The 1950s idea is retreaded:
In 1955, influential physicist John Wheeler proposed that, at the quantum level, spacetime is not constant but “foamy,” made up of ever-changing tiny bubbles. As for what these bubbles are “made” of, recent work suggests that spacetime bubbles are essentially mini-universes briefly forming inside our own.
The spacetime foam proposal fits nicely with the intrinsic uncertainty and indeterminism of the quantum world. Spacetime foam extends quantum uncertainty in particle position and momentum to the very fabric of the universe, so that its geometry is not stable, consistent, or fixed at a tiny scale…
Carlip is working on a quantitative model of spacetime foam to supplement the theoretical model currently on the table. He’s calling the model “minisuperspace,” and is hopeful that physicists researching other approaches in the quantum-cosmology intersection could find examples of the model in their own work, if they know to look for it. To start with, Carlip says he’ll be looking at some numerical simulations to support the foam model.Carly Minsky, “The Universe Is Made of Tiny Bubbles Containing Mini-Universes, Scientists Say” at Vice
In short, nothing specifically backs this “multiverse of ephemeral bubbles” inside our universe except a need to believe in it. Maybe that’s theoretical physics now.
See also: The multiverse is science’s assisted suicide
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