Revealing the Universe’s Mysterious Dark Age

<p>
 The universe wasn’t always like this. Today it’s filled with glittering galaxies, scattered across space like city lights seen from above. But there was a time when all was dark. Really dark.
</p>
<p>
 First, a very brief history of time: from the Big Bang, the universe burst onto the scene as a tiny but glowing inferno of energy. Immediately, it expanded and cooled, dimming into darkness as particles condensed out of the hot soup like droplets of morning dew. Electrons and protons coalesced into atoms, which formed stars, galaxies, planets, and eventually us.
</p>
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<p>
 But a crucial piece still eludes scientists. It’s a gap of several hundred million years that was filled with darkness—a darkness both literal and metaphorical. Astronomers call this period the dark ages, a time that’s not just bereft of illumination, but also devoid of data.
</p>
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/cosmic-dark-ages.width-990_lqTNQ5e.jpg"/>
<figcaption>A time-lapse visualization of what the cosmic web’s emergence might have looked like
        
    </figcaption>
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<p>
 The Big Bang left a glowing imprint on the entire sky called the cosmic microwave background, representing the universe when it was 380,000 years old. Increasingly precise measurements of this radiation have revealed unprecedented details about the earliest cosmic moments. But from then until the emergence of galaxies big and bright enough for today’s telescopes, scientists don’t have any information. Ever mysterious, these dark ages are the final frontier of cosmology.
</p>
<p>
 And it’s a fundamental frontier. It represents the universe’s most formative years, when it matured from a primordial soup to the cosmos we recognize today.
</p>
<p>
 Even without much direct data about this era, researchers have made great strides with theory and computer models, simulating the universe through the birth of the first stars. Soon, they may be able to put those theories to the test. In a few years, a suite of new telescopes with new capabilities will start peering into the darkness, and for the first time, astronomers will reach into the unknown.
</p>
<h2>
 The Final Frontier
</h2>
<p>
 Considering that it’s the entire universe they’re trying to understand, cosmologists have done a pretty good job. Increasingly powerful telescopes have allowed them to peer to greater distances, and because the light takes so long to reach the telescopes, astronomers can see farther back in time, capturing snapshots of a universe only a few hundred million years old, just as it emerged from the dark ages. Given that the universe is now 13.7 billion years old, that’s like taking a picture of the cosmos as a toddler.
</p>
<p>
 That makes the cosmic microwave background, or CMB, like a detailed ultrasound. This radiation contains the first photons that escaped the yoke of the universe’s primordial plasma. When the universe was a sea of radiation and particles, photons couldn’t travel freely because they kept running into electrons. But about 380,000 years after the Big Bang, the universe had cooled enough that protons were able to lasso electrons into an orbit to form hydrogen atoms. Without electrons in their way, the newly liberated photons could now fly through the cosmos and, more than 13 billion years later, enter the detectors of instruments like the Planck satellite, giving cosmologists the earliest picture of the universe.
</p>
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<p>
 It’s as if they have a photo album documenting a person’s entire life, but nothing from when the person learned to talk or walk—years of drastic changes.
</p>
</div>
<p>
 But from this point on, until the universe was a few hundred million years old—the limit of today’s telescopes—astronomers have nothing. It’s as if they have a photo album documenting a person’s entire life, with pictures of young adulthood, adolescence, childhood, and even before birth, but nothing from when the person learned to talk or walk—years of drastic changes.
</p>
<p>
 That doesn’t mean astronomers have no clue about this period. “People have thought about the first stars since the 1950s,” says Volker Bromm, a professor of astronomy at the University of Texas, Austin. “But they were very speculative because we did not know enough cosmology.” Not until the 1980s did researchers develop more accurate theories that incorporated dark matter, the still-unknown type of particle or particles that comprises about 85% of the matter in the universe. But the first key breakthrough came in 1993, when NASA’s COBE satellite measured the CMB for the first time, collecting basic but crucial data about what the universe was like at the very beginning—the so-called initial conditions of the cosmos. Theorists such as Martin Rees, now the Astronomer Royal of the United Kingdom, and Avi Loeb, a professor of astrophysics at Harvard, realized you could plug these numbers into the equations that govern how the first gas clouds and stars could form. “You could feed them into a computer simulation,” Loeb says. “It’s a well-defined problem.”
</p>
<p>
 Both Rees and Loeb would influence Bromm, then a graduate student at Yale. Rees and his early work in the 1980s, in particular, inspired Tom Abel, who was a visiting scientist during the 1990s at the University of Illinois, Urbana-Champaign. Independently, Abel and Bromm would make some of the first computer models of their kind to simulate the first stars. “That really opened the field,” Loeb says. “When I started, there were maybe one or a few people even willing to discuss this subject.”
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<p>
 Theorists like Bromm and Abel, now a professor at Stanford, have since pieced together a blow-by-blow account of the dark ages. Here’s how they think it all went down.
</p>
<h2>
 Then There Was Light
</h2>
<p>
 In the earliest days, during the time that we see in the CMB, the entire universe was bright and as hot as the surface of the sun. But the universe kept expanding and cooling, and after nearly 15 million years, it was as cool as room temperature. “In principle, if there were planets back then, you could’ve had life on them if they had liquid water on their surface,” Loeb says. The temperature continued to fall, and the infrared radiation that suffused the universe lengthened, shifting to radio waves. “Once you cool even further, the universe became a very dark place,” Loeb says. The dark ages had officially begun.
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<p>
 Meanwhile, the simulations show, things began to stir. The universe was bumpy, with regions of slightly higher and lower densities, which grew from the random quantum fluctuations that emerged in the Big Bang. These denser regions coaxed dark matter to start clumping together, forming a network of sheets and filaments that crisscrossed the universe. At the intersections, denser globs of dark matter formed. Once these roundish halos grew to about 10,000 times the mass of the Sun, Abel says—a few tens of millions of years after the Big Bang—they had enough gravity to corral hydrogen atoms into the first gas clouds.
</p>
<p>
 Those clouds could then accumulate more gas, heating up to hundreds of degrees. The heat generated enough pressure to prevent further contraction. Soon, the clouds settled into enormous, but rather dull, balls of gas about 100 light years in diameter, Abel says.
</p>
<p>
 But if the dark matter halos reached masses 100,000 times that of the sun, they could accrue enough gas that the clouds could heat up to about 1000 degrees—and that’s when things got interesting. The surplus energy allowed hydrogen atoms to merge two at a time and form hydrogen molecules—picture two balls attached with a spring. When two hydrogen molecules collide, they vibrate and emit photons that carry away energy.
</p>
<p>
 When that happens, the molecules are converting the vibrating energy that is heat into radiation that’s lost into space. These interactions cooled the gas, slowing down the molecules and allowing the clouds to collapse. As the clouds grew denser, their temperatures and pressures soared, igniting nuclear fusion. That’s how the first stars were born.
