Abstract
Upcoming weak lensing surveys will probe large fractions of the sky with unprecedented accuracy. To infer cosmological constraints, a large ensemble of survey simulations are required to accurately model cosmological observables and their covariances. We develop a parallelized multilensplane pipeline called UFALCON, designed to generate fullsky weak lensing maps from lightcones within a minimal runtime. It makes use of LPICOLA (Howlett et al. [1]), an approximate numerical code, which provides a fast and accurate alternative to cosmological NBody simulations. The UFALCON maps are constructed by nesting 2 simulations with mass resolution of about 5 × 10 ^{12} and 2 × 10 ^{13} h ^{1} M _{o} covering a redshiftrange from z=0.1 to 1.5 without replicating the simulation volume. We compute the convergence and projected overdensity maps for LPICOLA in the lightcone or snapshot mode. The generation of such a map, including the LPICOLA simulation, takes about 3 hours walltime on 220 cores. We use the maps to calculate the spherical harmonic power spectra, which we compare to theoretical predictions and to UFALCON results generated using the full NBody code GADGET2. We then compute the covariance matrix of the fullsky spherical harmonic power spectra using 150 UFALCON maps based on LPICOLA in lightcone mode. We consider the PDF, the higherorder moments and the variance of the smoothed field variance to quantify the accuracy of the covariance matrix, which we find to be a few percent for scales ℓ ∼ 10 ^{2} to 10 ^{3} . We test the impact of this level of accuracy on cosmological constraints using an optimistic survey configuration, and find that the final results are robust to this level of uncertainty. The speed and accuracy of our developed pipeline provides a basis to also include further important features such as masking, varying noise and will allow us to compute covariance matrices for models beyond ΛCDM.
Original language  English 

Article number  044 
Journal  Journal of Cosmology and Astroparticle Physics 
Volume  2019 
Issue number  1 
DOIs  
Publication status  Published  22 Jan 2019 
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Data availability statement for 'Fast generation of covariance matrices for weak lensing'.
Sgier, R. (Creator), Réfrégier, A. (Creator), Amara, A. (Creator) & Nicola, A. (Creator), IOP Publishing, 22 Jan 2019
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