What is a No-Boundary Quantum State?

          @misc{ pirsa_PIRSA:17110122,
            doi = {10.48660/17110122},
            url = {https://pirsa.org/17110122},
            author = {Hartle, James},
            keywords = {Cosmology, Quantum Gravity},
            language = {en},
            title = {What is a No-Boundary Quantum State?},
            publisher = {Perimeter Institute},
            year = {2017},
            month = {nov},
            note = {PIRSA:17110122 see, \url{https://pirsa.org}}
          }
          

Abstract

Contemporary final theories consist of a theory of the universe’s dynamics (I) like the avatars of string theory together with a theory of its quantum state (Ψ) like the no-boundary wave function of the universe (NBWF). This talk is concerned with the definition of a no-boundary quantum state at the semiclassical level where its predictions can be straightforwardly derived and compared with observation. A semiclassical no-boundary wave function is defined by an ensemble regular saddle points. The ensemble is restricted by simple considerations of symmetry such as time neutrality. We will briefly review the successful predictions of the NBWF so defined. We then address the question of moving beyond the semiclassical approximation by defining the NBWF in terms of a Euclidean or Lorentzian integral or by a connection with a dual field theory.