Abstract
The Einstein equation in a semiclassical approximation is applied to a spherical region of the universe, with the stress-energy tensor consisting of the mass density and pressure of the ΛCDM cosmological model plus an additional contribution due to the quantum vacuum. Expanding the equation in powers of Newton constant G, the vacuum contributes to second order. The result is that at least a part of the acceleration in the expansion of the universe may be due to the quantum vacuum fluctuations.