What is a backreaction?

Answer 1

A backreaction is a reaction, as a result of Newton's third law, back from a radiating gravitational wave.

Answer 2

Backreaction refers to the effect of quantum fields on the background spacetime geometry in general relativity. When quantum fields are present, their energy and momentum can influence the curvature of spacetime, leading to a feedback loop where the geometry affects the fields and vice versa. This interplay is important for understanding the dynamics of the universe on both small and large scales.