Redshift-space distortions as a probe of dark energy

Measurements of the growth rate of structure f at different redshifts can discriminate the origin of cosmic acceleration. Coupled to estimates of the expansion rate H(z) as provided by Type-la supernovae or Baryonic Acoustic Oscillations, accurate measurements of f(z) are able to distinguish "dark energy" models from theories with modified gravity, as, e.g., braneworld models. These two classes are degenerate when considering the expansion history alone. We have recently shown (Nature, 451 (2008) 541) that redshift-space distortions in the clustering pattern of galaxies measured from large redshift surveys represent an extremely promising technique to trace f (z) back in time. We have measured the distortion parameter beta at z similar to 0.8 using the currently largest field of the VVDS-Wide survey, that includes more than 10000 galaxy redshifts to I(AB) = 22.5 over 4 deg(2). We obtain 0 - 0.70 +/- 0.26, corresponding to a growth rate f = 0.91 +/- 0.36, consistent with the simplest cosmological-constant scenario. Error bars are still large compared with the difference among models. However, next-generation deep surveys with N > 100000 redshifts over similar to 100deg(2) or larger, will be able to use this test as an accurate probe of the origin of cosmic acceleration.