Euclid: impact of nonlinear prescriptions on cosmological parameter
  estimation from weak lensing cosmic shear
  Upcoming surveys will map the growth of large-scale structure with
unprecented precision, improving our understanding of the dark sector of the
Universe. Unfortunately, much of the cosmological information is encoded by the
small scales, where the clustering of dark matter and the effects of
astrophysical feedback processes are not fully understood. This can bias the
estimates of cosmological parameters, which we study here for a joint analysis
of mock Euclid cosmic shear and Planck cosmic microwave background data. We use
different implementations for the modelling of the signal on small scales and
find that they result in significantly different predictions. Moreover, the
different nonlinear corrections lead to biased parameter estimates, especially
when the analysis is extended into the highly nonlinear regime, with both the
Hubble constant, $H_0$, and the clustering amplitude, $\sigma_8$, affected the
most. Improvements in the modelling of nonlinear scales will therefore be
needed if we are to resolve the current tension with more and better data. For
a given prescription for the nonlinear power spectrum, using different
corrections for baryon physics does not significantly impact the precision of
Euclid, but neglecting these correction does lead to large biases in the
cosmological parameters. In order to extract precise and unbiased constraints
on cosmological parameters from Euclid cosmic shear data, it is therefore
essential to improve the accuracy of the recipes that account for nonlinear
structure formation, as well as the modelling of the impact of astrophysical
processes that redistribute the baryons.
