Regular Compact Universes from Semiclassical FLRW Spacetimes

We introduce the concept of a Cosmic Gate (CG) as a novel construction that connects two higher-dimensional semiclassical Loop Quantum Gravity (LQG) models of flat, regular Friedmann-Lemaître-Robertson-Walker (FLRW) spacetimes. The resulting universe is compact and nonsingular. In contrast to a spherically symmetric thin-shell wormhole, which joins the exteriors of two timelike hypersurfaces in a bulk geometry, a spherically symmetric Cosmic Gate connects their interiors. The junction requires a surface perfect fluid, whose stress-energy tensor is shown to correspond to a dark-energy-like equation of state while satisfying both the weak and null energy conditions, thereby avoiding exotic matter. Furthermore, since the underlying FLRW spacetimes in loop quantum cosmology (LQC) are regular due to the resolution of the big bang singularity by a quantum bounce, the resulting closed universe constructed via the Cosmic Gate remains nonsingular across its entire spacetime.