cosmic inflation

Physicists Hunt for the Big Bang’s Triangles

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The story of the universe’s birth — and evidence for string theory — could be found in triangles and myriad other shapes in the sky.

Once upon a time, about 13.8 billion years ago, our universe sprang from a quantum speck, ballooning to one million trillion trillion trillion trillion trillion trillion times its initial volume (by some estimates) in less than a billionth of a trillionth of a trillionth of a second. It then continued to expand at a mellower rate, in accordance with the known laws of physics.

So goes the story of cosmic inflation, the modern version of the Big Bang theory. That single short, outrageous growth spurt fits all existing cosmological data well and accounts for the universe’s largeness, smoothness, flatness and lack of preferred direction. But as an explanation of how and why the universe began, inflation falls short. The questions it raises — why the growth spurt happened, how it happened, what (if anything) occurred beforehand — have confounded cosmologists since the theory emerged in the 1980s. “We have very strong evidence that there was this period of inflation,” said Matthew Kleban, a cosmologist at New York University. “But we have no idea — or we have many, many ideas — too many ideas — what inflation was, fundamentally.”

To understand the origin of the universe, today’s cosmologists seek to identify the unknown driver of inflation, dubbed the “inflaton.” Often envisioned as a field of energy permeating space and driving it apart, the inflaton worked, experts say, like a clock. With each tick, it doubled the size of the universe, keeping nearly perfect time — until it stopped. Theorists like Kleban, then, are the clocksmiths, devising altogether hundreds of different models that might replicate the clockwork of the Big Bang.

Like many cosmological clocksmiths, Kleban is an expert in string theory — the dominant candidate for a “theory of everything” that attempts to describe nature across all distances, times and energies. The known equations of physics falter when applied to the tiny, fleeting and frenzied environment of the Big Bang, in which they struggle to cram an enormous amount of energy into infinitesimal space and time. But string theory flourishes in this milieu, positing extra spatial dimensions that diffuse the energy. Familiar point particles become, at this highest energy and zoom level, one-dimensional “strings” and higher-dimensional, membranous “branes,” all of which traverse a 10-dimensional landscape. These vibrating, undulating gears may have powered the Big Bang’s clock.

At his office on a recent afternoon, Kleban sketched his latest inflaton design on the blackboard. First, he drew a skinny cylinder to depict the string landscape. Its length represented the three spatial dimensions of macroscopic reality, and its circumference signified the six other spatial dimensions that string theory says exist, but which are too small to see. On the side of the cylinder, he drew a circle. This is Kleban’s timepiece: a membrane that bubbles into being and naturally expands. As its inflating interior forms a new universe, its energy incrementally ticks down in clocklike fashion each time the expanding circle winds around the cylinder’s circumference and overlaps itself. When the energy of the “brane” dilutes, the clock stops ticking, and inflation ends. It’s a scheme that some string cosmologists have hailed for its economy. “I think it’s pretty plausible that some version of this happens,” he said.

Though Kleban acknowledges that it’s too soon to tell whether he or anyone else is on to something, plans are under way to find out.

The record of the inflaton’s breakneck ticking can be read in the distribution of galaxies, galaxy clusters and superclusters that span the cosmos. These structures (and everything in them, including you) are artifacts of “mistakes in the clock,” as Matias Zaldarriaga, a cosmologist at the Institute for Advanced Study (IAS) in Princeton, N.J., put it. That is, time is intrinsically uncertain, and so the universe inflated at slightly different rates in different places and moments, producing density variations throughout. The jitter in time can also be thought of as a jitter in energy that occurred as pairs of particles spontaneously surfaced all over an “inflaton field” and stretched apart like two points on an inflating balloon. These particles were the seeds that gravity grew into galactic structures over the course of eons. The pairs of structures spanning the largest distances in the sky today came from the earliest quantum fluctuations during inflation, while structures that are closer together were produced later. This nested distribution across all cosmic distance scales “is telling you in detail that the clock was ticking,” said Nima Arkani-Hamed, a theoretical physicist at IAS. “But it doesn’t tell you anything about what it was made of.”

