The End of Everything: (Astrophysically Speaking)

by Katie Mack

  Unfortunately, despite decades of hard work and extraordinarily intricate calculations, we haven’t yet settled on a theory that’s broadly accepted by the physics community. Not only have none of the ideas put forward been confirmed by particle experiments, it’s not even clear that they can be. Ideally, we’d write down two theories and then work out that they make different predictions for experiments like those being carried out at the Large Hadron Collider. But this is a challenge when you’re trying to distinguish between theories whose effects only become clear at energies many orders of magnitude higher than what LHC collisions can produce. This has led physicists to suggest solutions ranging from abstract arguments aimed at narrowing down the total range of possible universes to philosophical debates about how to make advances in areas of theory in which experimental evidence may never appear.

  For those of us who hold out more hope for new data, our best bet for something that could give us hints about a theory of everything might well lie in cosmology—especially the study of the early universe. If you need data about particle interactions at extraordinarily high energies, finding new ways to examine the Big Bang is generally going to be easier than trying to build a particle collider the size of the Solar System.

  We’re already being nudged in this direction. So far, we’ve seen only a small handful of physical phenomena that we cannot explain within the Standard Model of particle physics (or very minor modifications thereof). The big ones, dark matter and dark energy, are strongly supported by observational evidence. But absolutely all of that evidence comes from cosmology and astrophysics. Figuring out what these mysterious cosmic components are and how they work might be the best hope we have to see where the theories should go next.

  Another thing pointing us toward cosmology is the strange imbalance between matter and antimatter in the universe. Where our current theories suggest matter and antimatter should exist in equal quantities, our experience in the world and our ability to avoid constantly being annihilated by everything we touch shows us that regular matter is winning by a very wide margin. How that came about is still a mystery, but it’s one for which the clues are likely to lie in deeper and more detailed studies of the early universe, when that asymmetry first occurred.

  Wherever we end up looking for data, in the quest for a theory of everything, we have two complementary approaches. One is to examine the phenomena we already see in nature that don’t fit into established physical theories, so we can build new and better theories to explain them. The other is to just try to break the theories we have—write down hypothetical extreme cases that might not have been tested yet, and see if we can find a new way to look at the data that will show us if the theory still works there. A combination of these two approaches is pretty much always how we move forward in physics. It’s how we went from Newtonian gravity, which works extremely well in everyday situations, to Einstein’s general relativity. GR would be massive overkill for a block sliding down an inclined plane, but is absolutely essential for explaining the bending of light around extremely massive objects in space or the tiny shifts in the orbit of Mercury deep in the gravitational well of the Sun.

  Newtonian gravity had to be replaced so we could move on to the superior general relativity; now it’s general relativity’s turn to be supplanted by the next big thing.

  But GR has been so resistant to these efforts, we might end up having to rearrange the entire universe instead.

  MAKING SPACE

  There’s a classic episode of Star Trek: The Next Generation in which, through a complicated series of events, Dr. Crusher ends up being the only person on the ship while it’s trapped in some kind of weird hazy bubble. So many strange things are happening, including the sudden disappearance of the rest of the crew, and it’s all at such odds with the readings of the sensors, her medical expertise tells her there’s a very good chance she’s hallucinating. But when her medical diagnostic fails to come up with any problem, she reaches the next logical conclusion: “If there’s nothing wrong with me,” she says, “maybe there’s something wrong with the universe!” As it happens (and, apologies for the spoiler, but the episode aired in 1990; you’ve had three decades to watch it), she’s absolutely right.

  For a while now, some physicists have suspected that the incongruous weakness of gravity might be forcing them to a similar conclusion. Maybe there’s nothing wrong with the strength of gravity. Maybe there’s something wrong with the universe that’s making gravity seem weaker than it really is.

  What could make gravity seem weak? The solution might end up being surprisingly mundane. It’s leaking. Into another dimension.

