Carlos Garcia-Garcia

Abstract:

An accurate calibration of the source redshift distribution p(z) is a key aspect in the analysis of cosmic shear data. This, one way or another, requires the use of spectroscopic or high-quality photometric samples. However, the difficulty to obtain colour-complete spectroscopic samples matching the depth of weak lensing catalogs means that the analyses of different cosmic shear datasets often use the same samples for redshift calibration. This introduces a source of statistical and systematic uncertainty that is highly correlated across different weak lensing datasets, and which must be accurately characterised and propagated in order to obtain robust cosmological constraints from their combination. In this paper we introduce a method to quantify and propagate the uncertainties on the source redshift distribution in two different surveys sharing the same calibrating sample. The method is based on an approximate analytical marginalisation of the p(z) statistical uncertainties and the correlated marginalisation of residual systematics. We apply this method to the combined analysis of cosmic shear data from the DESY1 data release and the HSC-DR1 data, using the COSMOS 30-band catalog as a common redshift calibration sample. We find that, although there is significant correlation in the uncertainties on the redshift distributions of both samples, this does not change the final constraints on cosmological parameters significantly. The same is true also for the impact of residual systematic uncertainties from the errors in the COSMOS 30-band photometric redshifts. Additionally, we show that these effects will still be negligible in Stage-IV datasets. Finally, the combination of DESY1 and HSC-DR1 allows us to constrain the “clumpiness” parameter to S8 = 0.768+0.021 −0.017. This corresponds to a ∼ √ 2 improvement in uncertainties with respect to either DES or HSC alone.

Abstract:

In the context of modified gravity, at the linear level, the growth of structure in the Universe will be affected by modifications to the Poisson equation and by the background expansion rate of the Universe. It has been shown that these two effects lead to a degeneracy which must be properly accounted for if one is to place reliable constraints on new forces on large scales or, equivalently, modifications to general relativity. In this paper we show that current constraints are such that assumptions about the background expansion have little impact on constraints on modifications to gravity. We do so by considering the background of a flat, Λ cold dark matter universe, a universe with a more general equation of state for the dark energy, and finally, a general, model-independent, expansion rate. We use Gaussian processes to model modifications to Poisson's equation and, in the case of a general expansion rate, to model the redshift-dependent Hubble rate. We identify a degeneracy between modifications to Poisson's equation and the background matter density, ωM, which can only be broken by assuming a model-dependent expansion rate. We show that, with current data, the constraints on modifications to the Poisson equation via measurements of the growth rate range between 10-20% depending on the strength of our assumptions on the Universe's expansion rate.