  We study the instability of de Sitter space-time (dS) under thermal radiation
in different vacua. We argue that the mode function solution of a scalar field
in four-dimensional dS can be separated into the incoming and outgoing modes.
Different vacua for dS are realized by different combinations of positive
frequency modes assigned to each solution. For a minimally coupled massless
scalar field, we explicitly compute the behavior of the mode function and the
corresponding energy-momentum tensor in the Unruh vacuum near the horizon, and
find that the horizon area increases (decreases) in time when the incoming
(outgoing) mode contributes to thermal flux.