  Here’s how it works. As you’re probably aware, we usually think of our universe as having three dimensions of space (east-west, north-south, up-down). In relativity, we count time as a dimension as well, and we talk about locations in a 4D spacetime (a position in space and some moment on the past-future continuum). In the large extra dimensions scenario, there is another direction, or several, that we can’t access. All of the space part of our spacetime is limited to a 3D “brane”—think membrane—and a larger space extends outside it in some new direction (or directions) our limited human brains can only conceptualize mathematically. I should also mention that the “large” in “large extra dimensions” is a somewhat misleading term. Generally, if our universe does have extra dimensions, it might be effectively infinite in our usual three dimensions but extend no more than a millimeter in the new directions. (Imagine a large sheet of very thin paper—it’s technically a three-dimensional object even though two of its dimensions are much larger than the third.) But to a particle physicist, accustomed to measuring distances that make atoms look big, millimeters may as well be miles. Accordingly, we refer to the extra space outside our own brane as the “bulk.”

  In this scenario, particle physics and gravity still act fundamentally differently from each other, but not because of their inherent strength. The difference is that all the forces of nature in particle physics—electromagnetism and the strong and weak nuclear forces—are confined to live on the brane. To them, the larger, higher-dimensional bulk doesn’t exist. But gravity is not so limited. Gravity acts directly on spacetime, and that includes the spacetime outside of our 3D brane. So the gravity produced by a massive object in our own space loses a tiny bit of its apparent strength by leaking into the bulk, like an ink mark fading as it seeps into a sheet of paper. The fact that the new dimensions are so small compared to our ordinary ones means that this leakage isn’t really noticeable until you’re measuring the gravitational effects of things at millimeter distances, which is extremely hard to do. After all, most of the time, if you’re near an object and you step a millimeter farther away from it, you’re not going to notice that your gravitational pull toward it has appreciably reduced.

  Once you figure out how to make measurements on millimeter scales, though, you can test whether or not the decrease in gravity is what you expected from the standard equations. Going back to the ink-on-paper analogy, if you drop a gallon of ink on a sheet of paper, it’ll still look like you have a gallon of ink. But if you measure it by the drop, you’ll notice losing some when it soaks into the paper’s fibers. If extra dimensions are the width of millimeters, and you can measure gravity changing even on those kinds of scales, then the amount of gravity you’re losing to that extra dimensional bulk becomes comparable to the amount you’re trying to detect. You’ll see a drop-off of gravitational strength that’s more rapid than what you’d predict from general relativity in a nonleaky space, and that will make it obvious that something isn’t right.

  So far, while we still don’t have any alternative consensus on an explanation for gravity’s weakness, we also haven’t found any solid indication that this leakage is actually happening, despite getting better and better at measuring gravity at very small scales. As appealing as extra dimensions may sound from a theoretical point of view, their existence is still more in the category of intriguing po
ssibility than a confirmed feature of our cosmos. And to a large degree, the original motivation for them has faded, as almost all the most compelling theories explaining gravity’s weakness through leakage have been ruled out, since they predict alterations at levels we should have already detected. Still, we’re continuing the search, because if extra dimensions do turn out to be real, they offer an entirely new view of gravity, and of the universe. Our entire universe being on a brane contained within a larger spacetime brings up the possibility that there are other universes out there, perhaps on nearby branes, which may be capable of influencing our own with their gravity. Even more dramatically, interactions between branes could provide a new scenario for the origin of our universe. And, ultimately, its destruction.

  Enter: the ekpyrotic cosmos.

