Hawking radiation from the cosmological horizon in a FRW universe
  It is well known that there is a Hawking radiation from the cosmological
horizon of the de-sitter spacetime, and the de-sitter spacetime can be a
special case of a FRW universe. Therefore, there may be a corresponding Hawking
radiation in a FRW universe. Indeed, there have been several clues showing that
there is a Hawking radiation from the apparent horizon of a FRW universe. In
our paper, however, we find that the Hawking radiation may come from the
cosmological horizon. Moreover, we also find that the Hawking radiation from
the apparent horizon of a FRW universe in some previous works can be a special
case in our result, and the condition is that the variation rate of
cosmological horizon $\overset{.}{r}_{H}$ is zero. Note that, this condition is
also consistent with the underlying integrable condition in these works from
the apparent horizon.
