Before spacetime: a proposal of a framework for multiverse quantum cosmology based on three cosmological conjectures

The three cosmological conjectures to which our work refers are: the phenomenon called geodesic incompleteness, the physical gravitational theta(G)-term that would characterize the one-parameter family of inequivalent vacua of quantum gravidynamics, and the hypothesis of multiversality (Wilczek 2013 Class. Quantum Grav. 30 193001), more specifically, a zero-energy multiverse. The known cosmological phenomenology leads under plausible assumptions to theorems which establish that the universe is past incomplete (Borde A et al 2003 Phys. Rev. Lett. 90 151301; Vilenkin and Zhang 2014 J. Cosmol. Astropart. Phys. JCAP06(2014) 034). Here, starting from Wilczek's definition of multiverse (Wilczek 2013 Class. Quantum Grav. 30 193001) (a larger physical structure of which the universe forms part) and that spacetime is much larger than the observable universe, in a new sense suggested by these theorems, we place the observable universe, labelled by U-theta G(1), within a multiverse ensemble, {U-theta G}. Its topological theta((1))(G)-term would characterize the observable universe from the Planck epoch until the present time, and it could have physical effects in, for example, black-hole physics. Our proposal is therefore a possible framework for a multiverse quantum cosmology, in which the temporal parameters (see figures in the main text) start from a 'timeless multiverse big bang' (TLMBB), where all members of the multiverse ensemble, {U-theta G}, disappear, together with their corresponding classical spacetimes. Since quantum cosmology can be viewed as one attempt among many to face with the question of finding a gravitational quantum theory, if the TLMBB were the appropriate ground to define the physical or mathematical underlying structure of quantum cosmology, then multiversality could come to have a predictive power within our observable universe.