Dynamical Aspects of Analogue Gravity: The Backreaction of Quantum   Fluctuations in Dilute Bose-Einstein Condensates
  We discuss the backreaction force exerted by quantum fluctuations in dilute Bose-Einstein condensates onto the motion of the classical background, derived by an ab initio approach from microscopic physics. It is shown that the effective-action method, widely employed in semiclassical quantum gravity, fails to give the full backreaction force. The failure of the effective-action method is traced back, inter alia, to the problem of the correct choice of the fundamental variables and the related operator ordering issues. 
Uwe R. Fischer