Abstract
We identify a fundamental obstruction to any theory of the beginning of the universe, formulated as a semiclassical path integral. Hartle and Hawking's no boundary proposal and Vilenkin's tunneling proposal are examples of such theories. Each may be formulated as the quantum amplitude for obtaining a final 3geometry by integrating over 4geometries. We introduce a new mathematical tool  PicardLefschetz theory  for defining the semiclassical path integral for gravity. The Lorentzian path integral for quantum cosmology with a positive cosmological constant is meaningful in this approach, but the Euclidean version is not. Framed in this way, the resulting framework and predictions are unique. Unfortunately, the outcome is that primordial tensor (gravitational wave) fluctuations are unsuppressed. We prove a general theorem to this effect, in a wide class of theories.
Original language  English 

Article number  171301 
Journal  Physical Review Letters 
Volume  119 
Issue number  17 
DOIs  
Publication status  Published  27 Oct 2017 
Keywords
 hepth
 grqc
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Neil Turok
 School of Physics and Astronomy  Higgs Chair of Theoretical Physics
Person: Academic: Research Active