Geometry and growth contributions to cosmic shear observables

Abstract

We explore the sensitivity of weak-lensing observables to the expansion history of the Universe and to the growth of cosmic structures, as well as the relative contribution of both effects to constraining cosmological parameters. We utilize ray-tracing dark-matter-only $N$-body simulations and validate our technique by comparing our results for the convergence power spectrum with analytic results from past studies. We then extend our analysis to non-Gaussian observables which cannot be easily treated analytically. We study the convergence (equilateral) bispectrum and two topological observables, lensing peaks and Minkowski functionals, focusing on their sensitivity to the matter density ${\mathrm{\ensuremath{\Omega}}}_{m}$ and the dark energy equation of state $w$. We find that a cancellation between the geometry and growth effects is a common feature for all observables and exists at the map level. It weakens the overall sensitivity by factors of up to 3 and 1.5 for $w$ and ${\mathrm{\ensuremath{\Omega}}}_{m}$, respectively, with the bispectrum worst affected. However, combining geometry and growth information alleviates the degeneracy between ${\mathrm{\ensuremath{\Omega}}}_{m}$ and $w$ from either effect alone. As a result, the magnitudes of marginalized errors remain similar to those obtained from growth-only effects, but with the correlation between the two parameters switching sign. These results shed light on the origin of the cosmology sensitivity of non-Gaussian statistics and should be useful in optimizing combinations of observables.