Comment on "Inflation with a graceful exit and entrance driven by Hawking radiation"

Modak and Singleton [Phys. Rev. D 86, 123515 (2012)] have presented Hawking-like radiation for cosmological inflation which has a natural "turn on" and a natural "turn off" mechanism. This Hawking-like radiation results in an effective negative pressure "fluid" which leads to a rapid period of expansion in the very early Universe. We discuss that the turn on mechanism can not happen for FRW model in early universe because its horizon is apparent horizon not event horizon. Hence, we cannot apply geometric optic approximation which is a necessary condition for tunneling method. It was shown that this model predict a value for $\fracρ{m_{pl}^4}$ which is bigger than the COBE normalization constraint in the Cosmic Microwave Background (CMB) at the horizon exit.

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