The End of Everything: (Astrophysically Speaking)

by Katie Mack

  So, we’re branching out. There are radio surveys attempting to illuminate the cosmic Dark Ages between the time of the CMB and the epoch of the first stars, in the hope that some departure from Concordance Cosmology might reveal itself more starkly. There are new kinds of gravitational wave detectors that rely on techniques as different as quantum interference between atoms and combining signals from pulsars. These might, in an indirect way, bring us information about the behavior of black holes or the physics of the early universe. Experiments looking at new ways to find dark matter might show us how to expand the Standard Model of particle physics, or shift our thinking in cosmology. Studies of the polarization of the CMB could show us signatures of cosmic inflation that completely change our understanding of the early universe. Or, the lack of such signals could motivate more studies into inflation alternatives like bouncing cosmologies. Laboratory experiments studying alternative ideas about the energy of the vacuum might finally solve the problem of dark energy, if it’s not a cosmological constant after all. It may even be possible, with observations spanning decades, to measure the expansion of the universe directly by staring at a distant source for so long that its apparent speed away from us changes.

  Pedro Ferreira is also optimistic about this diversity of approaches. “I think it all might look quite specialized and bitty,” he says, but having a huge number of people suddenly individually racking their brains to come up with something new could be exactly what we need. “Out of that explosion someone might have an idea. ‘Oh! This is the way to figure out the future.’ ”

  How long such a program will take is another question. If we’re just trying to distinguish between a cosmological constant and some other form of dark energy, we literally have all the time in the world, and then some. There’s really no theory out there in which dark energy can destroy our planet before our own Sun does the job.

  But vacuum decay is another matter. The Standard Model of particle physics, the very same one that has passed every experimental test we’ve come up with, places us in a precarious position on the edge of total universal instability. How likely this is to be an actual risk, or a quirk of the extrapolation of an incomplete theory, depends on who you ask. (For the record, I asked several experts and I got answers ranging from “it tells us our theory is wrong” to “the risk is really tiny” to “maybe we’ve just been lucky so far.” Take that as you will.) In any case, if we want to be able to say something more reassuring than “It’s useless to worry because you are not going to feel any pain,”IV we’re going to need a very specific kind of data.

  Fortunately, we have a pretty good idea where we can get it.

  DISCOVERY MACHINES

  There is no place on Earth with a more persistent, if wholly undeserved, association with the destruction of the cosmos than CERN. Best known as the home of the Large Hadron Collider, CERN is a sprawling campus of laboratories and office buildings covering about six square kilometers that straddles the France-Switzerland border near Geneva. It’s essentially an oddly specialized little border town, complete with its own fire department and post office, alongside laboratories and machine shops and a bona fide antimatter factory. Physicists at CERN have been accelerating and smashing protons since the 1950s, long before the LHC was built, carrying out increasingly complex and sensitive experiments to examine the nature of subatomic particles by obliterating them against each other. These kinds of experiments helped us to create the Standard Model of particle physics, and more than fifty years of continued experiments have failed to find any cracks in it wide enough to stick a new particle through.

  But CERN keeps trying. And not just because smashing things is, admittedly, quite a lot of fun.

  The name of the game in particle colliders is energy. Throwing particles at each other faster means the eventual collision is at a higher energy, and the higher energy your collisions, the larger the swath of possible new physics you can reach. You can think of collision energy as legal tender, to be exchanged, via E = mc2, for particle mass. If the total energy in a collision is higher than the equivalent mass of the particle you’re trying to create, then as long as your theory allows any kind of interaction between that particle and the ones you’ve smashed together, you have a chance of creating that particle. Extensions to the Standard Model tend to involve particles that are significantly heavier than those we’ve detected so far, which means that we need to reach higher and higher energies to find them. But even when you’ve reached the right energy threshold, it takes more than one creation of a particle to get a meaningful, statistically significant signal. The Large Hadron Collider had to run for years, smashing countless trillions of protons,V before it had collected enough data to say with acceptable certainty that a Higgs boson had been found.

  It’s this constant push into the energy frontier that leads to CERN’s unfortunate reputation as an existential menace. The thinking goes that if humanity has never before seen so much energy concentrated into one place, who knows what could happen? Some of the concerns include the unsettling scenarios we’ve discussed in previous chapters, like the creation of little black holes, or the triggering of catastrophic vacuum decay. Fortunately, in every disaster scenario presented so far, we can easily set aside the worries based on the fact that the LHC is hardly even a blip compared to the particle-obliterating violence going on all around us in the universe. But in the minds of certain especially fretful nonphysicists, not every worry is as well defined, or as easily assuaged, even though the LHC has operated totally harmlessly for over a decade. By the time I visited CERN in February 2019, internet jokes about the LHC tearing open a portal into another dimension, or shifting the universe into “the bad timeline,” seemed about as prevalent as ever.

