Back reaction is the effect of the created particles on the background in which they propagate.
The gravitational back reaction is just the response of the geometry to the gravitational interactions between the quantum-mechanically produced particles. In order to study this effect I have worked mainly with two techniques. We observe that the gravitational back reaction is a loop effect, and then we proceed to employ covariant quantization techniques. Thus, we have shown that back reaction does not depend in any obvious way on the gauge. Most important, we conducted the calculation in a way that keeps perturbation theory under control, so that higher order loops can be calculated in the future.