by Harry Cliff
However, all is not lost. We may never be able to build the Ultimate Collider, but the universe itself may offer us another way to get tantalizingly close to the Planck scale. For most of the last fifty years we have only been able to look back to a point 380,000 years after the big bang, when the primordial fireball cooled to form a transparent gas, releasing the blaze of light that would fade to become cosmic microwave background. The cosmic microwave background represents a firewall through which ordinary telescopes cannot penetrate. However, as of September 2015 we have a brand-new way to look at the universe, one that may allow us to peer right back to an instant after the beginning.
THE ECHO OF CREATION
Deep in the forests of southern Louisiana, where the warm, humid air makes the loblolly pines grow tall, a revolution in our understanding of the universe is underway. Just outside the small town of Livingston is a telescope like few others on Earth: a huge L made from two 4-kilometer-long concrete tubes that slice their way through the woodland at a right angle, like some giant geometer’s tool. It’s a strange look for a telescope, but that’s because this instrument doesn’t study the universe in light. It uses gravitational waves.
To reach LIGO (the Laser Interferometer Gravitational-Wave Observatory), you turn off Highway 190 at Livingston, bump across a poorly maintained railroad crossing, and wind your way through forestry land, passing the occasional house or trailer, some with broken-down cars rusting in the front yard. Rounding a bend onto the final 500-meter straightaway leading to the gates of the observatory, a sign instructs you to slow to a 10 mile per hour crawl, hinting at the extreme sensitivity of the instrument ahead.
LIGO burst into the news in February 2016 after announcing the first direct detection of gravitational waves, ripples in the fabric of spacetime predicted by Albert Einstein almost exactly a century earlier. Gravitational waves are a direct consequence of general relativity, which describes spacetime as a dynamic fabric that can be bent, stretched, and squashed by massive bodies like planets or stars. Its elastic nature also allows it to carry waves, ripples that expand and compress space and time as they pass by.
At 5:51 a.m. on the morning of September 14, 2015, just as LIGO was about to begin its first data-collection run following a major upgrade, the Livingston observatory picked up the first-ever signal of a passing gravitational wave. Seven milliseconds later, its twin instrument 3,000 kilometers away in Hanford, Washington, detected the same wobble in the fabric of spacetime as it tore northward through the Earth at the speed of light. The wave was the echo of a cataclysmic collision between two gargantuan black holes, each around thirty times the mass of the Sun, which had spiraled into each other 1.3 billion years ago in a galaxy far, far away. In the final fraction of a second, the merger produced a disturbance in spacetime so violent that it pumped out fifty times more power than the entire visible universe, converting three Suns’ worth of mass into pure gravitational energy. However, thanks to its extreme remoteness, by the time this almighty blast reached the Earth 1.3 billion years later, it caused the length of LIGO’s two giant arms to flex almost imperceptibly, by just one-thousandth of the width of a proton.
With this first signal, LIGO opened a new window on the universe. For the first time, it became possible to look into a hidden world, to study objects that emit neither electromagnetic radiation, nor neutrinos, nor any other subatomic particles. Colliding black holes and neutron stars, and perhaps things altogether strange and new, are now within reach.
After navigating the security gate, I was met by the head of the Livingston observatory, Joe Giaime, in front of the main LIGO building, a large metal-clad warehouse painted in two horizontal bands of blue and white to help it blend into its surroundings. Joe has been working on LIGO his entire career, starting out as a technician at MIT back in 1986. His doctoral supervisor was Rai Weiss, one of the founders of LIGO who would go on to share the 2017 Nobel Prize in Physics with Kip Thorne and Barry Barish for the discovery of gravitational waves.
Back in those early days, Joe didn’t realize quite how special the project he was embarking on was. He started a year before the name “LIGO” was even coined and helped put together a joint MIT-Caltech proposal that was submitted to the National Science Foundation in 1989. Just six years later, they had funding and were breaking ground at the twin sites in Louisiana and Washington, lightning fast for such a big science project.
Although he formally works in astrophysics, Joe spent thirty years without making a single observation of the heavens. He describes himself as an instrumentalist by inclination. “I cut my teeth building and designing things,” he said. Until 2015, his entire career was devoted to getting LIGO to the point where it would finally be able to start studying the universe.
Joe led me from the main building on a short walk to a bridge spanning one of LIGO’s giant arms. From there we could look in a dead-straight line through the forest to a point where the concrete tube met its end station, 4 kilometers in the distance. To our left, a second arm emerged from the main LIGO building and plunged through the forest at a right angle.
LIGO works by detecting tiny changes in the lengths of the two arms as a gravitational wave causes space to expand and compress as it passes by. Inside the main building, a laser is split into two beams that are fired down the two perpendicular arms, which then bounce off mirrors at the end stations and back down the same tubes to recombine in the main building. Generally, a gravitational wave will change the length of one arm more than another, so that when the lasers recombine, the positions of their peaks and troughs are ever so slightly out of whack, producing what is known as an interference pattern.