  COSMIC APPLAUSE

  The first time I encountered the ekpyrotic scenario for the origin (and fate) of the cosmos, it was at a very engaging physics lecture at Cambridge University by one of its originators, Neil Turok. The second time, it was in a science fiction story about aliens. It’s not often that somewhat esoteric theoretical constructions developed to solve complex problems in early universe physics show up in fiction, so it was something of a novelty at the time. The story, “Mixed Signals” by Lori Ann White and Ken Wharton, tells of a series of strange events that ultimately appear to be linked to gravitational waves. Specifically: weirdly powerful gravitational waves too regular to be due to the usual suspects—black holes or neutron stars colliding. Eventually the protagonists figure out the waves are a signal from intelligent beings, sent through the higher-dimensional bulk from another brane. The authors even name-check the ekpyrotic model, explaining that in this theory our universe is just one of several 3-dimensional branes in a higher-dimensional space, in which gravity but nothing else can travel. And if gravity can traverse the bulk, gravitational waves could be an excellent inter-brane communication mechanism.

  While the existence of other civilizations on nearby brane-universes was never technically ruled out as a possibility, the main purpose of the hypothesis was to explain the origin and destruction of this universe. Not long after the lecture and the sci-fi story, I ended up doing my PhD thesis on early universe physics with Paul Steinhardt, who worked with Neil Turok to come up with the ekpyrotic model. While I focused more on other theories of our cosmic origins, I encountered the ekpyrotic scenario on a regular basis at group meetings and discussions. (Somehow, aliens never really came up.)

  Since those days, the ekpyrotic scenario has been revised and generalized, and the latest version doesn’t include extra dimensions at all. But as often happens in science, a novel idea that may not work in the end can still spur on a different way of thinking about the problem—one that can lead us in a totally new (and hopefully better) direction. So let’s start with the original idea. It does, after all, present us with the possibility of an intriguingly dramatic cosmic end.

  The term “ekpyrotic” comes from the Greek for “conflagration,” a reference to the fiery origin and ultimate death of the universe in this scenario. In the standard, non-ekpyrotic story, the beginning of the universe included a period of cosmic inflation,II which we discussed in Chapter 2. Inflation causes a dramatic stretching of the cosmos during the first tiny fraction of a second, after which the decay of whatever was responsible for the stretching (we call it the inflaton field)III dumps a massive amount of energy into the cosmos to set up the “hot” phase of the Hot Big Bang. In the original version of the ekpyrotic model, on the other hand, the early universe is heated up by a spectacular collision of two adjacent 3D branes, one of which contains what will later become our whole cosmos. After the collision, the two branes go their separate ways, slowly drifting apart across the bulk and expanding. But they will come back. The ekpyrotic scenario is a cyclic one, with creation and destruction of the cosmos occurring over and over again.

  Personally, I find this whole thing makes much more sense if you employ the oldest tool in the physicist’s toolbox: hand-waving.

  Your left hand is our 3-brane: the 3-dimensional universe in which we live. (Obviously, none of this is to scale. This is hand-waving, after all.) Your right hand is another, “hidden” brane.IV First, place your hands together, with your fingers closed, in a prayer posture. This is the moment of cosmic creation. This is the collision that sets off the primordial fire. Both branes are filled with dense hot plasma at this moment, an unimaginably intense inferno forging the first atoms and carrying the humming plasma waves that, on our brane, we’ll later see as fluctuations in the cosmic microwave background light. Now, slowly separate your hands to a short distance apart, keeping them parallel, and spread your fingers. The branes have drifted apart across the higher-dimensional bulk, and the space in each brane is, independently, cooling and expanding in its own way. There’s no inflation phase in this model, just a steady expansion after the collision. And they’re not expanding into the bulk between them; they’re extending in their own branes parallel to each other. On our brane, your left hand, this is the cosmos we see today. While we can’t perceive our motion away from the other brane, we do see galaxies recede into the distance as the 3D space we live in expands, and our universe becomes more and more empty, heading toward a Heat Death. We don’t know what’s happening on your right hand, the hidden brane. Maybe there are civilizations there too, watching their own universe empty out as it traverses an unseen void. Maybe it’s a quiet, desolate place, where for whatever reason matter never learned to arrange itself into life. Maybe they have puppies who can talk. Unless we somehow detect a gravitational wave signal from the hidden brane, we may never know its true nature, or if it even exists.

