by Katie Mack
So, technically, I can’t tell you for certain that vacuum decay isn’t right about to happen. I also can’t tell you for certain that it hasn’t already happened somewhere in our own Solar System, or on the other side of the galaxy, or in another galaxy, creating a light-speed-expanding bubble that is silently coming for us as we speak. But I can tell you that if you’d like to prioritize your paranoia, you’re much more likely in your lifetime to be struck by lightning, an out-of-control car, rampaging cattle, or even a stray meteor than by the spontaneous appearance of a bubble of true vacuum.
There’s just one more thing, though.
We’ve already covered the facts that we can’t produce our own vacuum decay bubble with a high-energy particle collision, and that a spontaneous tunneling event is so unlikely we should probably try very hard to forget that we ever heard of it in the first place. But recently, physicists have come up with yet another way to destroy the universe with vacuum decay, and I have to say it’s kind of a cool one.
SMALL BUT DEADLY
In 2014, Ruth Gregory, Ian Moss, and Benjamin Withers, building on a bit of previous work on this topic, put out a new paper that caught my attention. It explained that while spontaneous vacuum decay is tediously slow, the presence of a black hole could speed the process up considerably and generally make things much more interesting. In fact, they argued, the really dangerous thing is a small black hole, because particle-sized black holes can dramatically increase the chance of vacuum decay occurring right on top of them. Maybe we don’t have to wait 10100 years after all.
The way it works is similar to the way a particle of dust can condense a bit of water around it in a humid room, or the way that clouds get seeded in the upper atmosphere. The dust particle is a nucleation site—something that differentiates that point from others and allows the process to happen more easily. In the case of clouds and water, the water molecules have an easier time sticking together if there’s something else that they can stick to first. So an impurity can set off a chain reaction where otherwise things might have continued on as they were. It turns out tiny black holes can be those nucleation sites for bubbles of true vacuum, but only if they’re very small.
Fortunately for the universe, tiny black holes are not easy to make, given our current understanding of gravitational physics. Generally speaking, we only expect black holes to form at masses greater than that of the Sun, as the result of the collapse of massive stars at the end of their lives. Those black holes might grow to much larger masses by pulling in material or by merging with each other, but shrinking is another matter entirely. They can only lose mass via Hawking evaporation (see Chapter 4), and that takes ages. A black hole as massive as the Sun has an expected lifetime somewhere in the vicinity of 1064 years. At some point toward the end of that time, the black hole might get small enough to trigger vacuum decay, but we have quite a while before we really have to worry about that. It’s also been hypothesized that in the early universe, tiny black holes might have formed due to the extreme densities of the Hot Big Bang, but so far we don’t have any evidence of those. If they did form, though, and if little black holes really could destabilize the vacuum, we wouldn’t be here. So if we take that into account, and believe in the possibility of vacuum decay, any theory that predicts tiny primordial black holes has to be wrong, because we exist.
Just for fun, a few of us have also been wondering whether there are ways to make those little black holes without them having been around since the very beginning of the universe. Making tiny black holes isn’t a new idea. In addition to being awfully cute in a terrifying theoretical kind of way, these mini monsters could teach us about how gravity works, whether or not black holes really do that cool evaporation thing, and even whether or not there might be additional dimensions of space that we can’t otherwise see.
For years, physicists have been combing the data from particle colliders, hoping to see some telltale sign that one of the collisions between protons managed to put so much energy into such a small space that it all immediately collapsed into a microscopic black hole. That black hole, if it appears, should be harmless, according to the traditional thinking, not taking the possibility of vacuum decay into account. Theory states that it should immediately evaporate via Hawking radiation, and that even if it didn’t, it would likely be moving at relativistic speeds in some direction that would take it far away from us in a very short time, because a collision is never so perfectly timed and aimed that the particles completely stop. Plus, for the kinds of collisions that happen in particle colliders to be capable of making tiny black holes, somehow the force of gravity felt by subatomic particles has to be stronger than Einstein’s laws of gravity would suggest. And the only way that could happen, as far as we know, is for there to be extra dimensions of space. We’ll talk more about this in the next chapter, but the short story is that having more than our usual three dimensions of space can make gravity a bit stronger on very small scales and can therefore allow LHC collisions to make little black holes.
So if we can make black holes in the LHC, we have evidence for space having more dimensions than we thought. Which to a physicist looking for signs of exciting new physics seems like it would be fantastic news! Of course, it would be a shame if those little black holes we are trying to make in the LHC could trigger vacuum decay and cause the universe to end…
Fortunately, they can’t. We are as close to absolute certainty about that as physicists ever get. The main thing that acquits them is the fact that, as previously stated, cosmic rays can make collisions much more powerful than anything we see in our own colliders. If we can smash protons together to make black holes, the universe has already done this countless times, and, look!—we’re still here! So either the black holes aren’t being made anywhere, or they were harmless all along.