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<p>
 Massive stars consume fuel like gas-guzzling SUVs. They live fast and die young.
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<p>
 These first stars, which formed by the time the universe was a couple hundred million years old, were much bigger than those in today’s universe. By the early 2000s, Abel’s simulations, which he says are the most realistic and advanced yet, showed that the first stars weighed about 30 to 300 times the mass of the sun. Using different techniques and algorithms, Bromm says he arrived at a similar answer. For the first time, researchers had a good idea as to what the first objects in the universe were like.
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<p>
 Massive stars consume fuel like gas-guzzling SUVs. They live fast and die young, collapsing into supernovae after only a few million years. In cosmic timescales, that’s the blink of an eye. “You really want to think of fireworks at these early times,” Abel says. “Just flashing everywhere.”
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<p>
 In general, the first stars were sparse, separated by thousands of light years. Over the next couple hundred million years, though, guided by the clustering of dark matter, the stars started grouping together to form baby galaxies. During this cosmic dawn, as astronomers call it, galaxies merged with one another and became bigger galaxies. Only after billions and billions of years would they grow into those like our own Milky Way, with hundreds of billions of stars.
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<h2>
 Lifting the Fog
</h2>
<p>
 But there’s more to the story. The first stars shone in many wavelengths, and especially strongly in ultraviolet. The universe’s expansion would’ve stretched this light to visible and infrared wavelengths, which many of our best telescopes are designed to detect. Problem is, during the time of the first stars, a thick fog of neutral hydrogen gas blanketed the whole universe. This gas absorbed shorter-wavelength ultraviolet light, obscuring the view from telescopes. Fortunately, though, this fog would soon lift.
</p>
<p>
 “This state of affairs can’t last for very long,” says Richard Ellis, an astronomer at the European Southern Observatory in Germany. “These ultraviolet photons have sufficient energy to break apart the hydrogen atom back into an electron and a proton.” The hydrogen was ionized, turning into a lone proton that could no longer absorb ultraviolet. The gas was now transparent.
</p>
<p>
 During this so-called period of reionization, galaxies continued to grow, producing more ultraviolet light that ionized the hydrogen surrounding them, clearing out holes in the fog. “You can imagine the hydrogen like Swiss cheese,” Loeb says. Those bubbles grew, and by the time the universe was around 800 million years old, the ultraviolet radiation ionized the hydrogen between the galaxies, leaving the entire cosmos clear and open to the gaze of telescopes. The dark ages were over, revealing a universe that looked more or less like it does today.
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<h2>
 Seeing into the Dark
</h2>
<p>
 Of course, many details have to be worked out. Astronomers like Ellis are focusing on the latter stages of the dark ages, using the most powerful telescopes to extract clues about this reionization epoch.
</p>
<p>
 One big question has been whether the ultraviolet light from early galaxies was enough to ionize the whole universe. If it wasn’t, astronomers would have to find another exotic source—like black holes that blast powerful, ionizing jets of radiation—that would have finished the job.
</p>
<p>
 To find the answer, Ellis and a team of astronomers stretched the Hubble Space Telescope to its limits, extracting as much light as possible from one small patch of sky. These observations reached some of the most distant corners of the universe, discovering some of the earliest galaxies ever seen, during the heart of this reionization era. Their observations suggested that galaxies—large populations of small galaxies, in particular—did seem to have enough ultraviolet light to ionize the universe. Maybe nothing exotic is needed.
</p>
<p>
 But to know exactly how it happened, astronomers need new telescopes, like the James Webb Space Telescope set for launch in 2018. “With the current facilities, it’s just an imponderable,” Ellis says. “We don’t have the power to study these galaxies in any detail.”
</p>
<p>
 Other astronomers are focusing not on the galaxies, but the hydrogen fog itself. It turns out that the spins of a hydrogen atom’s proton and electron can flip-flop in direction. When the spins go from being aligned to unaligned, the atom releases radiation at a wavelength of 21 centimeters, or 8.27 inches, a telltale signal of neutral hydrogen that astronomers call the 21-cm line. The expanding universe would have stretched this signal to the point where it became a collection of radio waves. The more distant the source of light, the more the radiation gets stretched. By using arrays of radio telescopes to measure the extent of this stretching, astronomers can map the distribution of hydrogen at different points in time. They could then track how those holes in the gas grew and grew until the gas was all ionized.
</p>
<p>
 “It’s surveying the volume of the universe on a scale that you can’t imagine doing in any way other than through this method—it’s really quite incredible,” says Aaron Parsons, an astronomer at the University of California, Berkeley, who’s leading a project called HERA, which will consist of 352 radio antennae in South Africa. Once online, the telescope could give an unprecedented view of reionization. “You can almost imagine making a movie of how the first stars galaxies formed, how they interacted, heated up, ionized, and turned into the galaxies we recognize today.”
</p>
<p>
 Other telescopes like LOFAR in the Netherlands and the Murchison Widefield Array in Australia will make similar measurements. But HERA will be more sensitive, Parsons says. And already with 19 working antennae in place, it might be closest to success, adds Loeb, who isn’t part of the HERA team. “Within a couple years, we should have the first detection of the 21-cm line from this epoch of reionization, which would be fantastic because it would allow us to see the environmental effect of ultraviolet radiation from the first stars and first galaxies on the rest of the universe.”
</p>
<p>
 This kind of data is crucial for informing computer models like the kind that Abel and Bromm have developed. But despite their successes, theorists are at the point where they need data to test whether their models are accurate.
</p>
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<p>
 Unfortunately, that data won’t be pictures of the first stars. Even the most powerful telescopes won’t be able to see the brightest of them. The first galaxies contain only a few hundred stars and are just too small and faint. “We’ll come ever closer,” Abel says. “It’s very difficult to imagine we’ll actually see those in the near future, but we’ll see their brighter cousins.”
</p>
<p>
 In fact, the darkest of times, during the couple hundred million years between the CMB and the appearance of the first stars, may always remain beyond astronomers’ grasp. “We currently don’t have any idea of how you could get any direct information about that period,” he says.
</p>
<p>
 Still, new telescopes over the next few decades promise to reveal much of the dark ages and whether the story theorists are telling is true or even more fantastic than they had thought. “Even though I’m a theorist, I’m modest enough to acknowledge the fact that nature is sometimes more imaginative than we are,” Loeb says. “I’m open to surprises.”
</p>
<p>
 Image credit: Maureen Teyssier/Rutgers University and Andrew Pontzen/University College London
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<p>Astrophysicists are pushing cosmologist theories to their limits to shed light on the universe&#x27;s mysterious dark age.</p>

Can We Make New Phones from Nothing But Old Ones?

