## Cosmology

### Galaxy Infall Kinematics as a Test of Modified Gravity

Infrared modifications of General Relativity (GR) can be revealed by comparing the mass of galaxy clusters estimated from weak lensing to that from infall kinematics. We measure the 2D galaxy velocity distribution in the cluster infall region by applying the galaxy infall kinematics (GIK) model developed by Zu and Weinberg (2013) to two suites of f(R) and Galileon modified gravity simulations.

Despite having distinct screening mechanisms, the f(R) and Galileon clusters exhibit very similar deviations in their GIK profiles from GR (Figure on the left). In combination with the stacked weak lensing measurements, this will provide powerful diagnostics of modified gravity theories and the origin of cosmic acceleration.

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### Redshift-Space Cluster-Galaxy Cross-Correlation: I. Modeling Galaxy Infall onto Millennium Simulation Clusters and SDSS Groups

We develop and test a method to recover galaxy infall kinematics (GIK) from measurements of the redshift-space cluster-galaxy cross-correlation function xi_cg, by calibrating an analytic model of the galaxy kinematic profiles comprised of a virialized component and an infall component. We show that convolving the real-space cross-correlation function with this velocity distribution accurately predicts the redshift-space xi_cg, and that measurements of xi_cg can be inverted to provide diagnostics of cluster mass profiles.

As a proof of concept we measure xi_cg for rich galaxy groups in the Sloan Digital Sky Survey and recover GIK profiles for groups in bins of central galaxy stellar mass. The figure on the left compares the best-fit (dashed contours) to the measurement of xi_cg (color contours) for one of the bins.

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### Cosmological Constraints from the Large Scale Weak Lensing of SDSS MaxBCG Clusters

We derive constraints on the matter density Omega_m and the amplitude of matter clustering sigma_8 from measurements of large scale weak lensing by clusters in the SDSS MaxBCG catalog. The weak lensing signal is proportional to the product of Omega_m and the cluster-mass correlation function xi_cm, breaking the degeneracies between cosmological and nuisance parameters.

We find sigma_8(Omega_m/0.325)^0.501=0.828 +/- 0.049 (beige contours) and the constraint is consistent with and orthogonal to the one inferred from WMAP CMB data (red contours), reflecting agreement with the structure growth predicted by GR for an LCDM cosmological model. A joint constraint assuming LCDM yields Omega_m=0.298 +/- 0.020 and sigma_8=0.831 +/- 0.020 (blue contours).

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