Optimising cosmic shear surveys to measure modifications to gravity on cosmic scales(2011). Donnacha Kirk, Istvan Laszlo, Sarah Bridle, Rachel Bean. We consider how upcoming photometric large scale structure surveys can be optimized to measure the properties of dark energy and possible cosmic scale modifications to General Relativity in light of realistic astrophysical and instrumental systematic uncertainities. In particular we include flexible descriptions of intrinsic alignments, galaxy bias and photometric redshift uncertainties in a Fisher Matrix analysis of shear, position and position-shear correlations, including complementary cosmological constraints from the CMB. We study the impact of survey tradeoffs in depth versus breadth, and redshift quality. We parameterise the results in terms of the Dark Energy Task Force figure of merit, and deviations from General Relativity through an analagous Modified Gravity figure of merit. We find that intrinsic alignments weaken the dependence of figure of merit on area and that, for a fixed observing time, a fiducial Stage IV survey plateaus above roughly 10,000deg2 for DE and peaks at about 5,000deg2 as the relative importance of IAs at low redshift penalises wide, shallow surveys. While reducing photometric redshift scatter improves constraining power, the dependence is shallow. The variation in constraining power is stronger once IAs are included and is slightly more pronounced for MG constraints than for DE. The inclusion of intrinsic alignments and galaxy position information reduces the required prior on photometric redshift accuracy by an order of magnitude for both the fiducial Stage III and IV surveys, equivalent to a factor of 100 reduction in the number of spectroscopic galaxies required to calibrate the photometric sample. Submitted to MNRAS.
Disentangling dark energy and cosmic tests of gravity from weak lensing systematics(2011). Istvan Laszlo, Rachel Bean, Donnacha Kirk, and Sarah Bridle. We consider the impact of key astrophysical and measurement systematics on constraints on dark energy and modifications to gravity on cosmic scales. We focus on upcoming photometric "Stage III" and "Stage IV" large scale structure surveys such as DES, SuMIRe, Euclid, LSST and WFIRST. We illustrate the different redshift dependencies of gravity modifications compared to intrinsic alignments, the main astrophysical systematic. The way in which systematic uncertainties, such as galaxy bias and intrinsic alignments, are modelled can change dark energy equation of state and modified gravity figures of merit by a factor of four. The inclusion of cross-correlations of cosmic shear and galaxy position measurements helps reduce the loss of constraining power from the lensing shear surveys. When forecasts for Planck CMB and Stage IV surveys are combined, constraints on the dark energy equation of state and modified gravity model are recovered, relative to those from shear data with no systematic uncertainties, if fewer than 36 free parameters in total are used to describe the galaxy bias and intrinsic alignment models as a function of scale and redshift. To facilitate future investigations, we also provide a fitting function for the matter power spectrum arising from the phenomenological modified gravity model we consider. Submitted to MNRAS.
Constraining Interactions in Cosmology's Dark Sector(2008). Rachel Bean, Eanna Flanagan, Istvan Laszlo, and Mark Trodden. We investigate possible dark sector interactions, in particular interactions between dark matter and dark energy or self interaction of dark matter. Using large scale structure observations, we then constrain their strengths relative to that of gravity. In the case of dark matter-dark energy interactions we find a maximum interaction strength of 7% that of gravity, while in the case of dark matter self interaction (via a Yukawa type coupling) we find that at a scale of 10 Mpc the strength of the interaction must be less than 5% that of gravity. Published in Phys. Rev. D 78 123514(2008)
Nonlinear growth in modified gravity theories of dark energy(2007). Istvan Laszlo and Rachel Bean. We carry out N-Body simulations of various modified gravity theories, including theories that modify the Poisson Equation thus altering the relationship of the gravitational potential to the matter distribution and theories that introduce anisotropic shear stress, altering the response of matter to the potential. We then compare the simulations to two main analytical fits for nonlinear power and find that although the fits are developed for standard gravity, they do remarkably well even in the case of the modified theories. Published in Phys. Rev. D 77 024048(2008).