by Katie Mack
Except, they weren’t.
After just a few months, physicists and astronomers outside the BICEP2 collaboration independently analyzed the data and found that the pattern could be completely explained by something much more mundane: ordinary cosmic dust, in our own Milky Way galaxy. If primordial gravitational waves had been discovered, they would have been evidence against the ekpyrotic model, since that model doesn’t include the inflationary universe-quake that could produce them. Unfortunately, their nondetection takes us back to square one. While inflation theory says that primordial gravitational waves must be produced, there’s nothing in the theory that says they have to be detectable. The most popular inflation models give you substantial gravitational waves, but it’s entirely possible to come up with one that produces a signal too weak to compete with the confusion of the cosmic dust.VII So the fact that the dust got in our way doesn’t prove that the inflation signal isn’t there, any more than it proves that it is.
Still, we might get clues from other sources. We might find evidence for or against braneworlds in the search for extra dimensions, or we might finally get a hint of those primordial gravitational waves. Even ordinary gravitational waves could hold clues, either by showing us a signal that travels through the bulk (via interdimensional aliens or not),IX or by helping us to map out the structure of spacetime by, essentially, watching how it wiggles. According to some studies, data from black hole collisions have already put a damper on theories involving gravity leaking out into a higher-dimensional void. So far, all our measurements are consistent with a plain old boring universe with only three spatial dimensions.
Whether or not we find extra dimensions, the idea of a cyclic universe will likely continue to hold appeal as an alternative to inflation. One reason is the problem of entropy, the ever-increasing disorder in the universe that ultimately leads to a Heat Death. We can calculate the amount of entropy in our observable universe, and we can look back through cosmic history to determine what it must have been at early times if it has been steadily increasing over the lifetime of the cosmos. The result is that the universe must have started at a shockingly low-entropy—highly ordered—state when our own cosmic history began. This is a deeply uncomfortable idea for a lot of cosmologists. How did the entropy get set so low at the beginning? It’s as if you walk into a room you’re sure no one has ever been in before and you find rows and rows of dominoes lying on the floor, overlapping as if they’ve just toppled upon each other in sequence. How did they all get so carefully set up in the first place?
A major bonus in certain cyclic and bouncing models is that they offer an opportunity to attribute that low initial entropy to something that happened before the bounce. The latest update to the ekpyrotic model, developed jointly by Paul Steinhardt and Anna Ijjas, explains the early universe’s low entropy by effectively taking all the entropy from a tiny patch of the pre-bounce universe and setting that as the initial entropy of the entire observable universe today.
This new model (which is so new that it appeared on the scene during the writing of this book) has some significant advantages over previous versions of the ekpyrotic scenario. In particular, it doesn’t require extra spatial dimensions or a singularity at the bounce. In fact, the contraction might be fairly mild—the reduction in size of the universe might be as low as a factor of two. The details are (obviously) complicated, but the basic idea is that what’s really cycling is the mix of ingredients in the universe, and the way observers would perceive its evolution. As mentioned before, it’s a scalar field filling the universe that drives the contraction/bounce, rather than a brane collision.
If this new cyclic model describes our universe, then in some far future epoch, we will start to see distant galaxies stop in their expansion and slowly turn around back toward us. It will look, at first, like the early stages of a Big Crunch, with the background radiation starting to heat up from “cool” to “this is not quite as cool” as the cosmos gets just a tiny bit more crowded. But just as we start to think maybe we should worry, we are, out of nowhere, suddenly and spectacularly obliterated when the scalar field violently converts its energy into radiation and starts the next Big Bang cycle of the universe.
Intriguingly, one aspect this brand-new hot-off-the-presses version of the ekpyrotic model shares with the old one is that rogue gravitational waves could be a kind of inter-universe signal. In the old version, it’s conceivable some gravitational waves could pass through the bulk from another brane. In this one, since the cosmos never gets truly small during the bounce, gravitational waves might pass from one cycle to the next. These signals would be incredibly hard to find, but if they existed, they could present us with clues about a universe before our own.
Watch this space.
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Of course, ekpyrotic models are not the only way to put some bounce in our cosmic step.
Roger Penrose, an early pioneer of modern cosmology who fundamentally changed the way we look at gravity in the universe, has his own proposal for a cycling cosmos, in which our Big Bang is born from a previous cycle’s Heat Death. It involves piecing together the far future spacetime of one universe and the singularity at the beginning of another. Penrose has been, for decades, one of the most prominent voices in cosmology pointing out the seriousness of the entropy problem in standard early universe scenarios. And he does not think inflation does the trick. He told me recently, “When I first heard about it, I thought, well, that theory won’t last a week.”