At least, that’s the idea. But the effect of a passing gravitational wave is so minute that it can be easily overwhelmed by vibrations from all manner of things here on Earth. The forest surrounding the Livingston observatory is owned by an international wood and paper company—the hot, humid Louisianan climate means that trees grow unusually fast here—and the felling of trees is an occasional source of background noise (to say nothing of British science writers and their noisy rental cars). Nonetheless, LIGO manages to live in what Joe described as “uncomfortable harmony” with the local logging industry.
It’s not just falling trees that LIGO has to contend with. The instrument is sensitive to impossibly tiny changes in the length of the arms, all the way down to 10-19 meters, one ten-thousandth of the width of a proton, or “the private space enjoyed by two quarks,” as Joe put it. However, there is a long list of sources of vibration that could shake the optics by far larger amounts, everything from footsteps in a nearby corridor to waves crashing on the continental shelf in the Gulf of Mexico. LIGO copes with all of these through an ingenious system of seismic isolation, including sets of quadruple pendulums that keep the mirrors as still as possible.
LIGO first achieved sensitivity in 2005, shortly after Hurricane Katrina devasted the nearby city of New Orleans and the surrounding area. Even in those early days there were hopes that LIGO might see a signal, but it would take a further ten years of laborious upgrade work to finally reach the level of precision that allowed them to catch their first wave.
Back in the main building, Joe showed me into the control room, with banks of desks and computer monitors facing larger screens on the front wall. Just as we entered the room there was a commotion among the staff, with some standing up from their desks to study the screens in more detail. “We’ve lost lock,” said Joe. Just at that moment, a seismic wave from a 7.1 magnitude earthquake near Indonesia’s Maluku Islands had hit LIGO, which even at a distance of 15,000 kilometers was enough to shake the optics out of alignment. “We won’t be able to do anything now for several hours while that dies down,” Joe told me, “while those waves go round and round the Earth.” Standing in the control room I couldn’t help but marvel that any of this worked at all. To be able to measure length changes ten thousand times smaller
than a proton, while contending with tremors from all and sundry including earthquakes on the other side of the planet, seems nothing short of miraculous.
Nevertheless, it does work, and beautifully. Despite operating for just a few short years, LIGO is already starting to change our understanding of the universe. Perhaps the most significant event so far was detected on August 17, 2017, nearly two years after it landed its first gravitational wave. This time, both LIGO observatories, along with their European counterpart, Virgo, in northern Italy, picked up a signal from a collision between two neutron stars, the ultradense husks left over from violent supernova explosions. As soon as the gravitational wave was detected, LIGO and Virgo sent out an alert to telescopes all over the world, which started to feverishly scan the sky for an accompanying electromagnetic glow. Unlike black holes, a collision between two neutron stars is expected to produce a powerful burst of electromagnetic radiation—which was indeed spotted eleven hours later, emanating from a galaxy 140 million light-years from Earth.
Not only was this the first time a gravitational and electromagnetic signal had been detected from the same collision, it also made astrophysicists reevaluate their ideas about where the chemical elements come from. As we saw, for a long time it was thought that the heavy elements beyond iron were made when giant stars went supernova. However, there had been a growing suspicion that neutron star mergers might in fact be their main source. Sure enough, spectroscopic studies of the light from the 2017 collision revealed telltale signs of the production of precious metals including gold and platinum, suggesting that a large fraction of the metal in a piece of jewelry came from exactly such a collision.
Back in Joe’s office with coffees in hand, we talked through LIGO’s plans for the next few years. “There’s a scaling law that’s wonderful and horrible,” he explained. Every time you double the instrument’s sensitivity, you can see twice as far into space, but because the volume of space you can scan increases with the cube of the range of the instrument, the number of events you can detect goes up by a factor of eight. This creates a temptation to always be making improvements instead of collecting data. “Everybody has this itch to make tiny little changes. You can convince yourself that the payback is so enormous that essentially you should never be running.”
In practice, they take a more pragmatic approach and spend half the time recording data and half improving the instrument, with an aim to double LIGO’s sensitivity by 2024. This will bring a huge unexplored region of the universe into view. But in the longer term there are even grander plans in the works.
By proving that gravitational waves really exist, LIGO effectively invented an entirely new type of astronomy. A number of large projects are now being planned that could have truly revolutionary impacts on our understanding of the universe and its history. Europe is currently drawing up a proposal for the Einstein Telescope, a huge underground triangular observatory with three 10-kilometer-long arms, while in the United States a supersized version of LIGO with 40-kilometer arms known as the Cosmic Explorer is being studied. But perhaps most ambitious of all is LISA (Laser Interferometer Space Antenna), three spacecraft flying around the Sun in an equilateral-triangle formation, firing laser beams back and forth between them, effectively creating an observatory with arms that are 2.5 million kilometers long. Having spent years in the doldrums, the LISA project has been reinvigorated by LIGO’s discovery of gravitational waves, with the European Space Agency planning to launch the mission sometime in the 2030s.