  Now, let your hands slowly approach again and then suddenly slam your hands back together. In this scenario, after the branes have drifted to their maximum distance and expanded, they are attracted back together, to bounce off each other again. That clap—the bounce—has destroyed everything on both branes, ended our universe, and created a new Big Bang. Both universes are back in the hot phase, filled with a plasma inferno, a chaotic state whose reborn space harbors little or no physical remnant of what it once may have held. Now separate your hands and do the whole cycle again. And again. And again. A braneworldV ekpyrotic universe is eternal, cataclysmic, cosmic applause.

  ROUND AND ROUND AGAIN

  Whether or not we really live on a braneworld, and whether or not there are other branes out there across some higher-dimensional bulk, are still open questions. The general idea of a cyclic universe holds some appeal, though, as it is one of a very few reasonable alternatives to inflation with any chance of duplicating its successes.VI What shapes the ekpyrotic model and inflation will eventually take are yet to be determined—the newest ekpyrotic models don’t require branes at all, while some versions of inflation now do. The big difference between ekpyrotic models and inflation is that where inflation solves a number of cosmological problems by introducing a period of rapid expansion in the very early universe, the ekpyrotic model does it through slow contraction just before the bounce. In the case of the braneworld model, this is during the phase when the branes are coming together. Like inflation, an ekpyrotic model might be compatible with the distribution of matter we see in the universe today, and can potentially explain why our cosmos appears to be so very uniform and flat (in the sense of not curving back on itself or having some other complicated large-scale geometry). The fact that everything is weirdly uniform makes sense if the branes are huge and parallel before the bounce—it means that the bang can happen everywhere in the same way at the same time, with only some slight quantum fluctuations adding the necessary blips to set up the higher-density regions that will grow up to become galaxies and clusters of galaxies and all of cosmic structure.

  As with inflation, however, a lot of theoretical details are still being worked out. The biggest is the question of what exactly happens during the bounce. Does a true singularity occur? Or does the bounce happen without the ulti
mate maximum density being be reached, allowing for the possibility that information of some kind could survive the event and pass on into the next cycle? The latest version of the model has very little contraction, so nothing like a singularity occurs. Instead of using a collision between branes, the contraction in that model is driven by a scalar field, which is something similar to the Higgs field, or (possibly) to what we think might have caused inflation. That model does offer the tantalizing possibility that information might pass between cycles, and we could in principle someday see evidence of that.

  Which brings us to the question of observational evidence. Since both the ekpyrotic model and inflation were designed to solve the same cosmological problems, it may take a little creativity to confirm or rule out either of them. Everything we’ve seen so far in the cosmos seems compatible with the standard inflation picture, but we haven’t seen a smoking gun for it, nor have we seen anything that proves or kills off an ekpyrotic alternative. Arguments have gone back and forth for years about whether cyclic models are more or less theoretically appealing than inflation, but observationally it’s still an open question. Some data to finally decide the issue would be extremely helpful here.

  Our best bet might be finding evidence for primordial gravitational waves: large-scale ripples in space that would originate not from merging black holes or neutron stars but from the violence of the inflationary epoch, when the first seeds of cosmic structure were laid down by quantum wiggles in the inflaton field. If found, these would be as close as we’re likely to get to the elusive smoking gun for inflation. Briefly, in 2014, the cosmology community lit up with excitement as the leaders of an experiment called BICEP2VII announced that they’d seen evidence for exactly that. Looking at the polarization of the light from the cosmic microwave background, they saw what appeared to be twisting patterns that could only have come from gravitational waves distorting space during the epoch of primordial fire. These patterns were held up as a discovery so revolutionary Nobel Prizes were virtually guaranteed. After all, even setting aside the implications for inflation, they were a solid observation of gravitational waves (more than a year before LIGO saw its first black holes collide) and, because of the connection to quantum wiggles, they were the first evidence of any kind for the quantum nature of gravity.