To reverse-engineer the clockwork, cosmologists are seeking a new kind of data. Their calculations indicate that galaxies and other structures are not merely randomly spread out in pairs across the sky; instead, they have a slight tendency to be arranged in more complex configurations: triangles, rectangles, pentagons and all manner of other shapes, which trace back not just to quantum jitter in the Big Bang’s clock, but to a much more meaningful turning of the gears.

Teasing out the cosmological triangles and other shapes — which have been named “non-Gaussianities” to contrast them with the Gaussian bell curve of randomly distributed pairs of structures — will require more precise observations of the cosmos than have been made to date. And so plans are being laid for a timeline of increasingly sensitive experiments. “We’re going to have far more information than we have now, and sensitivity to far subtler effects than we can probe now,” said Marc Kamionkowski, a cosmologist at Johns Hopkins University. In the meantime, theorists are making significant progress in determining what shapes to look for and how to look for them. “There’s been a great renaissance of understanding,” said Eva Silverstein, a string cosmologist at Stanford University who devised the dimensional-winding mechanism used by Kleban, as well as many clock designs of her own.

The rigorous study of non-Gaussianities took off in 2002, when Juan Maldacena, a revered, monklike theorist at IAS, calculated what’s known as the “gravitational floor”: the minimum number of triangles and other shapes that are guaranteed to exist in the sky, due to the unavoidable effect of gravity during cosmic inflation. Cosmologists had been struggling to calculate the gravitational floor for more than a decade, since it would provide a concrete goal for experimenters. If the floor is reached, and still no triangles are detected, Maldacena explained, “then inflation is wrong.”

When Maldacena first calculated the gravitational floor, actually detecting it seemed a distant goal indeed. At the time, all precise knowledge of the universe’s birth came from observations of the “cosmic microwave background” — the oldest light in the sky, which illuminates a two-dimensional slice of the infant universe as it appeared 380,000 years after the Big Bang. Based on the limited number of nascent structures that appear in this 2-D snapshot, it seemed impossible that their slight propensity to be configured in triangles and other shapes could ever be detected with statistical certainty. But Maldacena’s work gave theorists the tools to calculate other, more pronounced forms of non-Gaussianity that might exist in the sky, due to stronger effects than gravity. And it motivated researchers to devise better ways to search for the signals.

A year after Maldacena made his calculation, Zaldarriaga and collaborators showed that measuring the distribution of galaxies and groupings of galaxies that make up the universe’s “large-scale structure” would yield many more shapes than observing the cosmic microwave background. “It’s a 3-D versus 2-D argument,” said Olivier Doré, a cosmologist at NASA’s Jet Propulsion Laboratory who is working on a proposed search for non-Gaussianities in the large-scale structure. “If you start counting triangles in 3-D like you can do with galaxy surveys, there are really many more you can count.”

The notion that counting more shapes in the sky will reveal more details of the Big Bang is implied in a central principle of quantum physics known as “unitarity.” Unitarity dictates that the probabilities of all possible quantum states of the universe must add up to one, now and forever; thus, information, which is stored in quantum states, can never be lost — only scrambled. This means that all information about the birth of the cosmos remains encoded in its present state, and the more precisely cosmologists know the latter, the more they can learn about the former.

But how did details of the Big Bang get encoded in triangles and other shapes? According to Zaldarriaga, Maldacena’s calculation “opened up the understanding of how it comes about.” In a universe governed by quantum mechanics, all of nature’s constituents are cross-wired, morphing into and interacting with one another with varying degrees of probability. This includes the inflaton field, the gravitational field, and whatever else existed in the primordial universe: Particles arising in these fields would have morphed into and scattered with each other to produce triangles and other geometric configurations, like billiard balls scattering on a table.

Lucy Reading-Ikkanda for Quanta Magazine

These dynamical events would be mixed in with the more mundane quantum jitter from those particle pairs that popped up in the inflaton field and engendered so-called “two-point correlations” throughout the sky. A pair of particles might, for instance, have surfaced in some other primordial field, and one member of this pair might then have decayed into two inflaton particles while the other decayed into just a single inflaton particle, yielding a three-point correlation, or triangle, in the sky. Or, two mystery particles might have collided and split into four inflaton particles, producing a four-point correlation. Rarer events would have yielded five-point, six-point and even higher-point correlations, with their numbers, sizes and interior angles encoding the types and relationships of the particles that produced them. The unitarity principle promises that by tallying the shapes ever more precisely, cosmologists will achieve an increasingly detailed account of the primordial universe, just as physicists at the Large Hadron Collider in Europe hone their theory of the known particles and look for evidence of new ones by collecting statistics on how particles morph and scatter during collisions.

