Canada chimes in
ASTRONOMER Keith Vanderlinde has studied the universe from some fairly remote regions of the planet — the Atacama Desert in Chile, for example, and even the South Pole — but his most recent work, involving a unique Canadian- made telescope, has him working in a much more hospitable location: the Okanagan Valley in southern British Columbia. Because of his teaching duties at the University of Toronto, Vanderlinde makes most of his visits to the site in summer, and this part of the province, south of Okanagan Lake, is “infinitely more pleasant” than Antarctica, he says with a chuckle. “The drive from Kelowna down to the telescope is absolutely beautiful.”
The telescope in question is called the Canadian Hydrogen Intensity Mapping Experiment, or CHIME. It isn’t as versatile or as expensive as many other astronomy facilities around the world, and yet its reach will be unprecedented: it may soon shed new light on the mysterious “dark energy” that drives the acceleration of the universe.
“It’s a telescope quite unlike any other on the planet,” says Vanderlinde, a CIFAR Global Scholar alumnus.
CHIME is taking shape on the grounds of the Dominion Radio Astrophysical Observatory, about 20 kilometres south of Penticton. The site is already home to several world-class radio- astronomy telescopes, but even so, there has never been a telescope quite like CHIME. To begin with, there is no giant, steerable dish — nothing, for example, like the ones that Jodie Foster’s character uses to listen for alien signals in the movie Contact. Instead, CHIME consists of four steel half-cylinders, running in parallel, each resembling a skateboarder’s halfpipe. Each half-cylinder is 100 metres long and about 20 metres wide. For the past two years, researchers have been working on a half-scale version of the project; in early 2015, work on the full-size version got under way.
In total collecting area, CHIME is on par with the largest of the steerable-dish telescopes, like the one in Green Bank, West Virginia, which has a diameter of 100 metres. But the similarities end there: CHIME is not steerable at all. It just sits there, its half-cylinders aimed straight up at the sky. Running down the centre of each half-pipe is a “feed line,” each holding 512 separate radio antennas.
“CHIME is unlike other radio telescopes — really, unlike any other telescope out there — because it has no moving parts,” explains CIFAR Senior Fellow Matt Dobbs, a professor of physics and computer engineering at McGill University. “It lies there and looks straight up at the sky.” The telescope sees an entire north-south stripe of the sky, from the north horizon to the south, at any one time. Using sophisticated electronics and a good deal of computing power, astronomers can reconstruct which radio waves have come from which direction in the sky.
“That makes it essentially a digital telescope,” says Dobbs. With a standard radio telescope — indeed, with ordinary optical telescopes as well — you have to aim the device at the object you’re studying. Not so with CHIME. Instead, “you process all the information that’s coming in from the sky, and you construct an image by processing all of it simultaneously in real time. The power of CHIME is that it lets you look in all of those directions at the same time.” The key is ultra-sensitive timing: if you know exactly when a particular radio signal reaches each of the antennas, its source in the sky can be pinned down with great precision.
The telescope itself doesn’t move, but the Earth does, and astronomers will use that to their advantage. As the planet rotates, the gaze of the half-cylinders sweeps across the sky.
CHIME “can see the entire overhead sky from north to south, and then each night, as the earth revolves through one full rotation, we see the entire sky from east to west as well,” says Dobbs. As the earth rotates, CHIME builds a picture of the entire sky. Dobbs compares it to the luminous green stripe that moves along a flatbed scanner.
The telescope will produce more than just flat pictures; its images will also have depth. CHIME is particularly sensitive to the radiation associated with clouds of hydrogen gas, which make up much of the large-scale structure observed throughout the universe (hence the “H” in CHIME). CHIME’s antennas monitor some 2,000 radio frequencies (or channels) at once. Because of the ongoing expansion of the universe, hydrogen clouds that are farther away are moving faster away from us than those that are closer. So the frequency of radio waves emitted by hydrogen clouds correlates with their distance, with radio waves from farther clouds being longer than those from closer clouds.
“With CHIME, we’re able to see in 3D,” says Dobbs. The result is what he calls a “data cube.” “We can peel away different layers of that, like the layers of an onion, and each layer is like a different period in the expansion of the universe.”
