Quantum Leap 
Discovery 
The Final Frontier
To solve the mystery of dark energy—a phenomenon that could reveal the origins of the universe—Texas astronomers need a $34 million telescope and a little bit of luck. There are only two problems: it may not be dark, and it may not be energy. In fact, it may not exist at all.
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With its own water supply, fire marshal, staff homes, and lodge for visitors, formerly called the Transients Quarters (now the Astronomers’ Lodge), the observatory is its own little campus on a hill, one that abides by strange customs. People drive around at night with only their parking lights on, for instance, to keep things dark. It’s a quiet place, where an influential portion of the population sleeps during the day and works at night, and the sky commands attention—not just from astronomers but from anyone who finds herself on the mountain. The night I arrived at the observatory, a woman passing me on the road told me to be sure to take in the stunning sunset; the next night another person bid me look out at the storms to the west.
But there’s the sky we see, and then there’s the sky astronomers investigate. Enter through a side door of the HET building, and it’s as if you’ve stepped into a small manufacturing plant, with a concrete loading area and a flight of metal stairs leading to the site manager’s spare office. The manager in this case is Bob Calder, who worked for the Subaru telescope, in Hawaii, and the Smithsonian Astrophysical Observatory, in Cambridge, Massachusetts, before moving to Texas to oversee the operation and maintenance of the HET—though “maintenance” is perhaps too plain a word, too reminiscent of boilers or car engines, to convey the kind of care one has to take with a major telescope. Even cleaning is a delicate task. To focus on the very thin slices of sky required for astronomical work, the telescope’s large primary mirror must be absolutely smooth, free of any imperfections larger than one tenth the wavelength of the light hitting it. The aluminum coatings on the HET’s mirrors are the thickness of oil on water; they are cleaned on Mondays, Wednesdays, and Fridays by a four-person team that applies a carbon dioxide “snow” with a wand.
As site manager of the HET, Calder also greets the dignitaries and potential donors who come to tour the telescope, a duty that has, on occasion, served to remind him that he is not in Cambridge any longer. “We had a guy one time who tried to give me a wad of bills after the tour,” Calder recalled. “He goes, ‘Let me do something for your family.’ I suggested he donate to the observatory instead, and he said he was going to, but he still wanted to give me some money too.” (Calder declined the gift.)
He escorted me down through the control room, where the telescope is manipulated via computers and electronics, and inside the dome. It is a sort of cross between a science cathedral and a science factory, given over to one giant piece of industrial equipment aimed at the heavens: the HET. While it calls to mind some sort of futuristic vehicle poised to be launched into space, the telescope actually rotates on a fixed base, letting space come to it. Ninety-one hexagonal pieces form the primary mirror, a silver dish inside a steel cage; the light from the night sky hits this and then is relayed to an instrument above the mirror called the tracker. Itself a complex piece of machinery that took two years to build, the tracker can follow the image of a particular object as that image crosses the mirror. (Due to the rotation of the earth, the sky and its contents shift, relative to the position of the telescope, over the course of the night.)
Calder flipped a switch, engaging the air bearings below the telescope, and like a big hovercraft, it rose a few inches and then began to rotate. Moments later, a recording of squawking birds began to play, which is supposed to ward off real birds that might fly in through the ventilation shafts or through the top of the dome. Speaking over the sounds of the machinery and the squawking, Calder summarized the changes planned for the HET. “There’s one camera up there right now,” he said. “That whole top end is going to be replaced with a new top end that supports on the order of one hundred and fifty cameras, one hundred and fifty spectrographs.” Those spectrographs, he told me, would be deployed, over the course of 140 nights, to record one million galaxies.