Joe told me that these telescopes will be so sensitive that they will be able to see every black hole collision in the observable universe and look right back to the time when the first black holes formed from dying stars. One extremely exciting possibility is that they might discover a population of primordial black holes that didn’t form from collapsing stars but during the big bang itself. In the first second, when the universe was extremely hot and dense, it’s possible that fluctuations in the quantum fields could have created regions that were so dense that they collapsed into black holes, which in principle could have survived to this day. If the Einstein Telescope or the Cosmic Explorer saw black hole mergers taking place before the first stars formed, that would be an unmistakable smoking gun of their existence. Another possibility is that it could find black holes that weigh less than the Sun, a weight that marks them as too light to have formed from a collapsing star. Discovering primordial black holes would be huge, not only telling us about the conditions in the very first moments of the big bang but potentially also providing an explanation for some of what makes up dark matter.
However, perhaps the greatest prize of all would be seeing back into the fireball of the big bang directly. Until around 380,000 years after time zero, the entire universe was filled with a searing-hot plasma of subatomic particles. This fireball was opaque to light—any photons flying about before this time would have been endlessly pinballing off protons and electrons—which means that we can’t see back any further than this with ordinary telescopes. Gravitational waves, on the other hand, don’t get absorbed by matter and so would have been able to zip unimpeded through the universe from its very earliest moments.
For gravitational waves from the early universe to still be detectable today they would have to have been produced by unimaginably violent processes. One possibility we’ve already met: the collision between expanding Higgsy bubbles around a trillionth of a second after the big bang. This was the idea that matter may have won out over antimatter thanks to the Higgs field turning on unevenly through the universe, forming bubbles in the hot plasma. As these bubbles grew, they would have smacked into one another with incredible force, sending powerful ripples through the fabric of spacetime, the faint echoes of which could be picked up by one of the next generation gravitational wave observatories. If future astronomers were able to detect such a signal it would tell us directly about the physics going on the first trillionth of a second and potentially help us unravel the mystery of where the matter in our apple pie ultimately came from.
But maybe, just maybe, we’ll be able to see back even further. We said earlier that it’s almost impossible to imagine being able to build a particle collider powerful enough to study quantum gravity. But if you go back far enough perhaps the entire universe once acted as the ultimate collider. This time is believed to have occurred around a trillionth of a trillionth of a trillionth of a second after time zero, when the universe underwent a short period of extremely rapid expansion known as “inflation.”
Exactly how inflation happened, or indeed if it happened at all, is still uncertain, but it is thought that in an incredibly short time—a mere ten-billionths of a trillionth of a trillionth of a second—the universe ballooned to at least 10 trillion trillion times its previous size. To put that in some kind of perspective, if the period at the end of this sentence grew by the same factor it would end up a hundred times larger than the Milky Way.
Inflation explains a number of peculiar features of our universe, in particular why it looks remarkably uniform whichever direction you look in. This is really surprising because without inflation, two opposite regions of the sky should never have had been in contact long enough to have reached the same temperature and density. Inflation solves this problem by saying that even two points on opposite sides of the observable universe were once part of the same tiny patch of space. Perhaps even more remarkably, inflation also says that all the large-scale structure in the universe—in other words the way galaxies are distributed through space—ultimately came from tiny quantum fluctuations that occurred at distances far smaller than an atom and that then got blown up by inflation to absolutely enormous scales. These quantum fluctuations resulted in some areas of the universe being slightly denser than others, and these overdense regions eventually went on to collapse under gravity to form everything we see when we look up at the night sky. In other words, the billions of billions of galaxies in th
e observable universe were ultimately seeded by tiny wobbles down at the quantum level in the first instant of cosmic time.
Inflation is a generally accepted part of the cosmological story, and while many of its predictions have been confirmed there is still no unequivocal evidence that it actually happened. There’s no way to look back directly to a trillionth of a trillionth of a trillionth of a second after the big bang with ordinary telescopes, but the existence of gravitational waves may now make it possible. If inflation really did happen, it would have roiled spacetime, creating wild waves in the fabric of reality that should still be echoing through the universe. Today they would be stretched to incredibly long wavelengths and be impossibly faint, but nonetheless there is a chance that future planned observatories may be able to catch these whispers from the birth of the universe.
The trouble is that inflation isn’t just one theory—there are a whole bunch of different ways that inflation could have happened, each involving different numbers of quantum fields and different energy scales—and only some versions of inflation would produce powerful enough gravitational waves to be picked up directly. In the very simplest models, the waves would be too weak for even the giant LISA space observatory to detect. In that case, the one chance we might have to hear the echoes of inflation is to look for their effect on the oldest light in the universe, the cosmic microwave background.
Theorists have calculated that gravitational waves produced by inflation would have left twisting patterns, known as “B-modes,” in the cosmic microwave background. These are fiendishly difficult to detect, suffering from the double whammy of being both extremely weak and easily confused with more mundane backgrounds, like dust in our own galaxy. In fact, in 2014, the BICEP2 telescope at the South Pole caused a worldwide sensation when the team announced that they’d seen evidence for twists in the cosmic microwave background caused by gravitational waves from inflation. Breathless talk of a new age in our understanding of the cosmos and the imminent award of Nobel Prizes abounded, but as time passed the BICEP2 team were forced into a humiliating climbdown. It gradually became clear that they hadn’t properly accounted for the effect of galactic dust, and after reanalyzing their results the claimed signal melted into the background.