  * * *

  The CERN campus itself is not, for the most part, an especially impressive place. Once you get past the glitzy public reception lobby, it has the feel of a slightly run-down industrial installation, with a mishmash of low, drab, 1960s-era buildings with dark metal-shuttered windows. Each prominently numbered building houses its own lab or research group, the offices labeled with temporary paper nameplates to accommodate the constantly shuffling scientific staff. Across the entire campus, the physicists permanently employed by CERN number less than a hundred, with the rest of the labs and offices occupied by the thousands of visiting researchers from around the world, spending anywhere from a week to a few years carrying out the intense on-site work necessary to keep large-scale experiments running. Walking down the long dim hallways of one of these buildings, you might forget you’re at the most famous experimental facility in the world, and imagine yourself in the physics department of any ordinary university, peeking in at grad students and postdoctoral researchers tapping away at laptops, or scribbling equations and work schedules on whiteboards.

  When you see the experiments, though, that illusion of normalcy is swiftly and permanently broken.

  My own visit to CERN was divided between the organization’s two extremes. On some days, I was quietly ensconced in a bright second-floor office in the theory department, reading papers and taking breaks in the tea room to sketch out equations and chat with the other theorists about vacuum decay and my own research on dark matter. On other days, I was wearing a hardhat, 100 meters underground, standing on a metal walkway and gawking at a 25-meter-tall heavily instrumented cylinder of unimaginably complex machinery. The experiments at CERN are some of the most advanced and precise machines ever created by humanity, designed and built by teams of thousands over decades in order to tease out tiny changes in the motions and energies of particles that decay within microseconds. Meanwhile, theorists try to extract from equations of comparable but abstract complexity the implications of these experiments for the nature of space and the cosmos itself. It’s a heady place.

  It is also, however, an intensely bureaucratic place, being an institute governed by international treaties and run by a coalition of twenty-three different countries while hosting research
ers from every corner of the planet. This kind of cooperation is necessary for an effort of such magnitude and expense, but the upshot of CERN’s organizational structure is that the future of the facility and of any new experiments depends as much on international politics as on any scientific considerations. During my visit, the hot topic at the cafeteria was not some exciting new experimental result, but a series of back-and-forth newspaper editorials debating the merits of CERN’s proposal to build the so-called Future Circular Collider (FCC), a particle collider so big that the 27-kilometer LHC would become merely a pre-accelerator to bring the protons up to a speed where they could begin to circulate in the FCC ring. The FCC could reach energies of 100 TeV, which is about an order of magnitude higher than what’s currently possible at the LHC.

  As Freya Blekman pointed out to me during my visit, these experiments take decades to set up, and the data from current experiments can take equally long to analyze, so discussions of the next experimental direction have to happen now. The kind of data we are already getting with the LHC and its upcoming upgrades will take us ten or even fifteen more years to fully analyze. “So this is the time to decide,” Blekman says. “What do we want? Do we want an electron-positron collider? Should it be linear? Should it be circular? What are the pros and cons of each? Do we want to directly go to a higher-energy proton-proton machine?”

  The arguments for and against future colliders, especially the ambitious FCC, can get rather heated. Even if you set aside the cost (around 10 billion euros at a minimum), debates remain around the promise—or lack thereof—that a bigger collider will find new particles. It may be that the elusive “new physics” we’re searching for only shows up at energies so high that even gargantuan machines like the FCC have no hope of ever reaching them. Or it may be that just focusing on increasing the energy puts us on the wrong track entirely, and there’s some clue about new physics hiding in another regime that we have yet to explore, perhaps even in data we already have.

  Researchers I spoke to at CERN were adamant that increasing energy is essential to move us forward, even if only to better understand the Standard Model. Which does, after all, present us with the specter of vacuum decay. If that Sword of Damocles is going to be hanging over our heads, it would be nice to know exactly what it’s doing up there.

  André David, an LHC researcher in the Compact Muon Solenoid (CMS) collaboration who hosted my visit to the detector, pointed out that answering this question is a key motivation for the FCC and experiments like it. “One of the reasons why people are saying, ‘Oh we should go for the hundred-TeV collider’ is that then you actually get a chance at nailing this thing down.”

  As David points out, we already have a puzzle on the table: the nature of the Higgs field, and its (and our) fate. The data we’ve already obtained, and are working to analyze, could begin to trace out the nature of the Higgs in more detail, but with a new collider, we might finally answer the question of what the instability that threatens us with vacuum decay really means.

  As we discussed in Chapter 6, the Higgs potential is the mathematical structure that determines how the Higgs field evolves, and, importantly for us, whether it will send us all to our doom. It is, in a real sense, the holy grail of particle physics. But with current theories we have very little handle on what it looks like. Based on our current understanding, its shape depends sensitively on the competing influences of several different hard-to-calculate aspects of the Standard Model, and if some higher-energy theory exists, this could completely change the picture.

  Some researchers I spoke to, including CERN theorist (and leading supersymmetry advocate), John Ellis, suspect that the apparent instability of the Higgs is not really an existential threat, but rather a sign that there’s something about the theory that we don’t understand.