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<p>
 As if saying hello, the robot arm waves an iPhone at you. Then, in a few swift maneuvers, the arm and a suite of machinery begin dismantling the phone. First goes the outer casing. Next are the screws and other metal parts, dropping into neat piles. “Meet Liam,” says the voice-over on Apple’s promotional video. Like most things from Apple, Liam looks pretty cool.
</p>
<p>
 Apple created a robot named Liam to disassemble iPhones.
</p>
<p>
 Liam is a robot—29 robots working in concert, to be precise—whose job is to take apart iPhones. Apple unveiled the system last year in a bid to more efficiently recycle used phones. But recently, Liam has become part of an even more ambitious plan announced in April: eventually, Apple wants to make new products using only recycled products.
</p>
<p>
 Ideally, that means all your old Apple stuff would eventually become new Apple stuff. There would be no waste and no need to mine for more materials. Such an idea is called a closed-loop supply chain, part of a concept also referred to as a circular economy or a cradle-to-cradle philosophy, which considers the entire lifecycle of a product and was popularized by the architect and designer William McDonough.
</p>
<p>
 In a world with finite natural resources—such as the rare earth metals needed for all your favorite devices—dubious mining practices rife with environmental and human rights problems, and a global appetite for electronics that will only grow, a closed-loop approach may not be just a good idea, but a necessary one. According to a
 <a href="https://i.unu.edu/media/unu.edu/news/52624/UNU-1stGlobal-E-Waste-Monitor-2014-small.pdf">
  2014 report
 </a>
 from the United Nations, people generated 92.1 billion pounds of e-waste in 2014. That number is expected to balloon to 110 billion pounds in 2018—roughly the weight of 25,000 space shuttles. A closed loop could trim those numbers significantly.
</p>
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<p>
 People generated 92.1 billion pounds of e-waste in 2014.
</p>
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<p>
 As sustainability has become an industry buzzword, some companies have tried implementing a closed loop. But none are as big and influential as Apple.
</p>
<p>
 While Apple’s goals are laudable, the announcement didn’t exactly include a detailed plan—drawing some skepticism from experts. “It didn’t make people in the industry really excited because, well, you’re not looking at what it’s going to take to do that,” says Carole Mars, the senior research lead for the Sustainability Consortium at Arizona State University.
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<p>
 In fact, no one’s ever achieved a true closed loop on the scale of Apple’s ambitions. And even the company admits they’re not sure how it’ll be done. “We’re actually doing something we rarely do, which is announce a goal before we’ve completely figured out how to do it,” said Lisa Jackson, Apple’s vice president of environment, policy and social initiatives and former head of the EPA, told
 <a href="https://news.vice.com/story/apple-promises-to-stop-mining-minerals-to-make-iphones-it-just-isnt-sure-how-yet">
  VICE News
 </a>
 in April.
</p>
<p>
 The challenges, to say the least, are many.
</p>
<h2>
 Better Design
</h2>
<p>
 If there’s a guru for a closed-loop philosophy, it’s McDonough, who literally wrote the book on it—
 <i>
  Cradle to Cradle: Remaking The Way We Make Things
 </i>
 , with Michael Braungart in 2002. He’s designed numerous energy efficient buildings, including the Ford Motor Co.’s Rouge Factory, complete with a 10.4-acre living roof.
</p>
<p>
 In 1993, McDonough and his firm, MBDC, designed a greenhouse factory and offices for Herman Miller, a Michigan-based furniture company. By the end of the decade, the firm began helping Herman Miller embrace a cradle-to-cradle philosophy. “Bill challenged us to think about how we design our products to be part of a closed-loop system,” says Gabe Wing, director of environmental health and safety at Herman Miller.
</p>
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/herman-miller-factory.width-990_ClsPc6Z.jpg"/>
<figcaption>A Herman Miller employee assembles a chair in one of its factories.
        
    </figcaption>
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<p>
 According to Wing, the company has led the way when it comes to designing furniture that’s not only made from recycled and recyclable materials, but also easy to disassemble. To recycle, after all, is to break something down into its basic components for reuse. Anything that makes it harder to dismantle, like gluing disparate parts together, becomes a hindrance. If a product isn’t designed from the start with disassembly in mind, then recycling will be that much more difficult.
</p>
<p>
 “You have to start with a proper design part up front,” Wing says. “It’s just way too hard to go back and fix things.”
</p>
<p>
 That’s where Apple has garnered criticism. For all the praise heaped on Apple’s sleek gadgets and their precise assembly, critics say the company’s products are poorly designed when it comes time to take them apart. Batteries are glued in, disassembly instructions are secret, and even the screws are proprietary. Apple’s products, along with those from Samsung and Google, are the most difficult to dismantle and repair, according to a
 <a href="http://www.greenpeace.org/international/en/press/releases/2017/Apple-Samsung-products-among-least-repairable-Greenpeace-assessment-of-tech-brands/">
  report
 </a>
 from Greenpeace and the tech repair company iFixit, one of Apple’s consistent critics.
</p>
<p>
 Since no one knows Apple products better than Apple, they’re best positioned to dismantle iPhones—which is why they’ve designed Liam. The robot, however, is still a research project. For now, it only works on the iPhone 6. Only two of the systems exist; together, they’re capable of taking apart a total of 2.4 million phones per year. That might sound like a lot, but consider that when the iPhone 6 was launched, Apple sold 10 million during the opening weekend alone. Today, more than 1 billion Apple devices are in use worldwide.
</p>
<h2>
 A Melting Pot
</h2>
<p>
 But even if Apple builds thousands of Liams to take care of every different product, disassembly is just the start. Sure, the robot can break down a phone into parts. If you want to recover raw materials, though—like copper, tin, cobalt, and tungsten—you would have to melt them down, and separate the metals.
</p>
<p>
 That’s where it gets tricky. Today’s sophisticated phones require dozens of elements, including rare earth metals like indium, dysprosium, and neodymium, scattered throughout the device in tiny amounts. The glass on a touchscreen, for example, contains bits of indium. Just a few micrograms of hafnium is buried inside the semiconductor chip of your phone. Picking that out of a melting pot of parts isn’t impossible, but it’s not easy.
</p>
<p>
 “The chemistry is complicated,” says Alex King, the director of the Department of Energy’s Critical Materials Institute. “But in the end, you could always design a chemical process that separates one from another.”
</p>
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/iphone-7-parts.width-990_eYpldy4.jpg"/>
<figcaption>Disassembling modern smartphones can be a complicated process. Here, iFixit, a repair company, has taken apart an iPhone 7.
        
