Cosmology

Dark matter, constituting a fifth of the mass-energy in the Universe today, is one of the major "known unknowns" in physics. A number of different experimental and observational techniques exist to try to identify dark matter. However, these techniques are not only sensitive to the "physics" of dark matter (mass, cross sections, and the theory in which the dark matter particles live) but to the "astrophysics" of dark matter as well, namely the phase-space density of dark matter throughout the Milky Way and other galaxies and its evolution through cosmic time. In order to accurately map signals in experiments or observations to the particle-physics properties of dark matter, we need to understand the astrophysics of dark matter. In this talk, I will demonstrate how to get robust constraints on the particle-physics properties of dark matter either by careful modeling the astrophysics properties of dark matter or by elevating the astrophysics properties of dark matter as something to be extracted from future data sets alongside particle-physics parameters, and which approach (modeling vs. empirical) is more useful for given problems. As an example, I will show which aspects of the local dark-matter phase-space density can be understood through modeling and which aspects may be possible to infer empirically, and what the implications are for determining the particle-physics of dark matter from direct and indirect detection.

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.

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.

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.

I will discuss an alternative to inflation based on a Galileon field. The model starts in a (contracting or expanding) quasi Minkowski phase and all the energy of the Universe in generated suddenly in a sort of Genesis associated with a strong violation of the Null Energy Condition. The symmetries of the model force any additional scalar field to acquire a scale invariant spectrum of perturbations.