  It is shown that quantum vacuum fluctuations give rise to a curvature of
space-time of the order appropriate to explain the observed accelerated
expansion of the universe. The fact that the fluctuations produce curvature,
even if the expectation of the vacuum energy vanishes, is a consequence of the
non-linear character of the Einstein equation. A calculation is made, involving
plausible hypotheses within quantized gravity, which establishes a relation
between the two-point correlation of the vacuum fluctuations and the space-time
curvature.
