Cosmology

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.

How can we rule out whole classes of dark energy models? And what quantities, at what redshift, and with what accuracy, should be measured in order to rule out these classes of models? I present answers to these questions by discussing an approach that utilizes the principal component parametrization of dark energy. I show results based on current data, and future forecasted data from SNAP and Planck.

We show that the existence of the bullet cluster, 1E0657-56, is incompatible with the prediction of the standard Lambda CDM cosmology. The probability of finding the large infall velocity (3000 km/s) necessary for explaining the X-ray and weak lensing data of 1E0657-56 is between 3.3x10^{-11} and 3.6x10^{-9}. The existence of the bullet cluster poses a serious challenge to LCDM cosmology, unless a lower infall velocity solution for 1E0657-56 with <1800 km/s is found.

The Atacama Cosmology Telescope (ACT) has recently measured the CMB angular power spectrum from maps with arcminute resolution at 148 GHz. By fitting to a template for the SZ angular power spectrum, we constrain the model's amplitude A_SZ < 1.63 (95% confidence level) and the amplitude of matter perturbations sigma_8 < 0.86 (95% CL). In this talk, we review the fiducial template and present additional templates for the SZ angular power spectrum based on different models for the hot gas in dark matter halos. We also discuss how the TSZ, KSZ, and SZ power spectra scale with sigma_8.

X-ray surveys and deep Chandra observations recently provided ~10% accurate measurements of the total mass in nearly 100 galaxy clusters at z=0-0.9. These data clearly show the effect of Dark Energy on slow-down of the structure growth at z<1. THe combination of the structure growth measurements with other cosmological observations substantially improves the constraints on the Dark Energy equation of state parameter. More advanced applications include constraints on the deviations from General Relativity on 10-100 Mpc scales. Will the situation improve in the future when samples of 10^3-10^5 clusters are available? I will review the current ''bottlenecks'', and discuss possible strategies for using future cluster data for ''precision cosmology''.