Why is our universe three dimensional? Cosmic knots could untangle the mystery

Next time you're untangling your earbuds in frustration, here's an idea to help put it in perspective: knots may have played a crucial part in kickstarting our universe, and without them we wouldn't live in three dimensions. That's the strange story pitched by a team of physicists in a new paper, and the idea actually helps plug a few plot holes in the origin story of the universe.

Our universe has three spatial dimensions. That's such a basic fact of reality that most people don't ever stop to question why it's the case. But in theory, three dimensions seems like a somewhat arbitrary number. Why doesn't our universe have four, or five, or 11 dimensions? The question has plagued physicists, but trying to answer it has all-too-often been relegated to the "too-hard" basket.

After five years of tackling the problem, an international team of physicists has developed a theory that not only explains how the universe arose in its three dimensional state, but also solves several other mysteries of its birth and growth. The key is a fairly common element of the Standard Model of particle physics called a flux tube.

Flux tubes are flexible strands of energy that bind elementary particles together – linking quarks and antiquarks with the help of gluons. But as the particles drift apart, they can eventually break the flux tube between them. That gives off a burst of energy that creates a new quark-antiquark pair, which bind to the existing particles to form two complete pairs.

Flux tubes are a well known phenomenon, but for the new theory the physicists found that by kicking those up to a higher energy level, they can solve some mysteries about why the universe happens to be exactly the way we see it.

In the early days of everything, the universe was just a hot, thick primordial soup called quark-gluon plasma. With so many elementary particles in close proximity, they would have created a whole mess of flux tubes. Most of these tubes would have quickly been destroyed though, since matter and antimatter annihilate each other when they meet, taking the flux tubes with them.

But there are times when flux tubes can survive longer than the particles that they link. If those particles move in just the right way, they can twist their flux tubes into knots, which are stable enough to exist on their own. And if several of these flux tubes intertwine, they can form an even more stable network of knots, which would have quickly filled the early universe.

The team soon realized this idea explained two long-standing issues with the currently-accepted idea of how the universe came to be. The story goes that in its first few moments, the universe underwent a period of extremely rapid expansion – from the size of a single proton to a grapefruit in less than a trillionth of a second. After that, expansion happened much more slowly, although it is currently accelerating.

But two questions about that story have never been properly answered: what triggered that sudden burst of expansion, and then why did it slow back down again? When the team calculated how much energy would be tied up in their knotty network, they realized it gave a convenient explanation for both of those.

"Not only does our flux tube network provide the energy needed to drive inflation, it also explains why it stopped so abruptly," says Thomas Kephart, co-author of the study. "As the universe began expanding, the flux-tube network began decaying and eventually broke apart, eliminating the energy source that was powering the expansion."

The story neatly fits in with existing ideas of the origins of everything. After the flux tube network breaks down, it releases particles and radiation into the universe, which then continues to expand and evolve the way other theories have explained.

That also brings us back to the question of why the universe is three dimensional. According to knot theory, knots can only exist in three dimensions: as soon as you add a fourth, they quickly unravel. That means that during the early period, the knotted flux tubes would have only caused rapid expansion in the three spatial dimensions. By the time the flux tube network broke down, the groundwork had already been laid for a 3D universe to evolve, and any higher dimensions that exist would remain tiny and essentially undetectable.

While the theory is certainly intriguing, it's still a work in progress. Before the idea can be properly proposed, the researchers say they need to develop it further to allow it to make testable predictions about the nature of the universe.

And in the end, maybe tangled earbuds are a small price to pay, considering we might not exist without them.

The research was published in the European Physical Journal C.

Source: Vanderbilt University