# No-boundary wavefunction The no boundary proposal is a proposal for the beginning of the universe. The idea is to prepare the state or wavefunction using a Euclidean path integral that has no boundary in the past. ## Refs - original - [[1983#Hartle, Hawking]] - review - [[2023#Lehners]] - constraints of the contour - [[1990#Halliwell, Hartle]] - spatial topology of $S^2\times S^1$ - [[2025#Turiaci, Wu]] - factorisation properties - [[2023#Chua, Jiang]] - gauge fixing and field redefinition - [[2021#Partouche, Toumbas, de Vaulchier]] - boundary conditions - Dirichlet (bad) - unstable: weighting larger for larger fluctuations - [[FeldbruggeLehnersTurok201703]] - [[FeldbruggeLehnersTurok201705]] - [[FeldbruggeLehnersTurok201708]] - Neumann: good - [[TucciLehnersSberna2019]] - QG corrections - [[JonasLehners2020]][](https://arxiv.org/pdf/2008.04134.pdf) - scalar field - [[JonasLehnersMeyer2021]][](https://arxiv.org/pdf/2112.07986.pdf) - sum over regular, not necessarily compact, metrics - alternative proposals - "boundary proposal" - [[2024#Friedrich, Hebecker]] - phase - [[2025#Shi, Turiaci]] ## Comments - exists if the BH is *static* - so does not exist for Kerr - two point function blows up on the past horizon - okay for BHs formed from collapse because past horizon unphysical ## Properties - it is the ground state on the horizon wrt the null generator