Following Maldacena’s calculation of the gravitational floor, other researchers demonstrated that even many simple inflationary models generate much more pronounced non-Gaussianity than the bare minimum. Clocksmiths like Silverstein and Kleban have since been busy working out the distinct set of triangles that their models would produce — predictions that will become increasingly testable in the coming years. Progress accelerated in 2014, when a small experiment based at the South Pole appeared to make a momentous discovery about the universe’s birth. The announcement drummed up interest in cosmological triangles, even though the supposed discovery ultimately proved a grave disappointment.

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As news began to spread on March 17, 2014, that the “smoking gun” of cosmic inflation had been detected, Stanford University’s press office posted a celebratory video on YouTube. In the footage, the cosmologist Andrei Linde, one of the decorated pioneers of inflationary cosmology, and his wife, the string and supergravity theorist and cosmologist Renata Kallosh, answer their door to find their Stanford colleague Chao-Lin Kuo on the doorstep, accompanied by a camera crew.

“It’s five sigma, at point two,” Kuo says in the video.

“Discovery?” Kallosh asks, after a beat. She hugs Kuo, almost melting, as Linde exclaims, “What?”

Viewers learn that BICEP2, an experiment co-led by Kuo, has detected a swirl pattern in the cosmic microwave background that would have been imprinted by ripples in space-time known as “primordial gravitational waves.” And these could only have arisen during cosmic inflation, as corkscrew-like particles popped up in the gravitational field and then became stretched and permanently frozen into the shape of the universe.

In the next scene, Linde sips champagne with his wife and their guest. In the early 1980s, Linde, Alexei Starobinsky, Alan Guth and other young cosmologists devised the theory of cosmic inflation as a patch for the broken 1930s-era Big Bang theory, which described the universe as expanding outward from a “singularity” — a nonsensical point of infinite density — and couldn’t explain why the universe hadn’t become mottled and contorted as it grew. Cosmic inflation provided a clever fix for these problems, and BICEP2’s finding suggested that the theory was conclusively proved. “If this is true,” Linde says to the camera, “this is a moment of understanding of nature of such a magnitude that it just overwhelms. Let’s see. Let’s just hope that this is not a trick.”

To many researchers, the most exciting thing about the alleged discovery was the strength of the swirl signal, measured as r = 0.2. The measurement indicated that inflation occurred at an extremely high energy scale and at the earliest moments in time, near the time-energy domain where gravity, as well as the effects of strings, branes or other exotica, would have been strong. The higher the energy scale of inflation, the more cross-wiring there would be between the inflaton and these other primordial ingredients. The result would be pronounced triangles and other non-Gaussianities in the sky.

“After BICEP, we all stopped what we were doing and started thinking about inflation,” Arkani-Hamed said. “Inflation is like having a gigantic particle accelerator at much higher energy scales than you can get to on Earth.” The question became how such an accelerator would operate, he said, “and if there really was exotic stuff up there [near the inflation scale], how you could go about looking for it.”

As these investigations took off, more details of BICEP2’s analysis emerged. It became clear that the discovery was indeed a trick of nature: The team’s telescope at the South Pole had picked up the swirly glow of galactic dust rather than the effect of primordial gravitational waves. A mix of anguish and anger swept through the field. Two years on, primordial gravitational waves still haven’t been detected. In January, BICEP2’s successor, the BICEP/Keck Array, reported that the value of r can be no more than 0.07, which lowers the ceiling on the energy scale of inflation and moves it further below the scale of strings or other exotic physics.

Nonetheless, many researchers were now aware of the potential gold mine of information contained in triangles and other non-Gaussianities. It had become apparent that these fossils from inflation were worth digging for, even if they were buried deeper than BICEP2 had briefly promised. “Yeah, rwent down a little bit,” Maldacena said. But it’s not so bad, in his opinion: A relatively high scale is still possible.

