1809.09019
No evidence for modifications of gravity from galaxy motions on cosmological scales
He, Guzzo, Li, Baugh
The recent discovery of gravitational waves marks the culmination of a sequence of successful tests of GR since its formulation in 1915. Yet these tests remain confined to the scale of stellar systems or the strong gravity regime. A departure from GR on larger, cosmological scales has been advocated by the proponents of modified gravity theories as an alternative to the Cosmological Constant to account for the observed cosmic expansion history. While indistinguishable in these terms by construction, such models on the other hand yield distinct values for the linear growth rate of density perturbations and, as a consequence, for the associated galaxy peculiar velocity field. Measurements of the resulting anisotropy of galaxy clustering, when spectroscopic redshifts are used to derive distances, have thus been proposed as a powerful probe of the validity of GR on cosmological scales. However, despite significant effort in modeling such redshift space distortions, systematic errors remain comparable to current statistical uncertainties. Here, present the results of a different forward modeling approach, which fully exploits the sensitivity of the galaxy velocity field to modifications of GR. Use state-of-the-art, high-resolution N-body sims of a standard GR and a compelling f(R) model, one of GR's simplest variants, to build simulated catalogues of stellar-mass-selected galaxies through a robust match to the SDSS observations. Find that, well within the uncertainty of this technique, f(R) fails to reproduce the observed z-space clustering on scales 1-10 Mpc/h. Instead, the standard LCDM GR model agrees impressively well with the data. This results provides a strong confirmation, on cosmological scales, of the robustness of Einstein's GR.
1809.09148
Cosmology from cosmic shear power spectra with Subaru Hyper SUprime-Cam first-year data
Hikage, Oguri, Hamana, More, Mandelbaum, Takada, Köhlinger, Miyatake, et al
Measure cosmic WL shear power spectra with HSC survey first-year shear catalog covering 137 deg^2 of the sky. Thanks to the high effective galaxy number density of 16.5 arcmin^{-2} even after conservative cuts such as i<24.5 and photometric z cut of 0.3 <= z <= 1.5, obtain a high significance measurement of the cosmic shear power spectra in 4 tomographic z bins, achieving a total S/N of 16 in the multipole range 300 <= ell <= 1900. Carefully account for various uncertainties in the analysis including the intrinsic alignment of galaxies, scatters and biases in photometric z's, residual uncertainties in the shear measurement, and modeling of the matter PS. The accuracy of the PS measurement method as well as the analytic model of the covariance matrix are tested against realistic mock shear catalogs. For a flat LCDM model, find S_8 == sigma_8 (Omega_m/0.3)^alpha = 0.800+0.029-0.028 for alpha = 0.45 (S_8=0.780+0.030-0.033 for alpha=0.5) from the HSC tomographic cosmic shear analysis alone. In comparison with Planck CMB constraints, the results prefer slightly lower values of S_8, although metrics such as the Bayesian evidence ratio test do not show significant evidence for discordance between these results. Study the effect of possible additional systematic errors that are unaccounted in the fiducial cosmic shear analysis, and find that they can shift the best-fit values of S_8 by up to ~0.6 sigma. The full HSC survey data will contain several times more area, and will lead to significantly improved cosmological constraints.
1809.09540
The effects of calibration on the bias of shear measurements
Gillis, Taylor
Forthcoming large-scale surveys will soon attempt to measure cosmic shear to an unprecedented level of accuracy, requiring a similarly high level of accuracy in the shear measurements of galaxies. Factors such as pixelisation, imperfect PSF correction, and pixel noise can all directly or indirectly lead to biases in shear measurements, and so it can be necessary for shear measurement methods to be calibrated against internal, external, or simulated data to minimize bias. It is thus important to understand the nature of this calibration. In this paper, show that a typical calibration procedure will on average leave no residual additive bias, but will leave a residual multiplicative bias. Additionally, the errors on the post-calibration bias parameters will be changed, and on average increased, from the errors on the pre-calibration measurements of these parameters, but that this is generally worth the benefit in decreasing the expected value of the multiplicative bias. Find that in most typical cases, it is worthwhile to apply a first-order bias correction, while a higher-order bias correction is only worthwhile for methods with intrinsically high multiplicative bias (>10%) or when the simulation size is very small (<1e6 simulated galaxies).
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