Quantum back-reaction problems

Abstract

The macroscopic behavior of many physical systems can be approximately described by classical quantities. However, quantum theory demands the existence of omnipresent quantum fluctuations on top of this classical background -- which, albeit small, should have some impact onto its dynamics. The correct treatment of this quantum back-reaction is one of the main problems in quantum gravity and related to fundamental questions such as the initial (big bang) singularity or the cosmological constant. By means of the qualitative analogy between gravity and fluid dynamics, we try to shed some light onto these problems and show some of the difficulties associated with the calculation of the quantum back-reaction starting from the classical (macroscopic) equation of motion.