We tend to imagine absolute emptiness as something unique.
Depending on the theory used to describe the universe, this need not be so. There can be more than one notion of total emptiness.
This possibility has fascinating consequences.
These multiple vacua may share certain properties. In some cases, they may even be indistinguishable to observers trying to decide which one they inhabit. In other cases, they can differ substantially. Particles that exist in one vacuum may be significantly heavier than their counterparts in another.
At first sight, the existence of alternate notions of emptiness may seem immaterial, since one may think that our universe simply builds itself on top of one of them.
Their mere existence, however, can predict the existence of objects with fascinating properties.
If neighboring domains find themselves built on top of different notions of emptiness as the universe evolves from its hot past into its colder and more dilute future, then, under suitable conditions, the junction between them can freeze into a defect in emptiness.
A useful analogy is that of a ribbon. Imagine taking the two ends of a tangled ribbon, laying them flat on a surface, and then sliding them apart while keeping contact with the surface.
A ribbon lying dead flat throughout is the analog of the universe settling everywhere into the same vacuum. More generally, however, there might be a twist trapped somewhere in the middle.
Defects in emptiness are like such twists.
Depending on the theory and on how the neighboring domains are arranged, these defects can take different forms. The simplest ones are walls. Others require a continuous family of empty states wrapping smoothly around a certain region.
A useful way to picture this is to imagine the different empty states labeled by color, ranging continuously through red, yellow, green, blue, and violet, with the two ends identified so that the sequence closes back on itself. The defect is the place where all these colors are forced to come together and fuse into white.
In such cases, the defect can take different shapes. It may appear as a string stretching across the horizon, as a closed loop like a smoke ring, or ending on a domain wall. There can also be point-like magnetic knots carrying only a single pole, north or south.
Strings are tornadoes in emptiness, in the sense that different vacua wind around them like a vortex.
These defects can also have unusual gravitational properties. Domain walls tend to repel ordinary matter gravitationally, while strings produce little gravitational pull in the usual sense. And yet, strings can still reveal themselves by producing double images of objects behind them.
It is important to emphasize that the existence of these knots and tornadoes in emptiness is model dependent, and that none of them has yet been discovered in our universe. Their status is nevertheless very different from that of many more speculative ideas. Within theories that admit them, their existence, properties, and formation mechanisms are understood self-consistently. Moreover, many of the models that predict such defects are not arbitrary inventions, but are motivated by the attempt to explain some of the observed puzzles in the laws of nature.