In a paper last spring that drew on previous work by other researchers, Maldacena and Arkani-Hamed used symmetry arguments to show that a key feature of string theory could manifest itself in triangles. String theory predicts an infinite tower of “higher-spin states” — essentially, strings vibrating at an infinitely rising sequence of pitches. So far, no fundamental particles with a “spin” value greater than two have been discovered. Maldacena and Arkani-Hamed showed that the existence of such a higher-spin state would result in alternating peaks and troughs in the strength of the signal produced by triangles in the sky as they grow more elongated. For string theorists, this is exciting. “You can’t build a consistent interacting theory of such a particle except if you have an infinite tower of them” like the tower in string theory, explained Daniel Baumann, a theoretical cosmologist at the University of Amsterdam. Finding the oscillatory pattern in the triangles in the sky would confirm that this tower exists. “Just seeing one particle of spin greater than two would be indicative of string theory being present.”

Other researchers are pursuing similarly general predictions. In February, Kamionkowski and collaborators reported detailed information about primordial particles that is encoded in the geometry of four-point correlations, which “get interesting,” he said, because four points can lie flat or sweep into the third dimension. Observing the signals predicted by Arkani-Hamed, Maldacena and Kamionkowski would be like striking gold, but the gold is buried deep: Their strength is probably near the gravitational floor and will require at least 1,000 times the sensitivity of current equipment to detect. Other researchers prefer to tinker with bespoke string models that predict more pronounced triangles and other shapes. “So far we’ve explored only, I think, a very small fraction of the possibilities for non-Gaussianity,” Kamionkowski said.

Meanwhile, Linde and Kallosh are pushing in a totally different direction. Over the past three years, they’ve become enamored with a class of models called “cosmological alpha-attractors” that do not predict any non-Gaussianities above the gravitational floor at all. According to these models, cosmic inflation was completely pure, driven by a solitary inflaton field. The field is described by a Kähler manifold, which maps onto the geometric disk seen in Escher’s drawing of angels and devils. The Escherian geometry provides a continuum of possible values for the energy scale of inflation, including values so low that the inflaton’s cross-wiring to the gravitational field and other primordial fields would be extremely weak. If such a model does describe the universe, then swirls, triangles and other shapes might never be detected.

Linde isn’t bothered by this. In supporting the alpha-attractor models, he and Kallosh are staking a position in favor of simplicity and theoretical beauty, at the expense of ever knowing for sure whether their cosmological origin story is correct. An alpha-attractor universe, Linde said, is like one of the happy families in the famous opening line of Anna Karenina. As he paraphrased Tolstoy: “Any happy family, well, they look in a sense alike. But all unhappy families — they’re unhappy for different reasons.”

Will our universe turn out to be “happy” and completely free of distinguishing features? Baumann, who co-authored a book last year on string cosmology, argues that models like Linde’s and Kallosh’s are too simple to be plausible. “They are building these models from the bottom up,” he said. “Introducing a single field, trying to be very minimal — it would have been a beautiful model of the world.” But, he said, when you try to embed inflation into a fundamental theory of nature, it’s very hard to engineer a single field acting by itself, immune to the effects of everything else. “String theory has many of these effects; you can’t ignore them.”

And so the search for triangles and other non-Gaussianities is under way. Between 2009 and 2013, the Planck space telescope mapped the cosmic microwave background at the highest resolution yet, and scientists have since been scouring the map for statistical excesses of triangles and other shapes. As of their most recent analysis, they haven’t found any; given the sensitivity of their instruments and their 2-D searching ground, they only ever had an outside chance of doing so. But the scientists are continuing to parse the data in new ways, with another non-Gaussianity analysis expected this year.

Hiranya Peiris, an astrophysicist at University College London who searches for non-Gaussianities in the Planck data, said that she and her collaborators are taking cues from string cosmologists in determining which signals to look for. Peiris is keen to test a string-inflationary mechanism called axion monodromy, including variants recently developed by Silverstein and collaborators Raphael Flauger, Mehrdad Mirbabayi, and Leonardo Senatore that generate an oscillatory pattern in triangles as a function of their size that can be much more pronounced than the pattern studied by Arkani-Hamed and Maldacena. To find such a signal, Peiris and her team must construct templates of the pattern and match them with the data “in a very numerically intensive and demanding analysis,” she said. “Then we have to do careful statistical tests to make sure we are not being fooled by random fluctuations in the data.”

