Initial-state selection in de Sitter cosmology

Determine the physically appropriate initial state for de Sitter cosmology, including whether de Sitter invariance uniquely selects such a state or permits multiple choices.

Background

The paper identifies the specification of an initial state as a major unresolved issue in cosmology. Even when quantum field theory is required to be de Sitter invariant on a fixed background, the discussion emphasizes that this condition may not uniquely determine the state.

The subsequent sections study the Bunch–Davies and Hartle–Hawking wavefunctions as particular constructions, but the broader problem of selecting the physically correct initial state remains unresolved.

References

A major open problem in cosmology is the need for an initial state. Even if we require de Sitter invariance for QFT in fixed de Sitter, there could still be many choices.

— Notes on de Sitter space  (2609.19454 - Mühlmann, 16 Sep 2026) in Section 4, opening paragraph of 'de Sitter Wavefunction'

Our results therefore raise the question of how a zero-mode-free, BD-like vacuum is selected when the Universe begins in an approximately de Sitter regime, and what physical principle determines this choice.

— Breaking de Sitter Symmetry with Cosmic Topology: Cosmological Role of Infrared Vacuum Freedom  (2609.28637 - Negro et al., 23 Sep 2026) in Conclusion

Whether this assumption provides the most appropriate description of the perturbation state remains an open question and deserves further investigation.

— Note on decoherence of the cosmological background by thermal perturbations in the language of open quantum systems  (2610.11397 - Kulyabin, 8 Oct 2026) in Section 1, Introduction