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We study Hawking-like radiation in a Friedmann-Robertson-Walker (FRW)universe using the quasi-classical WKB/tunneling method which pictures thisprocess as a "tunneling" of particles from behind the apparent horizon. Thecorrect temperature of the Hawking-like radiation from the FRW spacetime isobtained using a canonical invariant tunneling amplitude. In contrast to theusual quantum mechanical WKB/tunneling problem where the tunneling amplitudehas only a spatial contribution, we find that the tunneling amplitude for FRWspacetime (i.e. the imaginary part of the action) has both spatial and temporalcontributions. In addition we study back reaction and energy conservation ofthe radiated particles and find that the tunneling probability and change inentropy, ${\cal S}$ are related by the relationship: $\Gamma\propto\exp[-\Delta{\cal S}]$ which differs from the standard result $\Gamma\propto\exp[\Delta{\cal S}]$. By regarding the whole FRW universe as an isolated adiabatic systemthe change in the total entropy is zero. Then splitting the entropy betweeninterior and exterior parts of the horizon ($\Delta {\cal S}_{total}=\Delta{\cal S}_{int} + \Delta {\cal S}_{ext}=0$), we can explain the origin of theminus sign difference with the usual result: our $\Delta {\cal S}$ is for theinterior region while the standard result from black hole physics is for theexterior region.

Hawking-like radiation as tunneling from the apparent horizon in a FRW Universe