  The presence of cosmological perturbations affects the background metric and
matter configuration in which the perturbations propagate. This effect, studied
a long time ago for gravitational waves, also is operational for scalar
gravitational fluctuations, inhomogeneities which are believed to be more
important in inflationary cosmology. The back-reaction of fluctuations can be
described by an effective energy-momentum tensor. The issue of coordinate
invariance makes the analysis more complicated for scalar fluctuations than for
gravitational waves. We show that the back-reaction of fluctuations can be
described in a diffeomorphism-invariant way. In an inflationary cosmology, the
back-reaction is dominated by infrared modes. We show that these modes give a
contribution to the effective energy-momentum tensor of the form of a negative
cosmological constant whose absolute value grows in time. We speculate that
this may lead to a self-regulating dynamical relaxation mechanism for the
cosmological constant. This scenario would naturally lead to a finite remnant
cosmological constant with a magnitude corresponding to $\Omega_{\Lambda} \sim
1$.
