Illustration by Kamran Samimi
A God’s-eye view of what it would look like if a false vacuum bubble universe formed and detached from the “real” universe. We hope that makes everything clear now.
A false vacuum is a region of space that appears to be totally stable, but in actuality could (theoretically) collapse at any moment and turn the vacuum inhabitants (us) into clouds of nonstandard particles in the time it takes for you to finish this sente... Ha, just kidding. But for real, Benjamin Shlaer is a postdoc researcher at the Tufts Institute of Cosmology and a man who knows from false vacuum decay, so we thought he’d be the perfect person to explain why our universe hasn’t just fallen apart at any poi… Ha! Got you AGAIN! Man, this is too easy.
Decay of a false vacuum is an extension of a theory that’s well known in chemistry and the physics of the phases of matter. The theory applies to the different “phases” of empty space as well, and those phases are called vacua. The same physics that governs water converting to steam as you boil it applies to empty space. Kind of like when you boil water and at the bottom of the pot there are these little bubbles fluctuating, we expect tiny “bubbles” of other vacua to be forming around us all of the time.
Tunneling is the process by which these bubbles (which we believe are always there and just very small) can occasionally—and this is why it takes a very long time to happen—fluctuate to be big, and when I say big, I’m still talking about below the scale of nuclear physics. So very small in conventional terms, but big enough that the roundness of the bubble and the surface tension of the bubble don’t cause them to collapse right away. When these get that big, then we say they tunneled. They had to borrow energy from somewhere to get that big, and there’s no apparent source of the energy—it’s just this quantum-mechanical borrowing of energy.
You might call them critical bubbles—a bubble that’s produced in the tunneling process that then grows. So a critical bubble is much bigger than most bubbles, which harmlessly collapse.
New bubbles occasionally appear in the vacuum and can grow to the size of the universe and leave behind a different vacuum, a different kind of empty space. If that happens, you’ll have a different physical construct of nature, by which I mean, roughly speaking, a different set of laws of physics. Certain things, like the value of the charge of electrons, or even the types of particles that exist—all these things could change. The universe could collapse or the laws of physics could change quite dramatically. The only thing that would definitely change would be the value of the vacuum’s energy density.
One theory is actually that this has already happened and what we call the current universe is the lower-energy vacuum state. If a new one occurs at some point in the future, then we’d have a new vacuum, and that would lead to a big crunch—a collapse of the universe. The way it would look is, first there would be a bubble that appeared somewhere far away and then expanded, and it would hit us pretty fast—basically at the speed of light. Then we’d be on the other side of this bubble wall. We might not really be held together by chemistry anymore if the laws of physics changed too much, but if the laws of physics didn’t change that much and we were still intact after the bubble passed by us, then we would find everything starting to be gravitationally attracted, and we’d all basically collide with all the galaxies and other matter in the universe. It’d be a catastrophic sort of crunch.
There’s nothing wrong with fearing that our vacuum is unstable, but the process takes so long we might just never observe it. It’s possible for there to be an instability that is so slow that we haven’t observed it yet because it hasn’t happened yet. But it could, in principle, still happen.
You could have different regions of space that are in different vacua, and in that case there’s always what we call a domain wall separating them—basically, a bubble wall that is usually accelerating. When you cross through that bubble wall you cross to the different vacua. Although usually it’s accelerating so fast that you can’t cross the barrier. But in principle, if it were a very slowly accelerating domain wall, for instance if the two different vacua have exactly the same vacuum energy, then you could actually cross back and forth and check out each vacuum. But you’d want to make sure that the same chemistry existed on both sides beforehand. You might cross into a vacuum where there were no stable protons or electrons and then you’d probably turn into some particles that would not be very comfortable to live in.