The standard cosmological model is based on general relativity and includes dark matter and dark energy. An important prediction of this model is a fixed relationship between the gravitational potentials responsible for gravitational lensing and the matter overdensity. Alternative theories of gravity often make different predictions for this relationship. We propose a set of measurements which can test the lensing/matter relationship, thereby distinguishing between dark energy/matter models and models in which gravity differs from general relativity. Planned optical, infrared and radio galaxy and lensing surveys will be able to measure EG, an observational quantity whose expectation value is equal to the ratio of the Laplacian of the Newtonian potentials to the peculiar velocity divergence, to percent accuracy. We show that this will easily separate alternatives such as ΛCDM, DGP, TeVeS and f (R) gravity.

Future weak lensing measurements of cosmic shear will reach such high accuracy that second order effects in weak lensing modeling, like the influence of baryons on structure formation, become important. We use a controlled set of high-resolution cosmological simulations to quantify this effect by comparing pure N-body dark matter runs with corresponding hydrodynamical simulations, carried out both in non-radiative, and in dissipative form with cooling and star formation. In both hydrodynamical simulations, the clustering of the gas is suppressed while that of dark matter is boosted at scales k > 1 hMpc −1. Despite this counterbalance between dark matter and gas, the clustering of the total matter is suppressed by up to 1 percent at 1 k 10 hMpc −1 , while for k ≈ 20 hMpc −1 it is boosted, up to 2 percent in the non-radiative run and 10 percent in the run with star formation. The stellar mass formed in the latter is highly biased relative to the dark matter in the pure N-body simulation. Using our power spectrum measurements to predict the effect of baryons on the weak lensing signal at 100 < l < 10000, we find that baryons may change the lensing power spectrum by less than 0.5 percent at l < 1000, but by 1 to 10 percent at 1000 < l < 10000. The size of the effect exceeds the predicted accuracy of future lensing power spectrum measurements and will likely be detected. Precise determinations of cosmological parameters with weak lensing, and studies of small-scale fluctuations and clustering, therefore rely on properly including baryonic physics.

Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the Universe. Modified gravity theories have richer observational consequences for large scale structures than conventional dark energy models, in that different observables are not described by a single growth factor even in the linear regime. We examine the relationships between perturbations in the metric potentials, density and velocity fields, and discuss strategies for measuring them using gravitational lensing, galaxy cluster abundances, galaxy clustering/dynamics, and the integrated Sachs-Wolfe effect. We show how a broad class of gravity theories can be tested by combining these probes. A robust way to interpret observations is by constraining two key functions: the ratio of the two metric potentials, and the ratio of the gravitational ''constant'' in the Poisson equation to Newton's constant. We also discuss quasilinear effects that carry signatures of gravity, such as through induced three-point correlations. Clustering of dark energy can mimic features of modified gravity theories and thus confuse the search for distinct signatures of such theories. It can produce pressure perturbations and anisotropic stresses, which break the equality between the two metric potentials even in general relativity. With these two extra degrees of freedom, can a clustered dark energy model mimic modified gravity models in all observational tests? We show with specific examples that observational constraints on both the metric potentials and density perturbations can in principle distinguish modifications of gravity from dark energy models. We compare our result with other recent studies that have slightly different assumptions (and apparently contradictory conclusions). Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the Universe. Modified gravity theories have richer observational consequences for largescale structures than conventional dark energy models, in that different observables are not described by a single growth factor even in the linear regime. We examine the relationships between perturbations in the metric potentials, density and velocity fields, and discuss strategies for measuring them using gravitational lensing, galaxy cluster abundances, galaxy clustering/dynamics, and the integrated Sachs-Wolfe effect. We show how a broad class of gravity theories can be tested by combining these probes. A robust way to interpret observations is by constraining two key functions: the ratio of the two metric potentials, and the ratio of the gravitational ''constant'' in the Poisson equation to Newton's constant. We also discuss quasilinear effects that carry signatures of gravity, such as through induced three-point correlations. Clustering of dark energy can mimic features of modified gravity theories and thus confuse the search for distinct signatures of such theories. It can produce pressure perturbations and anisotropi...

The galaxy intrinsic alignment is a severe challenge to precision cosmic shear measurement. We propose to self-calibrate the induced gravitational shear-galaxy intrinsic ellipticity correlation (the GI correlation, Hirata & Seljak 2004) in weak lensing surveys with photometric redshift measurement.(1) We propose a method to extract the intrinsic ellipticity-galaxy density cross correlation (I-g) from the galaxy ellipticity-density measurement in the same redshift bin.(2) We also find a generic scaling relation to convert the extracted I-g correlation to the demanded GI correlation. We perform concept study under simplified conditions and demonstrate its capability to significantly reduce the GI contamination. We discuss the impact of various complexities on the two key ingredients of the self-calibration technique, namely the method to extract the I-g correlation and the scaling relation between the I-g and the GI correlation. We expect none of them is likely able to completely invalidate the proposed self-calibration technique.

The kinetic Sunyaev–Zel'dovich effect, which is the dominant cosmic microwave background (CMB) source at arcmin scales and ν∼ 217 GHz, probes the ionized gas peculiar momentum up to the epoch of reionization and is a sensitive measure of the reionization history. We ran high‐resolution self‐similar and ΛCDM hydro‐simulations and built an analytical model to study this effect. Our model reproduces the ΛCDM simulation results to several per cent accuracy, passes various tests against self‐similar simulations, and shows a wider range of applicability than previous analytical models. Our model in its continuous version is free of simulation limitations, such as a finite simulation box and finite resolution, and allows an accurate prediction of the kinetic SZ power spectrum, Cl. For the Wilkinson Microwave Anisotropy Probe cosmology, we find l2Cl/(2π) ≃ 0.91 × 10−12[(1 +zreion)/10]0.34(l/5000) 0.23‐0.015(italicz reion−9) for the reionization redshift 6 < zreion < 20 and 3000 < l < 9000. The corresponding temperature fluctuation is several μK at these ranges. The dependence of Cl on the reionization history allows an accurate measurement of the reionization epoch. For the Atacama Cosmology Telescope (ACT) experiment, Cl can be measured with ∼1 per cent accuracy. Cl scales as (Ωbh)2σ4∼68. Given cosmological parameters, ACT would be able to constrain zreion with several per cent accuracy. Some multireionization scenarios degenerate in the primary CMB temperature and temperature–E polarization (TE) measurement can be distinguished with ∼ 10 σ confidence.