    </figcaption>
</figure>
<p>
 The issue isn’t chemistry or engineering; rather, it’s economics. It simply might not be worthwhile to extract such small amounts of metal. The process will probably cost more in energy and labor than the value of whatever bit of material you recover. So to recycle every element in an iPhone won’t be economical or practical. “They would be very lucky to be able to recycle a third of the elements in the phone,” King says.
</p>
<p>
 Recycling the majority of a phone as measured by mass, on the other hand, may be feasible, he says. Apple plans to first focus on recycling its high-quality aluminum. Liam can recover 1,900 kilograms of aluminum for every 100,000 iPhones. According to its
 <a href="https://images.apple.com/environment/pdf/Apple_Environmental_Responsibility_Report_2017.pdf">
  2017 environmental report
 </a>
 , the company is already using some of that recovered aluminum to build mini computers used in the final assembly facilities for its iPhone. They’re also trying to scale up methods to recover tin, and researching how to recover cobalt from their batteries.
</p>
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<h2>
 Logistics
</h2>
<p>
 Disassembly and recovery is still only part of what makes a true closed loop. After selling a product, most companies never see it again. But for a closed-loop system, Apple would have to somehow recover all those old iPhones and iPods that end up at the bottom of your drawer. Otherwise, Liam is a robot without a job.
</p>
<p>
 “Liam is a great proof of concept—it’s great R&amp;D,” Mars says. “But Apple is going to have a very hard time getting enough phones together to make robots like that feasible.”
</p>
<p>
 The company is encouraging customers to use its Renew program, which offers gift certificates in exchange for old devices. But that’s failed to entice many of us who still have old gadgets lying around. Gift certificates probably aren’t enough to close the loop.
</p>
<p>
 Indeed, the logistics of retrieving used products may be the toughest part of a closed loop, regardless of whether they’re phones or chairs. Even Herman Miller, for all its experience, has found the task harder than expected, Wing says.
</p>
<p>
 Despite spending the last two decades incorporating a cradle-to-cradle philosophy, creating a product line for which 70% meets a cradle-to-cradle standard, Herman Miller still can’t claim a closed-loop supply chain. “We’re a long way from being a truly closed loop,” Wing says.
</p>
<p>
 The company has a Repurpose program to help customers donate or recycle old furniture. A few years ago, it set a goal to reuse or recycle half of its products—125,000 tons worth per year—by 2023. Today, Wing says, they’re only at 6,000 tons.
</p>
<p>
 But they’re making progress, he says. He cites one of the company’s success stories, in which they worked with General Motors to donate 2,000 tons of used office furniture, worth $1 million, to 100 nonprofits throughout the Detroit area. The furniture wasn’t recycled, but it was reused and avoided the landfill.
</p>
<p>
 Maybe the best example of a successful closed-loop belongs to Dell. It began in 2008, when the company began building the casing of its Optiplex desktop computers with recycled plastic, made from things like bottles and CD cases. In 2013, they expanded their recycling program, aiming to recycle plastic from 2 billion pounds of electronics by 2020.
</p>
<div class="pullquote">
<p>
 “We’re a long way from being a truly closed loop.”
</p>
</div>
<p>
 Then, in 2014, Dell started recycling the plastic from computers—including their own—for use in new desktops. It was one of the first instances of a true closed loop at that scale, earning an award from the Green Electronics Council. (Dell isn’t the only one to reach this achievement in the industry: Hewlett-Packard’s printer cartridges, made from a simpler type of plastic, have been in a closed-loop for years.) “It was a really impressive feat,” Mars says. Today, Dell is recycling plastic from nearly 1.8 billion pounds of electronics, says Scott O’Connell, Dell’s director of environmental affairs. The company has incorporated 10 million pounds of closed-loop plastic in 91 products, from desktops to monitors, O’Connell says.
</p>
<p>
 To recover so much plastic, Dell relies on a take-back and recycling program that spans 83 countries and territories around the world. In the U.S., it has partnered with Goodwill to establish a network of collection locations. The company also tries to design products that can be disassembled and easily refurbished, with unglued, removable batteries, for example.
</p>
<p>
 But most of the plastic in Dell’s products still isn’t in a closed-loop. Only 18% is recycled plastic, and less than 5% comes from old computers. Despite the modest numbers, it’s still enough to save the company 10% in plastic costs, O’Connell says.
</p>
<p>
 The company is now looking to use closed-loop plastic in more products and build more recycling and recovery facilities in other parts of the world like Asia and Europe. They’ve also started closing the loop for carbon fiber, used for components such as laptop bases and LCD panel covers.
</p>
<h2>
 Future Loops
</h2>
<p>
 Dell’s public aspirations may not be as bold as Apple’s, but their shorter time scale and more modest ambitions allow them to be more concrete about their near-term goals. If you read Apple’s statements more closely, they are somewhat open-ended and vague. “We believe our goal should be a closed-loop supply chain, where products are built using only renewable resources or recycled material,” reads its environmental report. It’s certainly a more sustainable practice, but it isn’t exactly a closed loop—at least not in the conventional sense of the term.
</p>
<p>
 “This is a big squishy area in sustainability conversations in general,” Mars says. “Where do you draw the line about closed loop?”
</p>
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/new-old-iphones.width-990_8htqGLr.jpg"/>
<figcaption>Could Apple make new iPhones from old ones? Right now, it's cost prohibitive.
        
    </figcaption>
</figure>
<p>
 In the more traditional definition, you have to recycle your own products for reuse. Simply making products out of recycled material, while still a great benefit, isn’t closing the loop. “It’s definitely a loop,” Mars says, “but it’s an open loop.”
</p>
<p>
 Apple’s report, of course, describes using recycled aluminum that Liam extracts from iPhones and exploring new ways to recover tin. But it also says that in some cases, as long as you recover, recycle, and put the same amount of material back in the market, using recycled stuff is enough. This bypasses the nontrivial logistical problem of retrieving your own product.
</p>
<p>
 How tightly the loop is closed may be just a matter of semantics at this point. Other than Liam and the mentions of aluminum, tin, and cobalt in its report, Apple hasn’t elaborated on how it might achieve its goal. When reached for comment, an Apple spokesperson said there’s nothing new and that the company typically doesn’t speak publicly about future plans. One piece of public information that hints at their progress came in June, when Apple issued a $1 billion green bond to fund energy efficiency and renewable energy projects—including the effort to develop a closed-loop supply chain.
</p>
<p>
 Still, Mars says she would like to hear more detail over the next few months about what Apple’s first steps might be: What are their priorities and how do they plan to boost recycling? The answers, she says, will reveal how serious their ambitions are.
</p>
<p>
 If Apple does make strides, its influence could be felt throughout the industry. Just ramping up recycling could increase supply and demand for certain metals, stimulating the market for recycling companies. “If Apple tells its supplier you will use recycled materials, that ripples through the entire supply chain and actually makes recyclable materials more available,” King says.
</p>
<p>
 Then there are Apple’s loyal fans. “All Tim Cook has to do is stand up and say not having recycled content is unacceptable; everybody should do this,” Mars says. “You’ve started a movement.”
</p>
<p>
 What Apple can do is to make the marketplace aware of closed-loop. “Make this sexy. Make it desirable. Make it part of the cultural experience of Apple,” she says. “If you can get that ball rolling, then you can be very sure that the Samsungs and the LGs are going to be all over it.”
</p>
<p>
 In the end, though, a true 100% closed loop for electronics may not be possible. But one thing is clear: even getting close will take time. Perhaps decades. It will take years for the industry to figure out the logistics of retrieving products. It will take years to develop the technology, infrastructure, and market to recycle more materials.
</p>
<p>
 All that may have to coincide with a shift in how we consumers think about our gizmos. “This is moving from a product of consumption to a product of service,” Wing says. Eventually, instead of owning your computer, you might lease it. When you’re ready to upgrade, you return it for recycling.
</p>
<p>
 “I hope that this is the beginning of an awesome story with Apple,” Mars says. “I would love to see it live up to the expectations being put on them in the media. That would be huge.”
</p>
<p>
 Photo credit: Herman Miller,
 <a href="https://www.ifixit.com/Teardown/iPhone+7+Teardown/67382">
  iFixit (CC BY-NC-SA)
 </a>
 ,
 <a href="https://www.flickr.com/photos/janitors/15707819731/">
  Kārlis Dambrāns/Flickr (CC BY)
 </a>
</p>

