The universe might be on track to eat itself from the inside out.
Luckily for us, physicists studying the phenomenon, called “spacetime decay”, believe this is very unlikely. Still, the possibility is interesting enough to explore in mind-boggling detail, covering “bubbles of nothing” in spacetime, hidden extra dimensions, and a hypothetical observer hitching a ride on the outer surface of our universe.
The idea that in specific scenarios the universe would be entirely destroyed by an expanding bubble of nothing has been around since 1982, when theoretical physicist Edward Witten introduced the possibility of the universe eating itself in a paper in Nuclear Physics B journal. He wrote: “A hole spontaneously forms in space and rapidly expands to infinity, pushing to infinity anything it may meet.”
Given that a bubble of nothing has not in fact destroyed the universe, neither in the 13 billion years before Witten published his paper nor in the 38 years since, it would be reasonable for physicists to push it down the research priority list. But three physicists at the University of Oviedo in Spain and the University of Uppsala in Sweden argue that we can learn important lessons from an all-consuming, universe-destroying bubble in a wonderfully titled paper, “Nothing Really Matters”, submitted to the Journal of High-Energy Physics this month.
In particular, understanding the conditions for spacetime decay through a bubble of nothing is a step towards connecting the best theories about the tiniest building blocks of the universe—strings—with theories about space and time itself.
The unstable universe
It's commonly understood that a vacuum is a region of total emptiness, so it’s confusing to think that our entire universe which contains planet earth, distant galaxies and everything in between is almost entirely a vacuum. But the fact that our universe is mostly vacuum is part of the reason it exists in a relatively stable state.
In quantum field theory, which connects quantum physics and the dynamics of spacetime, a vacuum is better understood as the lowest possible energy state. “Excited” quantum states with energy above a vacuum state don’t stay excited for very long, and tend to quickly decay down to lower energy states by emitting photons and other packets of energy. Vacuums don’t have lower energy states to decay down to, and so exist happily in a stable state.
Since most of our universe is a vacuum and already in the lowest possible energy state, we shouldn’t have to worry about spacetime decay. In theoretical physics, however, assumptions like that are rarely stable themselves.
In the early 1970s, a few Russian physicists separately explored the idea that there’s a middle ground between a stable vacuum and an unstable non-vacuum: a vacuum-like state which seems stable because of the very long time period it will stay in this “metastable” state before decaying. Now referred to as a “false vacuum”, the suggestion was an attempt to resolve inconsistencies in theories about early conditions in the universe, the effects of gravity, and cosmological observations.
“[A bubble of nothing] describes a possible channel for 'universe destruction;' in that the bubble of nothing expands and can 'eat' all of spacetime, converting it into 'nothing'"
Although the new concept of a false vacuum was suggested to describe only a transition period before the Big Bang, more recent research into the Higgs Field (a quantum force field famously detected by particle accelerator CERN) suggests we might still be living in a false vacuum after all, since what was previously thought to be the stable (lowest energy) state of a Higgs field might not be the lowest energy state.
The possibility that the stability of our universe is a very long illusion has opened up looming questions about how and why the delicate false vacuum might decay. One answer is through a "bubble of nothing."
Infinite nothing and hidden dimensions
A bubble of nothing is one example of a "spacetime bubble" where spacetime has different properties inside and outside the bubble boundary. Other types of bubbles might have different strengths of dark energy inside and outside the bubble, for example, but bubbles of nothing have no interior at all, says paper author and researcher at Uppsala University, Marjorie Schillo.
If a bubble of nothing spontaneously forms in false-vacuum spacetime, it will grow and eventually swallow the entire universe. “[A bubble of nothing] describes a possible channel for 'universe destruction;' in that the bubble of nothing expands and can 'eat' all of spacetime, converting it into 'nothing.'” says Schillo.
But why would a bubble of nothing form in the first place? The answer lies in string theory, a popular and successful candidate for a “theory of everything” which postulates tiny entities called strings with properties that other fundamental particles don’t have. In particular, strings have a vibrational state which accounts for quantum gravity. In other words, the theory integrates phenomena in quantum physics with the behaviour and effects of gravitational fields. This result is much-sought after, and is a significant reason why string theory is so popular.
Such a tantalizingly complete theory relies on several assumptions that are not guaranteed. The maths of string theory only works if there are more than four dimensions: three spatial dimensions, a time dimension, and then lots of other dimensions that are so small that they can’t be detected, only derived mathematically. In string theory, the geometry of our universe only appears to be four dimensional spacetime because the extra dimensions are tightly compacted and hidden.
For mathematical reasons that are almost too technical to explain in words, bubbles of nothing won’t form in four dimensional spacetime, but they will form in “stringy” multidimensional spacetime. One model of stringy spacetime is called a Kaluza-Klein vacuum, and in this model the probability of a bubble of nothing destroying everything is one (i.e. certain) across an infinite space. Physicists actually aren’t sure if our universe is an infinite or finite volume, but reassuringly, the result that bubble-of-nothing universe destruction is 100 percent certain is seen as something to rectify, not something to worry about.
“It would be interesting to work out under what conditions an observer could 'ride' on the bubble of nothing and see a universe that is similar to the one we live in"
As Czech string theorist Luboš Motl notes in a surprisingly funny blog post, a bubble of nothing catastrophe should be used to rule out descriptions of our universe, since if it was going to happen, it should have already happened.
“We don't know whether our spacetime is exactly stable. It is plausible that it is threatened by a cosmic catastrophe,” he writes. “But because the Universe has lived for [around 14 billion years]… we know that the probability of the birth of a deadly [bubble of nothing] shouldn't be much larger than [an extremely tiny number far less than one]."
He goes on: “If a theory predicted a (much) larger probability density of a lethal destructive tumor, it would also predict that our Universe should have been (certainly) destroyed by now. But it wasn't so the theory would have a problem.”
Schillo agrees. She says that her research into bubbles of nothing aims, in part, at establishing what the implications are for string-theoretic descriptions of the universe, given that spacetime decay via a bubble of nothing is very unlikely.
“It is important to understand these decay channels because if we want a stringy vacuum to describe our universe, instabilities like the bubble of nothing must be either extremely rare or absent,” she says.
Riding on a bubble
The bubble of nothing serves another purpose, too. Schillo and others believe that the mathematical description of a universe-destroying bubble of nothing could also be used to model the origin of the universe.
The behaviour of a rapidly expanding bubble of nothing is a good approximation for the early inflation of the universe. More specifically, the outer surface of a growing bubble of nothing would look very much like the creation of the universe, if it were possible to watch universe creation from the outside.
This may sound far-fetched, but it’s a key focus of theoretical physics and early universe cosmology. “One of the future topics of research that I am most excited about is looking into the 'universe creation' aspect of this work," Schillo said.
“It would be interesting to work out under what conditions an observer could 'ride' on the bubble of nothing and see a universe that is similar to the one we live in," she explained. "Because the bubble expands, such an observer would see an expanding universe, and this could explain the observed dark energy.”
So, you definitely don't have to worry about a bubble of nothing gobbling up all of spacetime. But if you’ve ever wondered what the universe looked like when it first exploded, it’s certainly worth keeping up with bubble research.