Fluctuations of Energy Density and Validity of Semiclassical Gravity
  From calculations of the variance of fluctuations and of the mean of the energy density of a massless scalar field in the Minkowski vacuum as a function of an intrinsic scale defined by the world function between two nearby points (as used in point separation regularization) we show that, contrary to prior claims, the ratio of variance to mean-squared being of the order unity does not imply a failure of semiclassical gravity. It is more a consequence of the quantum nature of the state of matter field than any inadequacy of the theory of spacetime with quantum matter as source. 
B. L. Hu and Nicholas. G. Phillips