CHIME is an all-Canadian initiative with heavy involvement from CIFAR fellows in the Cosmology & Gravity program. It is led by researchers from the University of British Columbia (UBC), McGill and U of T, as well as from the Dominion Radio Astrophysical Observatory (DRAO). CIFAR Senior Fellow Mark Halpern of UBC is the principal investigator for the infrastructure funding. Along with Vanderlinde and Dobbs, other CIFAR scientists working with CHIME are Senior Fellow Gary Hinshaw from UBC and Senior Fellows J. Richard Bond and Ue-Li Pen from U of T. It was Pen’s work that first showed the potential of the hydrogen-mapping technique. R. Howard Webster Foundation Fellow Victoria Kaspi from McGill is the principal investigator for a proposed extension of CHIME to study transient radio signals. Others working on that extension are Senior Fellow Ingrid Stairs and Associate Scott Ransom.
To understand the problem the researchers are working on, you have to start with the Big Bang. That explosion of space and time, some 13.8 billion years ago, sent all matter rushing away from all other matter. At first, the cosmos was nearly homogeneous, but over the eons, gravity began to impose order on the chaos. Under gravity’s pull and in spite of the Big Bang’s initial outward push, matter attracted matter; clouds of gas and dust spawned the first stars, and those stars came together to form primordial galaxies. That, at least, was the standard picture until the late 1990s, when astronomers discovered that the universe is not only expanding but that expansion is accelerating. What’s causing that acceleration? Nobody knows. For now, physicists posit a substance that opposes gravity, pushing matter apart. They call it “dark energy.”
But there’s more to it than that: dark energy, it seems, has not been a significant part of the universe’s history. We know this because our clearest picture of the very early universe — the cosmic microwave background radiation, or CMB — shows no signs of dark energy’s effects. Back when the universe was very young, there was “no appreciable amount of dark energy,” says Dobbs. Today, on the other hand, “there’s a ton of it, that’s causing the universe to expand faster and faster.” Extrapolating between those two points, Dobbs explains, there must have been a moment, when the universe was between a third and a half of its current age, that dark energy “must have ‘turned on’ and taken over the expansion.”
So far, we have no way to probe that transitional period, because we simply don’t have any data from the universe’s “middle years.” In contrast, astronomers have studied the CMB, the “echo” of the Big Bang, in some detail. That radiation was, as noted, nearly homogeneous, but it was not, in fact, perfectly smooth. Instead, our best maps of the CMB show a pattern of tiny blotches, about one degree across. These go by the technical name of “baryonic acoustic oscillations”: essentially, spots of slightly higher density, or slightly lower density, compared to the average. Over billions of years, the higher-density “hot spots” evolved into the clusters of galaxies that make up the largest-scale structures we see in the universe today. But how did it happen?
“We’re looking for a growth spurt that began about halfway through the age of the universe, when dark energy took over,” Dobbs says.
Since everything about dark energy is mysterious, learning how its role evolved over cosmological time can only aid in efforts to pin it down. One possibility is that dark energy is the substance imagined by Albert Einstein a hundred years ago when he introduced a fudge factor into his theory of gravity (known as general relativity), thinking that this extra mathematical term was necessary to keep the universe stable. He called it the “cosmological constant.” (Later, when Edwin Hubble discovered that the universe was expanding, Einstein referred to the fudge as his “greatest blunder.”)
“It could be that dark energy is just this simple mathematical trick, this cosmological constant, that Einstein put into his equations,” Dobbs says. “That would be pretty boring.” A more exciting possibility, he says, is that there’s something wrong with our understanding of gravity, “that the equations that describe the gravitational interaction of matter on the largest scales aren’t quite right, that Einstein’s general relativity isn’t the whole picture.” Scientists already believe general relativity can’t be the final theory of gravity, because it’s incompatible with quantum mechanics. Unfortunately, there has been little progress in reconciling the two frameworks; “quantum gravity” remains elusive.
“Maybe we’ll see that dark energy is more complex than just the cosmological constant,” says Dobbs. “Maybe it will give us some hint as to how to rewrite the equations of gravity to perhaps include quantum mechanics.”
