PIRSA:25030159

Video URL

https://pirsa.org/25030159

No-boundary observations

Blommaert, A. (2025). No-boundary observations. Perimeter Institute for Theoretical Physics. https://pirsa.org/25030159

Blommaert, Andreas. No-boundary observations. Perimeter Institute for Theoretical Physics, Mar. 06, 2025, https://pirsa.org/25030159 

          @misc{ scivideos_PIRSA:25030159,
            doi = {10.48660/25030159},
            url = {https://pirsa.org/25030159},
            author = {Blommaert, Andreas},
            keywords = {Quantum Gravity},
            language = {en},
            title = {No-boundary observations},
            publisher = {Perimeter Institute for Theoretical Physics},
            year = {2025},
            month = {mar},
            note = {PIRSA:25030159 see, \url{https://scivideos.org/index.php/pirsa/25030159}}
          }

Andreas Blommaert IAS

Talk numberPIRSA:25030159
DOI10.48660/25030159
Source RepositoryPIRSA
Collection
Talk Type Scientific Series
Subject

Abstract

I discuss work in progress on the no-boundary state of the universe in quantum cosmology, emphasizing the key role played by the inclusion of an observer in obtaining a reasonable no-boundary state. For concreteness we focus on a 2d toy model, namely dS JT gravity. This model has avatars of important issues with the wavefunction of the universe in inflationary models. The issues are that the wavefunction predicts a small universe (whereas our universe is rather large), and that it is not normalizable. I show that the non-normalizability is resolved by promoting dS JT gravity to sine dilaton gravity, a type of UV completion of dS JT gravity of which I discuss the cosmological interpretation. I then argue that the issue of predicting a small universe (in this toy model) is resolved by including an observer in the theory. With the observer, the leading contribution is a bra-ket wormhole. The observer's no boundary state is a (maximally mixed) identity matrix, and thus prefers neither small nor large universes.