Constraining modified gravity and growth with weak lensing

The idea that we live in a Universe undergoing a period of acceleration is a new, yet strongly held, notion in cosmology. As this can, potentially, be explained with a modification to general relativity, we look at current cosmological data with the purpose of testing gravity. First, we constrain a phenomenological model [modified Dvali Gabadadze Porrati (mDGP)] motivated by a possible extra dimension. This is characterized by a parameter a which interpolates between alpha = 0 [lambda cold dark matter (LCDM)] and alpha = 1 (the DGP model). In addition, we analyse general signatures of modified gravity given by the growth parameter gamma and power spectrum parameter Sigma. We utilize large angular scale (theta > 30 arcmin) weak lensing data (Canada-France-Hawaii Telescope Legacy Survey wide) in order to work in the more linear regime and then add, in combination, baryon acoustic oscillations (B Lambda Os) and supernovae. We subsequently show that current weak-lensing data are not yet capable of constraining either model in isolation. However, we demonstrate that even at present this probe is highly beneficial, for in combination with BAOs and Supernovae we obtain alpha < 0.58 and 0.91 at 1 sigma and 2 sigma, respectively. Without the lensing data, no constraint is possible. This corresponds to a disfavouring of the flat DGP braneworld model at over 2 sigma. We highlight these are insensitive to potential systematics in the lensing data such as an underestimation of the shear at high redshift. For the growth signature gamma, we show that, in combination, these probes do not yet have sufficient constraining power. Finally, we look beyond these present capabilities and demonstrate that Euclid, a future weak-lensing survey, will deeply probe the nature of gravity. A 1 sigma error of 0.104 is found for alpha (l(max) = 500) whereas for the general modified signatures we forecast 1 sigma errors of 0.045 for gamma and 0.25 for Sigma(0)(l(max) = 500), which is further tightened to 0.038 for gamma and 0.069 for Sigma(0)(l(max) = 10 000).