Disentangling dark energy and cosmic tests of gravity from weak lensing
  systematics
  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.
