  In inflationary cosmological models driven by an inflaton field the origin of
the primordial inhomogeneities which are responsible for large scale structure
formation are the quantum fluctuations of the inflaton field. These are usually
computed using the standard theory of cosmological perturbations, where both
the gravitational and the inflaton fields are linearly perturbed and quantized.
The correlation functions for the primordial metric fluctuations and their
power spectrum are then computed. Here we introduce an alternative procedure
for computing the metric correlations based on the Einstein-Langevin equation
which emerges in the framework of stochastic semiclassical gravity. We show
that the correlation functions for the metric perturbations that follow from
the Einstein-Langevin formalism coincide with those obtained with the usual
quantization procedures when the scalar field perturbations are linearized.
This method is explicitly applied to a simple model of chaotic inflation
consisting of a Robertson-Walker background, which undergoes a quasi-de-Sitter
expansion, minimally coupled to a free massive quantum scalar field. The
technique based on the Einstein-Langevin equation can, however, deal naturally
with the perturbations of the scalar field even beyond the linear
approximation, as is actually required in inflationary models which are not
driven by an inflaton field such as Starobinsky's trace-anomaly driven
inflation or when calculating corrections due to non-linear quantum effects in
the usual inflaton driven models.
