  We study the effective energy-momentum tensor (EMT) for cosmological
perturbations and formulate the gravitational back-reaction problem in a gauge
invariant manner. We analyze the explicit expressions for the EMT in the cases
of scalar metric fluctuations and of gravitational waves and derive the
resulting equations of state. The formalism is applied to investigate the
back-reaction effects in chaotic inflation. We find that for long wavelength
scalar and tensor perturbations, the effective energy density is negative and
thus counteracts any pre-existing cosmological constant. For scalar
perturbations during an epoch of inflation, the equation of state is de
Sitter-like.