Some string models have already been ruled out by this data analysis. Regarding the public debate about whether string theory is too divorced from empirical testing to count as science, Silverstein said, “I find it surreal, because we are currently doing some traditional science with string theory.”

Now under construction, the Large Synoptic Survey Telescope in Chile will be used to map 20 billion cosmological objects starting in 2023.

Now under construction, the Large Synoptic Survey Telescope in Chile will be used to map 20 billion cosmological objects starting in 2023.

 

Moving forward, cosmologists plan to scour ever larger volumes of the universe’s large-scale structure. Starting in 2020, the proposed SPHEREx mission could measure non-Gaussianity sensitively enough in the distribution of 300 million galaxies to determine whether inflation was driven by one clock or two cross-wired clocks (according to models of the theory known as single- and multi-field inflation, respectively). “Just to reach this level would dramatically reduce the number of possible inflation theories,” said Doré, who is working on the SPHEREx project. A few years further out, the Large Synoptic Survey Telescope will map 20 billion cosmological structures. If the statistical presence of triangles is not detected in the universe’s large-scale structure, there is yet another, perhaps final, approach. By mapping an ultra-faint radio signal called the 21-centimeter line, which is emitted by hydrogen atoms and traces back to the creation of the first stars, cosmologists would be able to measure even more “modes,” or arrangements of structures. “It’s going to have information about the whole volume of the universe,” Maldacena said.

If or when triangles show up, they will, one by one, reveal the nature of the inflaton clock and why it ticked. But will enough clues be gathered before we run out of sky in which to gather them?

The promise of unitarity — that information can be scrambled but never lost — comes with a caveat.

“If we assume we can make perfect measurements and we have an infinite sky and so on,” Maldacena said, “then in principle all the interactions and information about particles during inflation is contained in these correlators” — that is, three-point correlations, four-point correlations and so on. But perfect measurements are impossible. And worse, the sky is finite. There is a cosmic horizon: the farthest distance from which light has had time to reach us, and thus, beyond which we cannot see. During inflation, and over the entire history of the accelerating expansion of the universe since then, swirls, triangles, quadrilaterals and other shapes have been flying past this horizon and out of sight. And with them, the subtlest of signals, associated with the rarest, highest-energy processes during inflation, are lost: Cosmologists will never be able to gather enough statistics in our finite patch of sky to tease them out, precluding a complete accounting of nature’s fundamental constituents.

In his paper with Maldacena, Arkani-Hamed initially included a discussion of this issue, but he removed most of it. He finds the possibility of a limit to knowledge “tremendously disturbing” and sees it as evidence that quantum mechanics must be extended. One possible way to do this is suggested by his work on the amplituhedron, which casts quantum mechanical probabilities (and with them, unitarity) as emergent consequences of an underlying geometry. He plans to discuss this possibility in a forthcoming paper that will relate an analogue of the amplituhedron to non-Gaussianities in the sky.

People vary in the extent to which they are bothered by a limit to knowledge. “I’m more practical,” Zaldarriaga said. “There are, like, tens or many tens or orders of magnitude more modes that in principle we could see, that we have not been able to measure just because of technological or theoretical inability. So, these ‘in principle’ questions are interesting, but we are way before this point.”

Kleban also feels hopeful. “Yeah, it’s a finite amount of information,” he said. “But you could say the same thing about evolution, right? There’s a limited number of fossils, and yet we have a pretty good idea of what happened, and it’s getting better and better.”

If all goes well, enough fossils will turn up in the sky to tell a more complete story. A vast searching ground awaits.

How the Universe Got Its Bounce Back

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Humans have always entertained two basic theories about the origin of the universe. “In one of them, the universe emerges in a single instant of creation (as in the Jewish-Christian and the Brazilian Carajás cosmogonies),” the cosmologists Mario Novello and Santiago Perez-Bergliaffa noted in 2008. In the other, “the universe is eternal, consisting of an infinite series of cycles (as in the cosmogonies of the Babylonians and Egyptians).” The division in modern cosmology “somehow parallels that of the cosmogonic myths,” Novello and Perez-Bergliaffa wrote.