<p>Apple has an ambitious plan to make new devices using only recycled products. What will it take?</p>

Finding Future Tech in an Ancient Art

<p>
 Take a piece of paper. Fold it in half, and you’ve turned a floppy sheet into a three-dimensional structure that can stand up, even if a bit wobbly, on its own.
</p>
<p>
 A few more deliberate folds, and you may even have an airplane capable of actual flight. Or, with a little more patience, a crane, a rhinoceros, or almost any object, shape, or design you can imagine.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/origami-woolly-mammoth.width-990_duIsbtB.jpg"/>
<figcaption>An origami woolly mammoth
        
    </figcaption>
</figure>
<p>
 From this simple act of folding, you can create enormous complexity—something origami artists have been doing for hundreds of years, transforming paper into art. But recently, that art has also become useful, as origami has begun to merge with engineering. Origami principles have inspired novel ways of packaging airbags, fashioning heart stents, folding solar sails designed to propel spacecraft, and even collapsible bullet-proof shields.
</p>
<div class="related"></div>
<p>
 And researchers are just getting started. They’re now using origami to design materials of the future: materials that shift from soft to stiff—and back again—with the flick of a switch, or smart materials that respond to their environments as if they were alive. Combined with robotics and devices, these materials may redefine what it even means to be a material, with potential applications in engineering, architecture, biology, medicine, and beyond.
</p>
<p>
 A lot of modern research in origami engineering traces back to an astrophysicist named Koryo Miura who, in 1970, came up with simple but elegant fold. It consists of creases that form a pattern of tiled parallelograms, allowing it to collapse and open with ease. Known as the Miura fold, or Miura-ori, he later repurposed the fold as
 <a href="https://repository.exst.jaxa.jp/dspace/bitstream/a-is/7293/1/SA0035000.pdf">
  a way
 </a>
 to package large, flat membranes for deploying into space. In fact, in 1995, a Japanese satellite used the fold to store its solar panels for launch and unfurl them once in orbit.
</p>
<p>
 In the years since, the Miura-ori has become one of the most well-studied folds, prompting Cornell engineer
 <a href="http://cohengroup.lassp.cornell.edu/">
  Itai Cohen
 </a>
 to call it the hydrogen atom of origami. Like hydrogen, the pattern is simple, yet imbued with complex and exotic properties. When you squeeze a Miura-ori on its sides, for example, the top and bottom also collapses inward. This feature, known as a negative Poisson ratio, is unusual. Most other things spill outward in the direction perpendicular to your squeezing. Hold a sandwich too tight, and your meat and fixins’ will squirt out the sides.
</p>
<p>
 You can also adjust the stiffness of the fold. Cohen and his colleagues have shown how defects—forcing certain creases to fold in the opposite way—can make the Miura-ori rigid. “You can just take the same fold pattern and poke it in certain places and suddenly it responds differently than if you poke it in different places,” says
 <a href="http://blogs.umass.edu/csantang/">
  Christian Santangelo
 </a>
 of the University of Massachusetts, Amherst. In this sense, Miura-ori represents a switchable material whose properties—stiffness, in this case—can be toggled and tuned on the fly.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/miura-ori-2.width-990_HbIDHla.jpg"/>
<figcaption>The Miura-ori
        
    </figcaption>
</figure>
<p>
 Something folded into a Miura-ori is an example of a metamaterial, a material whose properties depend not on what it’s made of, but on its structure. Maybe the most famous example is an invisibility cloak, a material embedded with nanoscale structures that can redirect light around an object, making it appear invisible.
</p>
<p>
 But unlike conventional metamaterials, whose properties are built-in, foldable structures introduce a powerful new dynamic: the ability to transform. “We can in principle dial in any mechanical properties you want,” Santangelo says. “By changing the structure, you can have something that’s very soft or very hard. Or, it starts out being very soft and gets hard.”
</p>
<h2>
 Snapology
</h2>
<p>
 Using a building-block approach,
 <a href="http://www.overvelde.com/">
  Johannes Overvelde
 </a>
 at AMOLF, a research institute in the Netherlands,
 <a href="https://www.seas.harvard.edu/directory/bertoldi">
  Katia Bertoldi
 </a>
 at Harvard, and others, have developed a
 <a href="https://www.researchgate.net/publication/297760817_A_three-dimensional_actuated_origami-inspired_transformable_metamaterial_with_multiple_degrees_of_freedom">
  different way
 </a>
 of achieving complex structures. By stacking identical 3D cube-like structures together, the researchers can build a larger structure that, through folding, morphs into different shapes and configurations.
</p>
<p>
 In this modular strategy, dubbed snapology, the properties of each building block can generally transfer to the whole structure. “If you start from all rigid unit cells, your metamaterial becomes rigid as well,” Overvelde says. “If unit cells are more foldable, then potentially the metamaterial is more foldable and more configurable.”
</p>
<p>
 In a proof-of-concept example, he and his colleagues used snapology to build a network of plastic tubes that
 <a href="https://www.researchgate.net/publication/310766212_Reconfigurable_origami-inspired_acoustic_waveguides">
  channel sound waves
 </a>
 . They can reconfigure the structure by hand or with air pressure, changing and controlling how the sound travels. Someday, Overvelde says, an acoustic waveguide like this may be embedded into the ceiling of a room or concert hall, reconfiguring itself to dampen noise or change how sound travels.
</p>
<p>
 But that’s just one design. The possibilities, he says, are nearly endless. The researchers have devised a
 <a href="http://www.nature.com/nature/journal/v541/n7637/full/nature20824.html">
  systematic strategy
 </a>
 to produce tens of thousands of reconfigurable shapes and structures based on snapology. In principle, an engineer could search through this database, hunting for designs with the right properties needed for a particular application.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/origami-42.width-990_Ta1fKFJ.jpg"/>
<figcaption>Origami can produce myriad shapes with distinct properties.
        
