The authors show that the bubbles S^(2) × S^(2) can be created from vacuum fluctuation in the background de Sitter universes of k=0,1,so the space-time foam-like structure might really be constructed from bubbles of S2 x S2 in the very early inflating phase of our universe.Still,whether such foam-like structure persisted during the later evolution of the universe is a problem unsolved at present.
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