW by a factor of ~1000. This exemplifies the sensitivity of cosmological observables in the non-linear regime as probes of gravity.

Modified gravity theories under consideration typically reduce to a scalar-tensor form in the appropriate limits. I will discuss in what sense a universal scalar coupling is stable against quantum corrections, when the scalar equivalence principle is violated, how to look for such violations, and the connection with cosmic acceleration.

The statistics of strong lensing by galaxy clusters are sensitive both to cosmology and the detailed physics that determines the structure of halos. To exploit these sensitivites requires large and well defined samples of lenses on these mass scales. I will report on efforts to provide such samples - we finally now have uniformly selected samples of several hundred lenses to work with.