    </figcaption>
</figure>
<p>
 For now, though, Overvelde has just explored what’s possible with various geometries and designs, making prototypes out of cardboard. But if the three-dimensional structures he envisions are ever going to be useful, engineers will have to fabricate them out of more durable materials—likely in a range of size scales–which isn’t a trivial matter. One solution is 3D printing, but the technology isn’t yet up to snuff.
</p>
<p>
 This is where folding 3D structures from 2D sheets, like Miura-ori, has an advantage. After all, the power of origami lies in the fact that it’s easier to fold a structure than to build it from the ground up, which is generally how conventional metamaterials are made. From laser cutting to lithographic techniques that can etch nanoscale features on chips, 2D-printing methods are more advanced and faster than 3D printing. “We can print anything we want, really, in 2D,” says
 <a href="http://erikdemaine.org/">
  Erik Demaine
 </a>
 , an origami artist and computer scientist at MIT.
</p>
<div class="ad"></div>
<h2>
 A Universal Algorithm
</h2>
<p>
 What might make it even easier is software that makes designs for you. Software such as TreeMaker, created by origami artist and physicist
 <a href="http://www.langorigami.com/">
  Robert Lang
 </a>
 , or Origamizer, by
 <a href="http://origami.c.u-tokyo.ac.jp/~tachi/index.html">
  Tomohiro Tachi
 </a>
 of the University of Tokyo, can determine crease patterns for different shapes, helping origami artists create increasingly sophisticated designs.
</p>
<p>
 Although powerful, the software can’t turn every shape into origami. So Tachi and Demaine set out to develop a universal algorithm that can find a crease pattern for any 3D surface. After a decade of work, they’ve now succeeded and presented their
 <a href="http://erikdemaine.org/papers/Origamizer_SoCG2017/">
  algorithm
 </a>
 in July of last year. Once they encode their algorithm into the next version of Origamizer, you’ll be able to input a 3D rendering of your face, the Manhattan skyline, or any other surface, and the software will spit out a corresponding crease pattern.
</p>
<p>
 Even the algorithm has its limits, however. It assumes you have an infinitely thin sheet of material. Paper is thin, but not that thin, so adapting to real-world origami may be a little tricky. The algorithm is optimized to give you a set of folds that most efficiently uses a sheet of paper. But for metamaterials, that’s not necessarily what you want.
</p>
<p>
 “Origamizer is all about folding your material into the right shape,” Demaine says. “But in reality, and especially in metamaterials, you also care about structural properties.” The new algorithm doesn’t yet allow you to make designs based on structural properties, but future incarnations might, he says. Still, the new-and-improved Origamizer marks a significant achievement and can help design metamaterials.
</p>
<h2>
 The Amazing Self-Folding Microbot
</h2>
<p>
 But design is just the start. The key to origami is, of course, folding. And a major goal among researchers is to develop a way for structures to fold by themselves. Switchable and reconfigurable metamaterials won’t be very useful if you have to fold it by hand—and they’ll be nearly impossible to manage if they require microscopic folds.
</p>
<p>
 Most strategies use materials that shrink or expand at different rates in response to heat, for example. If you layer the materials together and apply heat, only one of the materials will contract, forcing the entire sheet to buckle. By designing your sheet to buckle only along designated creases, you can get it to fold up into whatever pattern you want.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/origami-tessellation-1.width-990_zy7gRIt.jpg"/>
<figcaption>An origami tessellation
        
    </figcaption>
</figure>
<p>
 Finding the right shrinking or expanding materials isn’t easy, however. Researchers like Santangelo, for example, have experimented with hydrogels, which swell in water. “The problem with hydrogels is that they’re very slow and weak,” he says. “They’re great for proving proofs-of-principle—not so great when you have to do actual physical work.”
</p>
<p>
 But now Cohen thinks he has an answer. Working with Marc Miskin at Cornell, Cohen has found a way to get strong, rigid panels to fold in less than a second. These structures are also tiny—about the size of a biological cell—which would make them potentially useful for medicine: imagine sending tiny, self-folding robots into the body to attack a virus or a tumor. “It’s the first real platform for the kind of nanotechnology you see in science fiction,” Miskin says.
</p>
<div class="pullquote">
<p>
 “Integrating devices into self-folding structures may be the materials of the future.”
</p>
</div>
<p>
 The panels bend along hinges just two nanometers thick that are made of glass stuck to a single sheet of graphene, an atom-thin layer of carbon that’s as stiff and, at the atomic scale, stronger than diamond. To get it to fold, the researchers dip the material into a chemical bath and increase its acidity. A higher acidity drives larger ions to replace smaller ones in the glass, which forces the glass to expand—and the sheet to bend. A more basic solution reverses the process.
</p>
<p>
 Again, the big draw for a self-folding device is that it fits seamlessly into current 2D lithographic and semiconductor technology, making manufacturing easier, Cohen says. The fact that these devices fold up also means you can squeeze them into compact spaces. The self-folding action can even do physical work, such as propulsion through the body or exerting force on a cell.
</p>
<p>
 The researchers have built a
 <a href="http://www.pnas.org/content/early/2018/01/01/1712889115.full">
  prototype
 </a>
 that folds into a 15-micron-wide tetrahedron, just three times bigger than a red blood cell. As proofs-of-principle, they’ve built small helices, cubes, a Miura-ori unit, and a book that clasps shut. “This ends up being a blank slate you can now design real machines for,” Cohen says. “Now the question is what you should put on there to make the machine you want.”
</p>
<p>
 Future robots, he says, could measure the properties of cells, interact with them, or trigger cellular responses. Because graphene is electrically conductive, you can attach electronic sensors or other components. For example, the researchers are exploring how to incorporate light sensors, which could be useful for biomedical research. Because their self-folding devices are sensitive to ions, they also envision using them to gauge ion concentrations in the body—and in doing so, maybe even measure when neurons are firing in the brain.
</p>
<h2>
 Material or Machine?
</h2>
<p>
 Indeed, integrating devices into self-folding structures may be the materials of the future. Overvelde, for example, is now trying to embed sensors into his 3D reconfigurable structures. He hopes to design smart metamaterials that sense their environments and reconfigure themselves in response—a kind of emergent behavior akin to flocking birds or schools of fish. Because the response doesn’t depend on a central computer or device, the structure will still work even if sliced in half.
</p>
<p>
 This kind of technology may bring to mind
 <a href="https://www.youtube.com/watch?v=Aq5ydeWWr4A">
  T-1000
 </a>
 , the shape-shifting homicidal robot from the movie
 <i>
  Terminator 2
 </i>
 , or the green blob from
 <a href="https://www.youtube.com/watch?v=HJT2QOpnYb8">
<i>
   Flubber
  </i>
</a>
 . “That’s clearly science fiction at the moment,” says
 <a href="https://amolf.nl/research-groups/mechanical-metamaterials">
  Martin van Hecke
 </a>
 of AMOLF. But as devices become more integrated with materials, the boundary between the two will blur. “If you really think ahead of what we’ll have in 50 years, I would expect that some devices we wouldn’t even recognize as devices. They might look like a piece of Jell-O or inert plastic, and they would pop into something completely different. I don’t see a physical reason why that’s impossible.”
</p>
<p>
 As even today’s origami-inspired structures are showing, there’s less distinction between material and machine. “To me personally, that’s an exciting thing because it means, conceptually, a different type of object,” he says.
</p>
<p>
 That amalgamation of material and machine may even include Cohen’s cell-sized self-folding robots. He imagines linking them together, creating an interconnected swarm that behaves more like a material, comparable to the microbots in the movie
 <a href="https://www.youtube.com/watch?v=ep2-W1X65KI">
<i>
   Big Hero 6
  </i>
</a>
 .
</p>
<p>
 But as of now, their technology—and other origami-inspired metamaterial-slash-robot—is just an important proof-of-principle, Cohen says. “The rest of this is our vision,” he says. “The chance that it all comes true is zero. The chances that some comes true, or at least inspires something that does come true, are quite high.”
</p>
<div class="newsletter"></div>
<p>
 Photo credits:
 <a href="https://www.flickr.com/photos/william_hartman/14779617283/">
  William Hartman/Flickr (CC BY)
 </a>
 , NASA JPL,
 <a href="https://www.flickr.com/photos/cambridgeuniversity-engineering/4706414628/in/photostream/">
  Engineering at Cambridge/Flickr (CC BY-NC-ND)
 </a>
 ,
 <a href="https://www.flickr.com/photos/23913057@N05/">
  Goran Konjevod/Flickr (CC BY-NC)
 </a>
</p>

