What is a bubble from nothing?

Question

Recently I was reading a collection of lectures on Swampland Conjecture and came across an interesting subtitle about a bubble from nothing.

A bubble from nothing is a non-perturbative instability that mediates the decay of a vacuum into nothing, that is, the vacuum annihilates itself. This occurs when the compact size collapses to zero size at the bubble wall. Therefore, it is literally a bubble with nothing inside (even with space-time) that appears in a vacuum and begins to expand at the speed of light, leaving nothing behind.

What exactly does "nothing" mean? A state without space-time, but still a state of a physical system? Or "nothing" in the literal sense, that is, the absence of something? What it is from a physical point of view?
Link to article:
https://arxiv.org/abs/2102.01111

Rohbar · Accepted Answer

Closed string tachyon condensation can lead to the decay of space-time dimensions. The endpoint of these condensations are what is refered to as the "nothing" state. What this really is is not yet clear. From a string field theory point of view it is a CFT whose perturbative cohomology vanishes, i.e. there are no pertubative states remaining in the theory. Especially there are no spacetime dimensions. But one can still formulate a string field theory around this nothing, the so called vacuum string field theory. And this is still equivalent ( due to background independence) to any other SFT.
The condensation of the tachyon itself starts at a point in spacetime and extends at the speed of light, this is what is now known as a bubble of nothing, first introduced by Witten in the setup of toriodal compactifications.
So in short: There are no perturbative states in "nothing", but non-perturbativ instantons still exist.

alexarvanitakis · Answer

Actually it's a bubble of nothing. This was originally found by Witten, and is best explained by Witten. It's an instability of Kaluza-Klein spacetimes, where from the perspective of the lower-dimensional observer, a bubble forms, so that the lower dimensional spacetime is Minkowski minus a bubble of some radius
$$r(t)=sqrt{R^2+t^2}$$
where $R$ is the radius of the KK circle. As $t$ increases, the spacetime disappears into "nothing".

What is a bubble from nothing?

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