Later that evening I crouched on a platform below the Smith telescope, where Hill, MacQueen, and graduate student Josh Adams were bustling about a prototype of the instrument that is slated to be put on the HET. The instrument is called VIRUS, for Visible Integral-field Replicable Unit Spectrograph, and it will incorporate more than 145 copies of a simple spectrograph, each of which registers data from at least 246 fibers arrayed within something called an integral-field unit. Scattered on the floor of the platform were scissors and cables and screws and Allen wrenches, as if the scientists were fixing a motorcycle rather than testing a highly engineered piece of equipment, which was contained within an irregularly shaped black box about four feet wide and attached to the underside of the telescope’s shaft by a metal harness. I watched as they checked the connections between the telescope and the box, cooled a crucial part with liquid nitrogen, and installed a glass plate, all in preparation for an observing run.
“There are four big links in the chain of observational astronomy,” MacQueen explained as he connected the liquid nitrogen tank to the device. “Collecting the light, processing the light, detecting the light, and all the software side of analyzing the light.” HETDEX will, if all goes smoothly, increase by about thirty-fold the area of sky that can be observed at any one time, speeding up the first three steps and thereby transforming the HET into a far more efficient and comprehensive cartographer of the skies. Rather than pointing at and shooting known objects, the upgrade will allow the research team to methodically obtain spectra from a giant celestial swath, taking information from more than 40,000 pixel-like sections of sky per exposure. Those will be captured by an 800-megapixel camera whose technology is similar to that of any vacationer’s digital camera. “This is just a super-duper digital camera of sorts,” MacQueen said.
Yet this is a camera that takes pictures of the remote past. Imagine a man in California trying to construct a map of the United States, using only the information brought to him by slow-moving (but long-lived) ants, creeping along at a hundred yards per year. The ants from New Jersey would have departed more than 50,000 years ago, before humans arrived in North America; the Midwestern ants some tens of thousands of years ago; the central California ants just a few years ago—so the map would record the East Coast as wild and uninhabited while the West Coast would contain highways and burger joints. This is the sort of map astronomers construct of the universe: The light that arrives from greater distances relays information about earlier eras.
If the big-bang model of the universe is correct, though, we cannot receive any messages from the earliest time periods, any more than our Californian mapmaker could learn about France. For some 380,000 years after the big bang, the universe was so hot that particles couldn’t form atoms; electrons whizzed around every which way, and light couldn’t travel any distance without colliding with those electrons. While this early fireball of a universe remains opaque to us, it is theorized that there must have been random variations in how densely its particles were distributed. Some areas were more dense than average, creating hot spots that drove particles out to cooler regions, which in turn caused waves to travel at the speed of sound. The result was a series of ripples through the sea of particles.
When the universe cooled down enough for atoms to form and light to escape, the oscillations ceased. At that particular moment in the history of the cosmos, the ripples were frozen into the density distribution of matter—so that now when astronomers map the heavens, they find the imprints of the early ripples in the way that galaxies are clustered. (It’s as if you drew a pattern on a balloon and then inflated it. As the balloon expanded, the pattern would persist though its scale would change.) These imprints are what the HETDEX team plans to measure, to learn how they might have grown over time. By comparing the traces of the oscillations at different epochs, both with one another and with a pattern already discovered in something called the cosmic microwave background—which gives us a picture of the universe just after the light first escaped—they will be able to evaluate the changes in the scale of the universe and from that infer whether dark energy has made a constant contribution to the universe’s expansion or has varied over the eons.
Some cosmologists have questioned whether a concerted push to do dark-energy experiments would be detrimental to the wider field of astronomy, hindering discoveries in other areas and relegating too many scientists to supporting roles on large-scale endeavors. After all, maybe there isn’t any such thing as dark energy. Maybe no one really understands how gravity works. Or maybe the universe is not as homogenous as theorists have assumed. Maybe “dark energy” is the wrong way to frame the problem.
But even the possibility that dark energy might be a mistaken notion is no cause for pessimism, according to Gebhardt. “That would be the most exciting answer—if we’re just so completely out to lunch,” he said. “It means there is something fundamental about the nature of our universe that we don’t understand. So whatever the answer is, it’s going to be a fundamental shift in our understanding. In other words, it’s not going to be a boring answer, and who knows where it will lead?”
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