You might never have wondered why the Universe has just three spatial dimensions, but it is a question that has long vexed physicists.
An idea that mixes particle physics with something called knot theory could not only offer an explanation, it might also provide insight into what powered the colossal growth-spurt the Universe experienced just moments after its creation.
The fact static volumes of space have three degrees of freedom – width, depth, and length – seems so fundamental, it's difficult to picture a Universe any other way.
Try to imagine a 4D hypercube as a real object and you'll quickly feel your brain tap out.
We can't rule out the possibility that there are other dimensions of space, folding in on scales so compact we simply can't yet experience them. Models such as string theory make room for a mind-bending nine or more such dimensions.
But until we do find solid evidence of their existence, three is all we need to be concerned about on a macro scale of humans, planets, and galaxies.
An international team of physicists went searching for a reason behind this magical number, finding a strong contender for a solution in a model they call knotty inflation.
The researchers started with some pretty well established physics called flux tubes, which has its roots in James Clerk Maxwell's modelling of electromagnetism in the 19th century.
Flux tubes are more or less ways of describing channels of the imaginary force lines produced by magnetism.
We're familiar with positive and negative electrical charges, and the north and south poles of magnets, as reflections of the same basic phenomenon.
The quantum world also has a similar kind of sticky system. So flux tubes can also be used on a quantum scale to explain why specific kinds of particles called quarks bind together to give us chunky sub-atomic particles such as protons and neutrons.
Pulling apart a pair of magnets gives us two smaller magnets, each with a north end and a south end.
The quantum world has its own answer to that too, only with a twist; if you pull apart a pair of glued quarks, the flux tube stretches until it snaps. The energy in that field then results in the production of another quark and an antiquark.
Sometimes the pair immediately come together and cancel out. Sometimes each quark grabs onto a separated half of the original pair to make a new partnership.
Quarks were all the fashion when the Universe was just a few microseconds old. So it stands to reason that flux tubes were probably a big deal then as well.
This was the starting place for the physicist's tangled speculations.
"We've taken the well-known phenomenon of the flux tube and kicked it up to a higher energy level," says researcher Thomas Kephart from Vanderbilt University.
Snapping a flux tube and popping out a quark and an antiquark should be the end of the story, especially if the quark and antiquark wipe each other out again.
But there's a peculiar exception – these tubes aren't technically restricted to a single-dimensional straight line. Complex arrangements of flux tubes can have different stabilities, some of which are capable of outlasting the quark and antiquark they produce and hanging around as a knot.
And the best configurations for these knots? Yup – they're most stable in three dimensions. Anything more and they quickly find a way to break apart.
As the Universe expanded enough for the quarks to bind together, empty space could have been filled with a tangle of these flux tube knots.
The researchers did the math on the energy contained by this fabric of flux tubes and found it was enough to drive a period known as inflation.
Our Universe looks pretty much the same in every direction. This makes cosmologists think it went through a dramatic burst of expansion fairly early on, taking it from roughly the size of a proton to about the size of a grapefruit in a fraction of a second.
"Not only does our flux tube network provide the energy needed to drive inflation, it also explains why it stopped so abruptly," says Kephart.
"As the Universe began expanding, the flux-tube network began decaying and eventually broke apart, eliminating the energy source that was powering the expansion."
Given the stable tubes were knotted in three dimensions, any other degrees of freedom were left in their dust.
While this research is a neat model, it might not be long before we have more solid evidence that our Universe had a rather twisted history.
This research was published in the European Physical Journal C.