The acceleration in the expansion rate of the Universe evidenced by the Hubble diagram of distant supernovae implies the existence of a component with negative effective pressure, what is commonly called "dark energy". In fact, the accelerated expansion can be reproduced also by models where there is no extra dark energy component, but the gravitational theory is modified. These two options, new physics or a modification of general relativity, are indistinguishable by experiments that probe the expansion history H(z) only, as it is the case of supernovae. The two scenarios, however, yield distinct predictions for the growth rate of cosmic structures. I will present recent results obtained from the VVDS redshift survey, that allow us to measure the growth rate at z~0.8 using the technique of redshift-space distortions, a novel approach at these redshifts. I will discuss the implications of this result and the prospects of using the same method in future larger redshift surveys of galaxies to unveil the origin of cosmic acceleration.