The other reason is that it seems like there’s a mass threshold tiny black holes have to reach before they’re even hypothetically dangerous. The kinds of black holes a particle collider could make would be safely below that level, as would, most likely, many of the collisions that would happen out in space. As a side bonus, some of us have already been working on using this fact, along with our continued existence, as a way to argue that there have to be limits on the possible size extra dimensions could be.IX (Just personally, as a cosmologist interested in testing different theories of physics, it’s always fun to be able to hold up the lack of a cosmic apocalypse as a data point.)
* * *
So, setting aside the little black holes for now, where does that leave us with vacuum decay? All the other possible ends of the universe we’ve looked at offer us at least the small comfort of being so far in the future that we can, with a great deal of confidence, leave them to be worried over by whatever post-human entities might inhabit the cosmos after we’re gone. Vacuum decay is special in that it could technically happen at any moment, even if the chances of that are astronomically low. It also comes with a uniquely extreme, almost gratuitous finality.
In 1980, two theorists, Sidney Coleman and Frank De Luccia, calculated that a true vacuum bubble would contain not only a totally different (and lethal) arrangement of particle physics, but also a kind of space that is, by its nature, gravitationally unstable. Once the bubble formed, they explained, everything inside would collapse gravitationally within microseconds. Then they wrote:
This is disheartening. The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in a new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.X
THE JOY OF NOT KNOWING
Of course, vacuum decay is, relatively speaking, a fairly new
idea that incorporates so many kinds of extreme physics that it’s entirely conceivable that our perspective on it will shift dramatically over the next few years. It may be that more detailed, rigorous calculations will give us different answers. These questions are difficult and complicated, and we still have a way to go before a consensus is reached.
If we conclude that our vacuum really is metastable, this may be incompatible with the theory of cosmic inflation. The quantum fluctuations during inflation, or the ambient heat afterward, seem like they should have been sufficient to trigger vacuum decay in the first moments of the cosmos, negating our very existence. Clearly, that didn’t happen. Which suggests either we don’t understand the early universe, or vacuum decay was never possible at all.
Whether or not you trust early universe theories, taking vacuum decay seriously depends on placing a great deal of trust in the Standard Model of particle physics, which we know cannot be the whole story. Dark matter, dark energy, and the incompatibility of quantum mechanics and general relativity all point to there being something more to the universe than what we can currently write down. Whatever comes along to replace the Standard Model might, by the by, save us from even having to vaguely worry about a wayward bubble of quantum death.
Or it may be that extensions of fundamental physics present entirely new ways for the universe to end. The possibility of extra dimensions of space—the same ones that tantalize collider physicists hoping to make miniature black holes—extends the universe into new realms of the unknown. Like any explorer reaching the edges of the map, we reach out not knowing what we might find. Higher dimensions of space might allow us to solve some long-standing problems with our theories of gravity, but they also come with a warning, scrawled in the margins of the ever-growing cosmic map: here be monsters.
I. Quarks come in six different “flavors,” which have different masses and charges. The flavors are: up, down, top, bottom, charm, and strange. They were named in the 1960s.
II. In Kurt Vonnegut’s Cat’s Cradle, a new form of ice is created, “ice-nine,” that is more stable than liquid water. In the story, every bit of water a particle of ice-nine touches turns to ice-nine, creating an existential threat to life and the world.
III. Believe me, I know.
IV. This is due to something called the Askaryan Effect, in which an ultra-high-energy neutrino punches through the lunar regolith and creates a burst of radio waves that we can hopefully pick up with radio telescopes. So far, our telescopes haven’t been sensitive enough, but we should be able to pick up these signals with the next generation of instruments.
V. This part of the book was written in North Carolina in August.
VI. Two people reaching at the same time for bread plates on opposite sides of them results in a pile-up that physicists would call a topological defect. In this specific case, it would be a domain wall, which, if let loose on the cosmos, would dominate the universe and lead to a Big Crunch. This is why I always wait for someone else to select the bread before making my attempt.
VII. We previously discussed this transition, and what it meant for the very early universe, in Chapter 2.
VIII. Of course, the Higgs field doesn’t have preferences; it’s just governed by its potential. But the way it would dive into a true vacuum if it could would definitely give an impression of enthusiasm.
IX. “Some of us” here being, specifically, me and my colleague Robert McNees, in our 2018 paper in Physical Review D. It was a fun one.
X. This discussion remains, to me, one of the most beautiful pieces of physics poetry I’ve ever seen in an academic journal.
CHAPTER 7: Bounce
HAMLET: O God, I could be bounded in a nutshell, and count myself a king of infinite space, were it not that I have bad dreams.
William Shakespeare, Hamlet
On September 14, 2015, at 9:50 a.m. and 45 seconds UTC, you were, for the briefest moment, just a little bit taller.
