Back(reaction) to the future in the Unruh-de Sitter state

Lars Aalsma, Maulik Parikh, Jan Pieter van der Schaar

Research output: Contribution to journalArticlepeer-review

20 Scopus citations


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.

Original languageEnglish (US)
Article number136
JournalJournal of High Energy Physics
Issue number11
StatePublished - Nov 1 2019


  • 2D Gravity
  • Black Holes
  • Effective Field Theories

ASJC Scopus subject areas

  • Nuclear and High Energy Physics


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