The presence of cosmological fluctuations influences the background cosmology in which the perturbations evolve .
This back-reaction arises as a second order effect in the cosmological perturbation expansion .
The effect is cumulative in the sense that all fluctuation modes contribute to the change in the background geometry , and as a consequence the back-reaction effect can be large even if the amplitude of the fluctuation spectrum is small .
We review two approaches used to quantify back-reaction .
In the first approach , the effect of the fluctuations on the background is expressed in terms of an effective energy-momentum tensor .
We show that in the context of an inflationary background cosmology , the long wavelength contributions to the effective energy-momentum tensor take the form of a negative cosmological constant , whose absolute value increases as a function of time since the phase space of infrared modes is increasing .
This then leads to the speculation that gravitational back-reaction may lead to a dynamical cancellation mechanism for a bare cosmological constant , and yield a scaling fixed point in the asymptotic future in which the remnant cosmological constant satisfies \Omega _ { \Lambda } \sim 1 .
We then discuss how infrared modes effect local observables ( as opposed to mathematical background quantities ) and find that the leading infrared back-reaction contributions cancel in single field inflationary models .
However , we expect non-trivial back-reaction of infrared modes in models with more than one matter field .