The gravitational wave crest that washed through you had been traveling across the cosmos, warping space itself in its wake, for 1.3 billion years, ever since it was set off by the violent merging of two black holes each 30 times more massive than the Sun. You might not have noticed the boost—after all, you grew by less than one millionth the width of a proton—but physicists at the Laser Interferometry Gravitational-Wave Observatory (LIGO) did. The first detection of gravitational waves was the culmination of a decades-long search, requiring the development of new technologies and the creation of the most sensitive equipment in the history of experimental physics. Finally detecting those ripples in spacetime was heralded as the ultimate vindication of Einstein’s general theory of relativity.
But even more significantly, it was the dawning of a new age of astronomical observation. It opened up the universe to a totally new way of seeing. Instead of collecting light or high-energy particles from distant sources, we could now reach out and feel the vibration of space itself, creating for the first time a window onto the kind of distant cosmic violence that can shake the very foundations of reality.
Since that first discovery, gravitational wave astronomy has continued to show us the inspirals and catastrophic mergers of black holes and neutron stars and allowed us to study the workings of gravity with an unprecedented level of precision. But gravitational waves may hold the key to something even more fundamental. They may give us a new view of the shape and origin of our universe, and present us with the possibility of determining whether or not there might be something outside it. Something that may ultimately destroy it all.
THE UNBEARABLE WEAKNESS OF GRAVITY
We’ve known for a long time that something has to be wrong with gravity. It works too well. Einstein’s general relativity has so far performed perfectly in every situation in which it’s been tested. For decades, physicists have tried to find some kind of deviation, somewhere, anywhere, that would show us how the simpleI equations written down in Einstein’s theory inevitably break down. Somewhere, in some extreme regime, like at the edge of a black hole or among the particles at the center of a neutron star, the equations must have some kind of a crack. We haven’t found it in any of our searches so far, but we’re sure it has to be there.
Figure 19: Illustration of the effect of a passing gravitational wave. As a gravitational wave hits face-on, it stretches the space it’s moving through vertically while squeezing it horizontally, and then vice versa, with each wave crest. If you are in the path of the wave, you are alternately a bit taller and thinner, and a bit shorter and wider, over and over until the wave passes. The magnitude of the stretching of your body is only about one millionth the width of a proton.
We have good reasons for being suspicious. Compared to the other forces, gravity is an oddball. Not only does it look totally different from a mathematical point of view, it’s way too weak. Sure, when you get together enough mass for a galaxy, or a black hole, it seems fairly strong. But in daily life, it’s easily the weakest force you encounter. Every time you lift a coffee cup you’re overcoming the gravitational pull of the entire planet. It takes putting the mass of the Sun into something the size of a city before gravity can even begin to compete with the atomic and nuclear forces holding atoms together.
Comparing forces is about more than just a strength test, though. The idea that all the forces can somehow be reframed as different aspects of the same thing, in extremely high-energy environments, is generally considered to be key to truly understanding how physics works. We live in hope that there’s some ultimate theory out there—a theory of everything—that unites all the forces of particle physics and gravity to explain, well, everything.
But so far, gravity won’t play along. We have a rock-solid theory of the electroweak force (a unification of electromagnetism and the weak nuclear force), confirmed by experiments. We also have some perfectly promising leads on a Grand Unified Theory uniting the electroweak and strong nuclear force. But every time we try to bring gravity in, its feebleness ruins the whole pic
ture. Even aside from that, gravity and quantum mechanics (which describes the workings of all the other forces) explicitly clash in their predictions about things like what should happen at the edge of a black hole. Finding a way to bring gravity into line would help immensely.
So there seem to be a few options here. One obvious one is to abandon the whole idea of unification, and just let gravity be its own special snowflake of a theory, unconnected to the rest of physics. It’s totally possible that there’s no theory of everything, and we’re never going to piece it all together in any sensible way. But just typing that makes my physicist toes curl, so maybe we can set that to the side for now in the “BREAK GLASS IN CASE OF EXISTENTIAL EMERGENCY” cabinet.
A much more appealing and intellectually exciting idea is that the problem is our theory of gravity: general relativity needs to be altered or replaced, and when that happens, it’ll all fit together. There’s been no shortage of impressive, well-motivated attempts in this direction. Quantum gravity theories, of which string theory and loop quantum gravity are the most famous examples, continue to be hot topics among theorists trying to find a way to unite particle physics with gravity and tie it all up with string. Or loops. You get the idea. In each of these scenarios, you end up with a gravity theory that can be quantized—expressed in terms of particles and fields rather than forces or spatial curvature—and these particles and fields mesh nicely with those of the quantum field theories that explain interactions between quarks and electrons and photons and the whole subatomic world. In this picture, gravitational forces would be manifestations of the exchange of particles called gravitons, just like an electric field is due to photons moving between objects. And gravitational waves, which we currently think of as the stretching and squeezing of spacetime, could also be envisioned as the motion of gravitons expressing their wavelike nature.