In recent decades, it hasn’t seemed like much of a contest. The Big Bang theory, standard stuff of textbooks and television shows, enjoys strong support among today’s cosmologists. The rival eternal-universe picture had the edge a century ago, but it lost ground as astronomers observed that the cosmos is expanding and that it was small and simple about 14 billion years ago. In the most popular modern version of the theory, the Big Bang began with an episode called “cosmic inflation” — a burst of exponential expansion during which an infinitesimal speck of space-time ballooned into a smooth, flat, macroscopic cosmos, which expanded more gently thereafter.

With a single initial ingredient (the “inflaton field”), inflationary models reproduce many broad-brush features of the cosmos today. But as an origin story, inflation is lacking; it raises questions about what preceded it and where that initial, inflaton-laden speck came from. Undeterred, many theorists think the inflaton field must fit naturally into a more complete, though still unknown, theory of time’s origin.

But in the past few years, a growing number of cosmologists have cautiously revisited the alternative. They say the Big Bang might instead have been a Big Bounce. Some cosmologists favor a picture in which the universe expands and contracts cyclically like a lung, bouncing each time it shrinks to a certain size, while others propose that the cosmos only bounced once — that it had been contracting, before the bounce, since the infinite past, and that it will expand forever after. In either model, time continues into the past and future without end.

With modern science, there’s hope of settling this ancient debate. In the years ahead, telescopes could find definitive evidence for cosmic inflation. During the primordial growth spurt — if it happened — quantum ripples in the fabric of space-time would have become stretched and later imprinted as subtle swirls in the polarization of ancient light called the cosmic microwave background. Current and future telescope experiments are hunting for these swirls. If they aren’t seen in the next couple of decades, this won’t entirely disprove inflation (the telltale swirls could simply be too faint to make out), but it will strengthen the case for bounce cosmology, which doesn’t predict the swirl pattern.

Already, several groups are making progress at once. Most significantly, in the last year, physicists have come up with two new ways that bounces could conceivably occur. One of the models, described in a paper that will appear in the Journal of Cosmology and Astroparticle Physics, comes from Anna Ijjas of Columbia University, extending earlier work with her former adviser, the Princeton professor and high-profile bounce cosmologist Paul Steinhardt. More surprisingly, the other new bounce solution, accepted for publication in Physical Review D, was proposed by Peter Graham, David Kaplan and Surjeet Rajendran, a well-known trio of collaborators who mainly focus on particle physics questions and have no previous connection to the bounce cosmology community. It’s a noteworthy development in a field that’s highly polarized on the bang vs. bounce question.

The question gained renewed significance in 2001, when Steinhardt and three other cosmologists argued that a period of slow contraction in the history of the universe could explain its exceptional smoothness and flatness, as witnessed today, even after a bounce — with no need for a period of inflation.

The universe’s impeccable plainness, the fact that no region of sky contains significantly more matter than any other and that space is breathtakingly flat as far as telescopes can see, is a mystery. To match its present uniformity, experts infer that the cosmos, when it was one centimeter across, must have had the same density everywhere to within one part in 100,000. But as it grew from an even smaller size, matter and energy ought to have immediately clumped together and contorted space-time. Why don’t our telescopes see a universe wrecked by gravity?

“Inflation was motivated by the idea that that was crazy to have to assume the universe came out so smooth and not curved,” said the cosmologist Neil Turok, director of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and co-author of the 2001 paper on cosmic contractionwith Steinhardt, Justin Khouryand Burt Ovrut. In the inflation scenario, the centimeter-size region results from the exponential expansion of a much smaller region — an initial speck measuring no more than a trillionth of a trillionth of a centimeter across. As long as that speck was infused with an inflaton field that was smooth and flat, meaning its energy concentration didn’t fluctuate across time or space, the speck would have inflated into a huge, smooth universe like ours. Raman Sundrum, a theoretical physicist at the University of Maryland, said the thing he appreciates about inflation is that “it has a kind of fault tolerance built in.” If, during this explosive growth phase, there was a buildup of energy that bent space-time in a certain place, the concentration would have quickly inflated away. “You make small changes against what you see in the data and you see the return to the behavior that the data suggests,” Sundrum said.

