By imposing Robin boundary conditions in the Hartle-Hawking formulation of quantum gravity (middle), one can exclude off-shell geometries with singularities (right), allowing for a consistent path-integral summation, the sum being dominated by a no-boundary geometry (left).
From the article:
No-Boundary Proposal as a Path Integral with Robin Boundary ConditionsAlice Di Tucci and Jean-Luc Lehners
Phys. Rev. Lett. 122, 201302 (2019)
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