Mysterious radio bursts
CHIME is designed specifically to address the problem of the dark energy, but it may also shed light on another, very different, astrophysical problem. For about eight years now, astronomers have been puzzled by quick bursts of radio waves from random directions in the sky, seemingly from cosmological distances. They last only a few milliseconds, fading away just as quickly as they appear, and as far as we know, they don’t repeat: each has been a one-time occurrence. Astronomers have dubbed them “fast radio bursts,” or FRBs. The first six were detected with the Parkes Radio Telescope in Australia, beginning in 2007; later, another was detected using the Arecibo Observatory in Puerto Rico.
“The origin of these FRBs is completely unknown,” says Victoria Kaspi of McGill. Kaspi has worked extensively on seemingly similar phenomena, like pulsars and gamma-ray bursts; even so, FRBs present a mystery.
“They are certainly pointing to some new and so-far unexplained phenomenon, which is always an exciting thing,” says CIFAR Senior Fellow Ingrid Stairs.
Kaspi wasn’t involved with the original CHIME proposal, but as soon as she heard about the telescope’s unique design, she wondered if it could help in the investigation of FRBs. It wouldn’t involve any changes to the telescope itself, she realized; all that was needed was some extra electronics.
“It became clear to me that CHIME could be an incredibly useful tool for solving this brand-new mystery,” Kaspi says. “You could tap off the signal and use it for a totally different purpose, even while they’re working on their goals, at the same time. And it would just take a little extra money.” The telescope team recently submitted a proposal for funding to expand CHIME — not in physical size, but in the “back end” — in how the data will be processed. If her project goes ahead, CHIME could be “a world-class telescope for studying these fast radio bursts.”
One problem in the hunt for FRBs is that the telescopes involved have very narrow fields of view; they can examine only a very small piece of sky at any one time. It’s no wonder, then, that only a handful of FRBs have been detected so far. But a simple calculation from those few observations suggests that some 10,000 FRBs ought to be visible each day, if only we could monitor the entire sky. “We think that they’re actually an incredibly common phenomenon. They’re just really hard to detect,” she says. “That’s where CHIME comes in, because CHIME can see a huge area of the sky at any one time.”
The giant Arecibo dish, Kaspi points out, can monitor only a tiny circular patch of the sky, about a hundredth of the area of the full moon. CHIME, by comparison, will be able to cover about 300 square degrees of the sky at any one time. It could end up detecting about 30 FRBs per day, she says. “With CHIME, we could blow this problem away. We could start detecting hundreds of these, thousands of these, every year.”
As with dark energy, almost everything about FRBs is a mystery. How are they distributed across the sky? Are they associated with galaxies or some other structure that could be seen via optical telescopes?
“It’s really fun to have a brand-new problem to work on,” says Kaspi. “Lots of groups around the world are trying to do this, but CHIME could be the world leader.”
Video game technology
CHIME’s design is almost shockingly simple, and yet, although people have talked about such a design over the years, no one has actually tried to build it until now. “Just very recently it became possible to build a very good amplifier for cheap. Suddenly there’s not a big expense associated with the receiver, and the big expense is the moving reflector,” Halpern says. As Vanderlinde puts it, “We’re beating everyone else to the punch, and at a tiny fraction of the price.” Indeed, CHIME’s total cost is only $11.5 million. That’s very low compared to most other major projects in physics and astronomy. For instance, the European Extremely Large Telescope, planned for completion in 2024, will cost more than a billion euros.
The telescope’s electronics are cheap for a reason. They employ large-scale signal processing and the manipulation of large volumes of data, which are the same processes that lie at the heart of the booming cellphone and video game industries. “CHIME takes these two incredibly commercially important technologies and says, ‘Hey, we can make a fantastic telescope that way!’” Kaspi says. “It’s a case of Canada doing some very interesting science, but it’s science that also has these tremendous economic benefits and applications.”
Some of the electronics are “off-the-shelf ” parts, such as video cards used in high-end computer gaming systems, but the telescope has also employed custom design and programming. When it goes live, it will boast the largest radio correlator in the world, processing radio bandwidth equivalent to all of the world’s cellphone signals combined. “This sort of thing can’t be purchased off the shelf,” Dobbs says.
So, the next time you’re on your mobile phone, or notice your kids playing Grand Theft Auto again, think about how that same technology isn’t just good for chatting with the spouse or simulating mayhem and murder. It may also be helping to solve some of the deepest mysteries in the universe — thanks to an innovative Canadian telescope in the heart of the Okanagan Valley. •
Dan Falk is a science journalist based in Toronto.
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