However, where exactly that infinitesimal speck came from, and why it came out so smooth and flat itself to begin with, no one knows. Theorists have found many possible ways to embed the inflaton field into string theory, a candidate for the underlying quantum theory of gravity. So far, there’s no evidence for or against these ideas.

Cosmic inflation also has a controversial consequence. The theory — which was pioneered in the 1980s by Alan Guth, Andrei Linde, Aleksei Starobinsky and (of all people) Steinhardt, almost automatically leads to the hypothesis that our universe is a random bubble in an infinite, frothing multiverse sea. Once inflation starts, calculations suggest that it keeps going forever, only stopping in local pockets that then blossom into bubble universes like ours. The possibility of an eternally inflating multiverse suggests that our particular bubble might never be fully understandable on its own terms, since everything that can possibly happen in a multiverse happens infinitely many times. The subject evokes gut-level disagreement among experts. Many have reconciled themselves to the idea that our universe could be just one of many; Steinhardt calls the multiverse “hogwash.”

This sentiment partly motivated his and other researchers’ about-face on bounces. “The bouncing models don’t have a period of inflation,” Turok said. Instead, they add a period of contraction before a Big Bounce to explain our uniform universe. “Just as the gas in the room you’re sitting in is completely uniform because the air molecules are banging around and equilibrating,” he said, “if the universe was quite big and contracting slowly, that gives plenty of time for the universe to smooth itself out.”

Although the first contracting-universe models were convoluted and flawed, many researchers became convinced of the basic idea that slow contraction can explain many features of our expanding universe. “Then the bottleneck became literally the bottleneck — the bounce itself,” Steinhardt said. As Ijjas put it, “The bounce has been the showstopper for these scenarios. People would agree that it’s very interesting if you can do a contraction phase, but not if you can’t get to an expansion phase.”

 

Bouncing isn’t easy. In the 1960s, the British physicists Roger Penrose and Stephen Hawking proved a set of so-called “singularity theorems” showing that, under very general conditions, contracting matter and energy will unavoidably crunch into an immeasurably dense point called a singularity. These theorems make it hard to imagine how a contracting universe in which space-time, matter and energy are all rushing inward could possibly avoid collapsing all the way down to a singularity — a point where Albert Einstein’s classical theory of gravity and space-time breaks down and the unknown quantum gravity theory rules. Why shouldn’t a contracting universe share the same fate as a massive star, which dies by shrinking to the singular center of a black hole?

Both of the newly proposed bounce models exploit loopholes in the singularity theorems — ones that, for many years, seemed like dead ends. Bounce cosmologists have long recognized that bounces might be possible if the universe contained a substance with negative energy (or other sources of negative pressure), which would counteract gravity and essentially push everything apart. They’ve been trying to exploit this loophole since the early 2000s, but they always found that adding negative-energy ingredients made their models of the universe unstable, because positive- and negative-energy quantum fluctuations could spontaneously arise together, unchecked, out of the zero-energy vacuum of space. In 2016, the Russian cosmologist Valery Rubakov and colleagues even proved a “no-go” theorem that seemed to rule out a huge class of bounce mechanisms on the grounds that they caused these so-called “ghost” instabilities.

Then Ijjas found a bounce mechanism that evades the no-go theorem. The key ingredient in her model is a simple entity called a “scalar field,” which, according to the idea, would have kicked into gear as the universe contracted and energy became highly concentrated. The scalar field would have braided itself into the gravitational field in a way that exerted negative pressure on the universe, reversing the contraction and driving space-time apart —without destabilizing everything. Ijjas’ paper “is essentially the best attempt at getting rid of all possible instabilities and making a really stable model with this special type of matter,” said Jean-Luc Lehners, a theoretical cosmologist at the Max Planck Institute for Gravitational Physics in Germany who has also worked on bounce proposals.

What’s especially interesting about the two new bounce models is that they are “non-singular,” meaning the contracting universe bounces and starts expanding again before ever shrinking to a point. These bounces can therefore be fully described by the classical laws of gravity, requiring no speculations about gravity’s quantum nature.

