# Back(reaction) to the Future in the Unruh-de Sitter State

## TL;DR

This paper introduces the Unruh-de Sitter state in de Sitter space, analyzes its properties, and explores its implications for the stability of de Sitter space and inflationary cosmology.

## Contribution

It defines a new quantum state in de Sitter space inspired by black hole physics and investigates its backreaction and potential to destabilize de Sitter space.

## Key findings

- Unruh-de Sitter state exhibits a negative energy flux.
- De Sitter space may be destabilized on a timescale related to gravitational entropy.
- The endpoint of instability is a singular geometry outside effective field theory.

## Abstract

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.