Hawking-like radiation as tunneling from the apparent horizon in an FRW universe

We study Hawking-like radiation in a FriedmannRobertsonWalker (FRW) universe using the quasi-classical WKB/tunneling method, which pictures this process as a "tunneling" of particles from behind the apparent horizon. The correct temperature of the Hawking-like radiation from the FRW spacetime is obtained using a canonical invariant tunneling amplitude. In contrast to the usual quantum-mechanical WKB/tunneling problem, where the tunneling amplitude has only a spatial contribution, we find that the tunneling amplitude for FRW spacetime (i.e. the imaginary part of the action) has both spatial and temporal contributions. In addition we study backreaction and energy conservation of the radiated particles and find that the tunneling probability and the change in entropy, S, are related by the relationship T ∞ exp[-δS], which differs from the standard result, T ∞ exp[-δS]. By regarding the whole FRW universe as an isolated adiabatic system, the change in the total entropy is zero. Then, splitting the entropy between the interior and exterior parts of the horizon (δS Total) =δSint δSext =0, we can explain the origin of the minus sign difference with the usual result: our δS is for the interior region, while the standard result from black hole physics is for the exterior region. © 2010 World Scientific Publishing Company.