Graham, Kaplan and Rajendran, of Stanford University, Johns Hopkins University and the University of California, Berkeley, respectively, reported their non-singular bounce idea on the scientific preprint site arxiv.org in September 2017. They found their way to it after wondering whether a previous contraction phase in the history of the universe could be used to explain the value of the cosmological constant — a mystifyingly tiny number that defines the amount of dark energy infused in the space-time fabric, energy that drives the accelerating expansion of the universe.

In working out the hardest part — the bounce — the trio exploited a second, largely forgotten loophole in the singularity theorems. They took inspiration from a characteristically strange model of the universe proposed by the logician Kurt Gödel in 1949, when he and Einstein were walking companions and colleagues at the Institute for Advanced Study in Princeton, New Jersey. Gödel used the laws of general relativity to construct the theory of a rotating universe, whose spinning keeps it from gravitationally collapsing in much the same way that Earth’s orbit prevents it from falling into the sun. Gödel especially liked the fact that his rotating universe permitted “closed timelike curves,” essentially loops in time, which raised all sorts of Gödelian riddles. To his dying day, he eagerly awaited evidence that the universe really is rotating in the manner of his model. Researchers now know it isn’t; otherwise, the cosmos would exhibit alignments and preferred directions. But Graham and company wondered about small, curled-up spatial dimensions that might exist in space, such as the six extra dimensions postulated by string theory. Could a contracting universe spin in those directions?

Imagine there’s just one of these curled-up extra dimensions, a tiny circle found at every point in space. As Graham put it, “At each point in space there’s an extra direction you can go in, a fourth spatial direction, but you can only go a tiny little distance and then you come back to where you started.” If there are at least three extra compact dimensions, then, as the universe contracts, matter and energy can start spinning inside them, and the dimensions themselves will spin with the matter and energy. The vorticity in the extra dimensions can suddenly initiate a bounce. “All that stuff that would have been crunching into a singularity, because it’s spinning in the extra dimensions, it misses — sort of like a gravitational slingshot,” Graham said. “All the stuff should have been coming to a single point, but instead it misses and flies back out again.”

The paper has attracted attention beyond the usual circle of bounce cosmologists. Sean Carroll, a theoretical physicist at the California Institute of Technology, is skeptical but called the idea “very clever.” He said it’s important to develop alternatives to the conventional inflation story, if only to see how much better inflation appears by comparison — especially when next-generation telescopes come online in the early 2020s looking for the telltale swirl pattern in the skycaused by inflation. “Even though I think inflation has a good chance of being right, I wish there were more competitors,” Carroll said. Sundrum, the Maryland physicist, felt similarly. “There are some questions I consider so important that even if you have only a 5 percent chance of succeeding, you should throw everything you have at it and work on them,” he said. “And that’s how I feel about this paper.”

As Graham, Kaplan and Rajendran explore their bounce and its possible experimental signatures, the next step for Ijjas and Steinhardt, working with Frans Pretorius of Princeton, is to develop computer simulations. (Their collaboration is supported by the Simons Foundation, which also funds Quanta Magazine.) Both bounce mechanisms also need to be integrated into more complete, stable cosmological models that would describe the entire evolutionary history of the universe.

Beyond these non-singular bounce solutions, other researchers are speculating about what kind of bounce might occur when a universe contracts all the way to a singularity — a bounce orchestrated by the unknown quantum laws of gravity, which replace the usual understanding of space and time at extremely high energies. In forthcoming work, Turok and collaborators plan to propose a model in which the universe expands symmetrically into the past and future away from a central, singular bounce. Turok contends that the existence of this two-lobed universe is equivalent to the spontaneous creation of electron-positron pairs, which constantly pop in and out of the vacuum. “Richard Feynman pointed out that you can look at the positron as an electron going backwards in time,” he said. “They’re two particles, but they’re really the same; at a certain moment in time they merge and annihilate.” He added, “The idea is a very, very deep one, and most likely the Big Bang will turn out to be similar, where a universe and its anti-universe were drawn out of nothing, if you like, by the presence of matter.”

It remains to be seen whether this universe/anti-universe bounce model can accommodate all observations of the cosmos, but Turok likes how simple it is. Most cosmological models are far too complicated in his view. The universe “looks extremely ordered and symmetrical and simple,” he said. “That’s very exciting for theorists, because it tells us there may be a simple — even if hard-to-discover — theory waiting to be discovered, which might explain the most paradoxical features of the universe.”