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

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.

arXiv comment24 pages, 2 figures. v2: extended discussion about source at the origin, added references, matches published version

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• Quantum gravity of the Heisenberg algebra
• Information paradox and island in quasi-de Sitter space
• De Sitter Quantum Breaking, Swampland Conjectures and Thermal Strings
• (In)stability of de Sitter vacuum in light of distance conjecture and emergence proposal
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• Von Neumann algebra description of inflationary cosmology
• On the Quantum Bousso Bound in JT gravity
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