  José Ramón Espinosa, a theorist studying vacuum decay, hopes to find ways to better understand the Higgs potential and what our precarious placement on the knife edge of stability might mean, without simply waiting for a true vacuum bubble to show up.VI “There’s no reason for the potential to be like this,” he says. “We live in this very, very special place. So for me this is kind of intriguing; maybe this is trying to tell us something.” The key to understanding the Higgs potential ultimately depends on what are called running couplings—the interactions between particles and fields and how they change with higher-energy collisions. “This might be one of the main messages of the LHC if we don’t find anything else,” Espinosa says. “Of course, if the LHC finds new physics, then most likely this is going to interfere with the running of the couplings. Then anything can happen. Maybe the potential is stable, maybe it’s even more unstable. We don’t know.”

  In addition to the small (but important!) point of determining the fate of the cosmos, a better understanding of the Higgs field could show us how mass works, or why the fundamental forces manifest with the strengths we measure. It could even point the way toward a theory uniting the forces, or help us to understand quantum gravity.

  Having some kind of guidance from observations or experiments on how to improve on Concordance Cosmology or the Standard Model would be very helpful. Because over on the pure theory side of things, things are getting very, very weird.

  THROUGH A GLASS DARKLY

  I recently came across an old black-and-white photo of Paul Dirac, Nobel Laureate and pioneer of quantum mechanics, standing on the grounds of Princeton’s Institute for Advanced Study with an axe slung over his shoulder. During his many visits there from the 1930s to the 1970s, he was known to wander through the woods behind the institute, clearing new paths for the resident theorists to walk and talk and think about the nature of reality. My own guide through those same muddy trails was Nima Arkani-Hamed, which seems appropriate, because he is a theorist determined to take an axe to our current understanding of quantum mechanics, and to the entire notion of spacetime itself.

  Arkani-Hamed has been working on a way of calculating the interactions between particles using a completely new framework, one that starts from a kind of abstract mathematics in which space and time are not, strictly, included. The work is still in its early stages, and so far applies more to certain idealized systems than to experimental results. But if it works out, the implications could not be more mind-blowing. “What we’re seeing is just, it’s in baby, baby toy, toy, toy examples, right? You can justifiably use as many diminutives as you want on what’s actually been accomplished, and I would be entirely sympathetic,” he tells me. “But for what it’s worth, there is starting to be one or two examples of actual concrete physical systems not so far from what we see in the real world where we can actually figure out how to describe them without either spacetime or quantum mechanics.” I tell him I’m trying to wrap my head around what it means to live in a universe where space and time aren’t real. He laughs. “Join the club.”

  Before you dismiss the idea as eccentric-theorist hyperbole, I should point out that Arkani-Hamed is not the only one talking like this. “I’m sure you’ve heard this from many people,” Clifford Johnson tells me, nonchalantly, a few months later, “but I think we’re getting better at realizing one of the things we’ve been saying in string theory for a long time, which is that spacetime isn’t fundamental.”

  Oh, yeah. That small detail. Sure.

  Johnson’s approach to the question is a bit different. There are some intriguing hints in quantum gravity theories of unexpected connections between physics on small and large scales, in ways that don’t make sense in our usual thinking about how spacetime works. A simplified explanation might be that if you imagine doing experiments in a hypothetical kind of space that has a certain radius, let’s call it R, the results of that experiment would look exactly like the results of the same experiments in a much smaller space, with a radius equal to 1 divided by R. In string theory, this is called T-duality, and it’s a weird enough coincidence that it seems like it has to be telling us something deep. “If you ask people about this quest
ion,” Johnson says, “the answer that people would give is that in some sense, none of it is real. In the sense that by undermining large and small, what you’re really doing is undermining the whole business of spacetime in the first place.”

  Some theorists have tried to reassure me. Sean Carroll, a cosmologist at Caltech who’s interested these days in the underpinnings of quantum mechanics, thinks we are all being a little rash to dismiss spacetime as not strictly real. “It’s real but not fundamental,” he tells me. “Just like this table is real but not fundamental. It’s a higher level of emergent description. That doesn’t mean it’s not real.” Basically we shouldn’t get too hung up on this because it’s not like spacetime isn’t there, it’s just that if we really understood what it was made of, it would look, at a deeper level, like something else entirely.

  This does not, in fact, reassure me.VII As a physicist I always try to maintain some level of dispassionate reserve when it comes to my subject, but the notion that spacetime is only real in the sense that it is something we can talk about and sit on but not in the sense of being what the universe is actually made of still makes me feel like it might collapse underneath me at any moment.

  Whether or not this has relevance to how or when the universe will end is still an open question. However real spacetime is or isn’t, we all live there, and what happens to spacetime is bound to affect us. But if thinking about emergent spacetime or new formulations of quantum mechanics leads us to some deeper fundamental theory, it might drastically change our outlook. Maybe, as Johnson suggests, connections between large and small scales could imply a new fate for the cosmos. Or maybe, if we were able to revise quantum mechanics, we would finally find an explanation for dark energy. Even if we settle on a cosmological constant and a Heat Death future, according to Arkani-Hamed, we will still need a major shift on the theory side to be able to talk about what quantum fluctuations might do then, in terms of Boltzmann Brains or Poincaré recurrences. “In my mind it’s very unlikely that all these things are explained and understood within the framework of quantum mechanics,” he says. “I think we need some extension of quantum mechanics to help us talk about it.”