<p>The ancient art is driving a revolution in materials science and robotics.</p>

Did the Universe Start with a Bounce Instead of a Bang?

<p>
 For a few physicists, the Big Bang wasn’t the beginning of the universe.
</p>
<p>
 Rather, they say, the universe existed before that point, stretching forever into the past as well as the future. While the universe is expanding today, it was contracting in the time before the Big Bang. In this picture, the Big Bang isn’t so much a bang but a bounce, a moment when a shrinking universe reversed course and began to grow.
</p>
<p>
 And according to their theory, the universe could bounce again. Today’s expansion could be followed by collapse in the far future, followed by another bounce. Some physicists have suggested this bouncing could be infinite, reviving a cyclic cosmology first proposed in the 1930s.
</p>
<div class="related"></div>
<p>
 This is, of course, counter to the way most cosmologists see it, where everything started with the Big Bang: From a tiny point of extreme density and temperature, the universe expanded and cooled, giving rise to stars, galaxies, and planets. Before the Big Bang? There was nothing, they say.
</p>
<p>
 But how that infinitesimal, hot, and dense point came to be remains an unanswered question. Bouncing theories could promise to explain the origin of the cosmos. Whether a single bounce or endless bounces, a handful of cosmologists have spent the last couple decades tinkering with these ideas. But to others, bounce theories are simply speculative and controversial, and to some, they’re discredited and wrong.
</p>
<p>
 Much of the debate between Big Bang and Big Bounce proponents revolves around the viability of inflation, the mainstream view of how the universe has come to be the way it is today. And although any cosmologist would agree that inflation is, at the very least, incomplete, the vast majority considers it the best model yet. Still, bounce proponents see fundamental flaws in this model.
</p>
<p>
 “Inflation’s not doing too well,” says Neil Turok, director of the Perimeter Institute for Theoretical Physics. “It’s had its day. It was useful when it was invented in the early 1980s.” But now, he says, we need a new theory, and that theory could be a bouncing universe.
</p>
<h2>
 A Cosmic Growth Spurt
</h2>
<p>
 The standard story of inflation goes like this: shortly after the Big Bang, the universe ballooned rapidly—much faster than its normal expansion. This sudden growth was necessary to create the smooth, flat, and uniform universe that scientists see today.
</p>
<p>
 Cosmologists first developed inflation in the early 1980s, before balloon-borne experiments and satellites returned increasingly precise data on the state of the early universe. These observations measured the leftover radiation from the Big Bang, a ubiquitous glow called the cosmic microwave background. The radiation is patchily distributed, with some spots hotter and cooler than others, an auspicious result since the exact nature of this patchiness was precisely what inflation predicted.
</p>
<div class="ad"></div>
<p>
 Inflation also predicted the mass density of the universe, also measured from the cosmic microwave background. “We’ve measured the mass density to better than a half percent accuracy, and it agrees perfectly with what inflation predicts—which is just gorgeous,” says Alan Guth, a physicist at MIT and the first who proposed inflation in 1980.
</p>
<p>
 “It’s really remarkable how much this simple idea of inflation has done,” says Robert Brandenberger, a physicist at McGill University. Although he’s exploring alternatives to inflation, the theory is the most self-consistent one out there, he says. “It’s successful because it predicted many things—and I emphasize predicted. Early in my career, we didn’t have the data. I saw inflation pass many more tests.”
</p>
<p>
 Still, while these successes have been more than encouraging for inflation, the evidence has yet to convince everyone. One prediction that might quell some dissent would be the detection of primordial gravitational waves, ripples in the fabric of space and time that originated from fluctuations of the gravity field in the early universe. It almost happened: In March 2014, the BICEP2 experiment at the South Pole claimed to have seen these gravitational waves. But that heralded discovery
 <a href="http://www.pbs.org/wgbh/nova/next/physics/bicep2/">
  vanished
 </a>
 when astronomers realized the signal could have been entirely due to dust in the galaxy.
</p>
<div class="pullquote">
<p>
 “If you have a theory that can’t be disproved, you should be dissatisfied with that.”
</p>
</div>
<p>
 Inflation is not without its theoretical issues either. Some critics say that inflation requires initial conditions that are too specialized and contrived to be realistic. To get inflation started, the early universe had to be just right.
</p>
<p>
 Another point of contention is that inflation could imply the existence of an infinite number of universes. In the early 1980s, physicists discovered that inflation goes on forever, stopping only in some regions of space. But in between these pockets, inflation continues, expanding faster than the speed of light. These bubbles are thus closed off from each other, effectively becoming isolated universes with their own laws of physics. According to this theory, we live in one of these bubbles.
</p>
<p>
 While inflation proponents embrace this so-called multiverse, detractors say it’s absurd. If anything can happen in these bubble universes, then scientific predictions become meaningless. “If you have a theory that can’t be disproved, you should be dissatisfied with that,” Turok says. “That’s the state with inflation and the multiverse, so I would say this is not a scientific theory.”
</p>
<p>
 Even ardent supporters of inflation would agree the theory is incomplete. It doesn’t say anything about the moment of the Big Bang itself, for example, when the known laws of physics break down at what’s called a singularity.
</p>
<p>
 What inflation still lacks is a deeper foundation. Physicists have tried connecting inflation with string theory—the best candidate for a so-called theory of everything. But it’s still a work in progress. “With inflation, we basically add something by hand and we say it works, but we don’t have a more theoretical understanding of where it could come from,” says Steffen Gielen of the Perimeter Institute, who works with Turok on bouncing models.
</p>
<h2>
 Bouncing Ideas
</h2>
<p>
 The suggestion that the Big Bang wasn’t the absolute beginning originates from the first half of the 20 th century, when physicists proposed a cyclic universe. But at the time, no one understood the details for how the universe could enter and emerge from each bounce.
</p>
<p>
 Todays’ physicists still have their work cut out for them, but now they have all the tools of modern particle physics and string theory. In 1992, Maurizio Gasperini and Gabriele Veneziano first used these modern ideas to revisit a pre-Big-Bang universe. Ten years later, Turok and Paul Steinhardt, a physicist at Princeton University and one of inflation’s pioneers turned critic, expanded on that work. They have since become two of the most outspoken detractors of inflation and proponents of a bouncing universe.