Penrose’s alternative model, called Conformal Cyclic Cosmology, conjectures that entropy works differently in the vicinity of singularities. If the conjecture is true, it implies that the entropy would be very low at the boundary between cycles from which our universe begins, and it doesn’t require inflation. Penrose’s model also contains the intriguing possibility that some imprint of the events that occurred in past cycles might appear in astronomical observations, showing up as features in the cosmic microwave background. In fact, Penrose and his collaborators have claimed that evidence for such features can already be seen in the data, though this has been met with skepticism. Whether or not these possible CMB hints will someday be seen as a compelling sign of a pre–Big Bang universe is yet to be determined.
Meanwhile, ekpyrotic-model co-developer Neil Turok has shifted focus to dive into a totally new model of the universe in which the Big Bang is merely a transition point. This proposal, developed by Latham Boyle, Turok, and their former student Kieran Finn, is motivated by taking symmetry arguments in particle physics to a cosmic level; it suggests that our universe and a time-reversed version of the cosmos meet at the Big Bang like two cones touching tip to tip. In a recent paper, they describe the picture as “a universe-antiuniverse pair, emerging from nothing.” It’s possible that cone-tip singularity might contain its own solution to the entropy problem, though the model and its details are (at time of this writing) still under development. Nonetheless, it makes some specific predictions for the nature of dark matter, and thus might be testable with upcoming experiments.
* * *
So where do we go from here? Was the Big Bang unique, or just a violent transition point? Will our cosmic existence be dramatically cut short by another universe coming down on us like a higher-dimensional flyswatter? Will data from cosmology or particle physics ever reveal the true nature of spacetime? How close are we to finding out what our cosmic far future holds, and what new information do we need to answer the question, once and for all?
How will it all end?
Like everything in science, our understanding of the cosmos is a perpetual work in progress. But that progress has, over the last few decades, been extraordinary, and new insights are coming fast. Over just the next few years, humanity will gain new tools that will give us an unprecedented view of our cosmic history, allowing us to piece together the story of our origins and open new windows onto the Big Bang, dark matter, dark energy, and our trajectory into the future. In the final chapter of this s
tory, we’ll get a glimpse of what those new tools might show us, and how work on the cutting edge of physics is already pointing us toward a universe far stranger than we ever could have imagined.
I. “Simple” here may be a matter of perspective. Working with the equations of general relativity requires a deep understanding of differential geometry, which is the sort of thing you pretty much only study if you’re doing graduate work in physics or mathematics. But if you ARE such a person, the equations are as elegant and transparent as fine-blown glass.
II. That thing where a first-draft theory is dramatically redesigned but still useful applies to inflation too. The original version of inflation is widely considered to be a stroke of genius in spite of being, in the end, a total failure. It didn’t work at all, and was totally revamped by other physicists within about a year. What its originators did exactly right was to propose a general class of solutions that became the spark of a firestorm of creative ways to finally make the Big Bang work. The revamped version, what we sometimes refer to as “new inflation,” became the basis of the kind of inflation we all talk about today.
III. Because we like to give particles and their associated fields names that end in “on.”
IV. Each of these branes is designated in the official literature as an “end-of-the-world” brane, because it lies at the boundary of the space. This seems fitting.
V. The term “braneworld” specifically refers to models in which there are higher dimensions and our observable universe lives on a 3D brane within a larger space. It’s sort of a type of multiverse, but usually when people talk about a multiverse they’re referring to something different, like regions of a larger (3D) space where the laws of physics might be different, or even the Many Worlds interpretation of quantum mechanics, which is another matter entirely. Any construction that allows there to be more to reality than our observable cosmic volume is a kind of multiverse theory.
VI. I am using “cyclic” and “bouncing” more or less interchangeably here, but a bouncing model doesn’t have to be cyclic, in the sense that there could be only one “bounce”—a transition from some past long-lived pre–Big Bang phase to our current universe, which then dies out on its own without producing a new universe afterward.
VII. The second iteration of the Background Imaging of Cosmic Extragalactic Polarization experiment.
VII. Technically, depending on the model, you could also get some tiny tiny level of primordial gravitational waves in an ekpyrotic universe during the slow contraction phase. But they’d be WAY too small to ever show up in observations.
IX. The idea that there might be matter on the hidden brane has been discussed in the literature, but as far as I’m aware, detecting black hole collisions across the bulk has not. Perhaps it would require too many levels of speculation for a serious study. But I think it sounds like fun.
CHAPTER 8: Future of the Future
How big the hourglass?
How deep the sand?
I shouldn’t hope to know, but here I stand.
