  Motivated by black hole physics, we define the Unruh state for a scalar field
in de Sitter space. Like the Bunch-Davies state, the Unruh-de Sitter state
appears thermal to a static observer. However, it breaks some of the symmetries
of de Sitter space. We calculate the expectation value of the energy-momentum
tensor in the Unruh-de Sitter state in two dimensions and find a non-vanishing
flux of outgoing negative energy. Extrapolating the result to four dimensions,
we argue that this backreacts on the initial de Sitter geometry
semi-classically. Notably, we estimate that de Sitter space is destabilized on
a timescale set by the gravitational entropy; analogous to black hole
evaporation, the endpoint of this instability is a singular geometry outside
the regime of effective field theory. Finally, we suggest that the Unruh-de
Sitter state may be a natural initial state for patches of de Sitter space, and
discuss the implications for slow-roll and eternal inflation, and for de Sitter
thermodynamics.