</p>
<p>
 A bouncing universe, they argue, could produce the cosmos we see today—but without inflation. The universe doesn’t need a period of super-expansion to reach the smooth, flat state we see today; it can do so while contracting. And because every corner of a shrinking universe would have been in contact with one another, the whole cosmos could settle into a uniform temperature—again, just as we see it today.
</p>
<p>
 Because so much of the early universe is unknown, theories of cosmology can vary widely. Inflation, for instance, isn’t one particular theory but a class of models, each a bit different in detail. Likewise, physicists have theorized many ways for how a universe can bounce.
</p>
<p>
 In one case, dubbed a matter bounce, the universe only bounces once. The collapse into the bounce is like a reverse-order Big Bang. Another version, called an ekpyrotic model, can be cyclical, with contraction followed by expansion followed by contraction, and so on. The anamorphic universe might be similarly cyclical.
</p>
<p>
 Pretty much all models require some sort of new physics. The differences between these models depend on the details, whether it’s new theories or exotic types of matter that halt the inertia of collapse and guide the universe through the bounce. Figuring out what happens at the bounce poses a big challenge, because that point is where the laws of physics fail, just as they do at the start of an inflationary universe.
</p>
<p>
 At the bounce, the universe collapses into a singularity, in which Einstein’s theory of gravity, general relativity, breaks down. Relativity isn’t currently compatible with quantum mechanics, which is needed at the small scales of the singularity. To unite the two, physicists have been searching for a theory of quantum gravity, which doesn’t yet exist.
</p>
<p>
 Over the past year, though, physicists have claimed modest progress on how to handle the singularity. Turok and Gielen have outlined how a simplified, toy model of a universe could undergo a quantum bounce. A bouncing universe containing only radiation—not unlike the radiation-dominated cosmos at the Big Bang—could cross the singularity in a way like quantum tunneling: According to quantum mechanics, a particle can spontaneously appear on the other side of a barrier that would otherwise be impenetrable in non-quantum physics. A collapsing universe can act like a particle and tunnel through the barrier-like singularity, appearing on the other side as the expanding universe we know today—and evading the singularity’s problems.
</p>
<div class="pullquote">
<p>
 We’re still far from solving the problem of the singularity.
</p>
</div>
<p>
 Meanwhile, Steinhardt and Anna Ijjas of Princeton University have proposed a way the universe could bounce without evoking quantum mechanics. They’ve shown that some exotic, negative energy could prevent a universe from collapsing into a singularity in the first place. By avoiding a singularity, the universe never gets small enough for quantum mechanics to come into play, so you don’t need quantum gravity. The universe then proceeds to expand.
</p>
<p>
 But while these two proposals might be a small advance, neither marks a radical leap from what’s been done before, Brandenberger says. We’re still far from solving the problem of the singularity. “If we solve the singularity problem by evoking exotic matter, the question is just twisted,” he says. In other words, instead of explaining the singularity, you now have to explain the exotic matter.
</p>
<p>
 Without new physics, a bounce doesn’t seem likely, according to Guth. “One has to adopt rather special features that one would have to assume in the underlying laws of physics to make the bounce possible,” he says. “To me, that doesn’t seem like a good bet.”
</p>
<p>
 But it’s still too early to judge, Turok says. The theories aren’t mature enough to be testable yet. Eventually, though, models could start making predictions. Future, more detailed measurements of the cosmic microwave background might support a particular model of inflation or a bouncing universe. Perhaps the most promising evidence would come in the form of primordial gravitational waves, which are about the best indicators of what happened in the moments after the Big Bang (or bounce).
</p>
<p>
 Depending on what these waves look like, researchers can start ruling out models of both bouncing universes and inflation. While the BICEP2 findings in 2014 were a false alarm, researchers hope other instruments will succeed, including its successor, BICEP3. The Atacama B-mode Search is now operating in the Atacama Desert in Chile, and researchers are planning future experiments with names such as the Primordial Inflation Polarization Explorer, Qubic, and Polarbear.
</p>
<h2>
 The Right Path
</h2>
<p>
 In the end, however, it may not simply come down to an either-or choice between bouncing models or inflation, even though proponents of bouncing models sell their idea as an alternative. “What they’re doing is much more closely allied to inflation than they would have you think,” says Andrew Liddle, a cosmologist at the University of Edinburgh. “I don’t think it’s that radical of a departure.” Many of the mathematical tools used in bouncing models are similar to those used for inflation, he says. And when you apply observations like the cosmic microwave background, both bouncing models and inflation give similar results.
</p>
<p>
 You can even have both a bounce and inflation. “Now, sociologically, many people who study bounce cosmologies do so because they’re interested in finding an alternative to inflation,” says Sean Carroll, a physicist at the California Institute of Technology. “That’s fine, but if you just said, without any preexisting agendas, does the universe have a bounce, and if so, could it also involve inflation? I think you’d say sure.”
</p>
<p>
 Still, the debates between bounce proponents and the most outspoken inflation supporters can get contentious, each somewhat dismissive of the other side. The conflict is a reminder that science—and perhaps theoretical physics, in particular—is ultimately a human endeavor, filled with egos and subjectivity. Legacies and Nobel Prizes could be at stake.
</p>
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 “In the absence of data, you’re welcome to your opinion—opinion is all you have,” Carroll says. “All of these ideas have significant challenges and question marks next to them.” While a problem may be a deal-breaker for one person, it’s only a minor stumbling block to another. When blazing a new trail, the right path is often subjective.
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 Image/video credits: NASA, Allison Eck
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<p>A small group of physicists hoping to explain the origin of the cosmos are exploring theories other than the Big Bang.</p>

Marcus Woo

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