  Quantized fields (e.g., the graviton itself) in de Sitter (dS) spacetime lead
to particle production: specifically, we consider a thermal spectrum resulting
from the dS (horizon) temperature. The energy required to excite these
particles reduces slightly the rate of expansion and eventually modifies the
semiclassical spacetime geometry. The resulting manifold no longer has constant
curvature nor time reversal invariance, and back-reaction renders the classical
dS background unstable to perturbations. In the case of AdS, there exists a
global static vacuum state; in this state there is no particle production and
the analogous instability does not arise.