Hozier, “No Plan”
In 1969, Martin Rees was not yet Astronomer Royal, Lord Rees, Baron of Ludlow. He was a postdoc cosmologist at Cambridge University, thinking about the end of everything, publishing a six-page paper titled “The Collapse of the Universe: An Eschatological Study,” which he would later describe as “rather fun.” In the introduction, Rees explained that while the observational evidence was still uncertain, it indicated that “the universe is indeed fated to collapse. All structural features of the cosmic scene would be destroyed during this devastating compression.” Part of what made the paper fun to Rees was calculating that, in the coming collapse, all the stars will be destroyed by ambient radiation, from the outside in. Who wouldn’t enjoy the thought of stars catching fire?
Despite Rees’s arguments in favor of a Big Crunch, the data remained ambiguous for decades. Was the universe closed (recollapsing) or open (eternally expanding)? In 1979, Freeman Dyson, at the Institute for Advanced Study in Princeton, decided to explore the other side of the argument, saying, “I shall not discuss the closed universe in detail, since it gives me a feeling of claustrophobia to imagine our whole existence confined within the box.” The open universe model was a pleasantly roomy alternative. In his paper “Time Without End: Physics and Biology in an Open Universe,” he worked through quantitative predictions of what an open universe might mean for humanity, working out a method by which future beings might, through regulating their activity and entering periods of hibernation, avoid oblivion into the infinite future as the rest of the cosmos dissolves around them.I While most of the paper consists of calculations and theoretical discussions, the introduction contains some sharp words aimed at the physics mainstream for unfairly disdaining the whole endeavor of studying the cosmic end times. “The study of the remote future still seems to be as disreputable today as the study of the remote past was thirty years ago,” he wrote, pointing out the scarcity of serious papers approaching the subject.II He continued with a cosmological call to arms: “If our analysis of the long-range future leads us to raise questions related to the ultimate meaning and purpose of life, then let us examine these questions boldly and without embarrassment.”
I can’t exactly say that cosmic eschatology has, after all this time, finally received its proper level of respect as an academic discipline. It’s still rather rare to find papers in the physics literature that examine our ultimate fate with the same rigor and depth as they do our origins. But studies of both ends of the timeline help us, in different ways, to examine the principles of our physical theories. Beyond the insight they might provide into our future or past, they can help us understand the fundamental nature of reality itself.
“By thinking about the end of the universe, just like with its beginning, you can sharpen your own thinking about what you think is happening now, and how to extrapolate. I feel like extrapolations in fundamental physics are essential,” says Hiranya Peiris, a cosmologist at University College London. In 2003, she led one of the teams interpreting the first detailed view of the cosmic microwave background with the Wilkinson Microwave Anisotropy Probe (WMAP) satellite and she has since then maintained a position at the leading edge of observational cosmology. In recent years, she’s set her sights on using observational data, simulations, and tabletop analogs to test some of the key elements of early- and late-universe physics like the creation of “bubble universes” in cosmic inflation and the mechanics behind vacuum decay. In studying all these questions, her motivation is the same. “I know this period needs to be understood. How what we’re doing now will map directly onto those periods is still not clear, but I think we’ll learn something about fundamental theory by doing this work.”
We certainly have a lot to learn. Cosmology and particle physics are in an awkward position at the moment; both have, in some ways, been victims of their own success. In each field, we have a very precise and comprehensive description of the world that works extremely well in the sense that nothing has been found to contradict it. The downside is that we have no idea why it works.
The reigning paradigm in cosmology is called the Concordance Model, or ΛCDM. In this picture, the universe has four basic components: radiation, regular matter, dark matter (specifically “cold” dark matter, CDM), and dark energy in the form of a cosmological constant (denoted in equations by the Greek letter lambda, Λ). The quantities of all these components are precisely measured, with the cosmological constant currently making up the largest slice of the cosmic pie. We have a good understanding of how these things have all varied over time as the universe has expanded, and we have an amazingly detailed description of the very early universe that includes a period of very rapid expansion called inflation. We also have a tried-and-tested theory of gravity, Einstein’s general relativity, which in the Concordance Model is taken to be completely correct. In this picture, because the cosmological constant is currently dominating the evolution of the cosmos, we can straightforwa
rdly apply our understanding of gravity and the components of the universe to determine our cosmic evolution. Doing this leads us unambiguously to a Heat Death in the far future. And that’s that.
The problem with the Concordance Model is that the most important elements of it—dark matter, the cosmological constant, and inflation—are completely mysterious. We don’t know what dark matter is; we don’t know how inflation happened (or if it even really did happen); and we have no reasonable explanation for why the cosmological constant exists or why it takes a value that seems to fly in the face of what we expect from particle physics. At the same time, we haven’t found anything in the data to contradict the model. No evidence that dark energy evolves in some way (which would go against a cosmological constant), no evidence that dark matter is anything experimentally detectable (and no evidence that it’s not), and despite a century of putting it through the experimental ringer, no evidence for gravity behaving like anything other than Einstein’s general relativity.