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Unveiling the hidden universe

by CIFAR Jul 24 / 12

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It may be the bottom of the world, but Antarctica, according to Matt Dobbs, is near the top of the list of great places to do science. Dobbs, a physicist and cosmologist at McGill University and a Fellow in CIFAR’s Cosmology & Gravity program, has made two five-week trips to the ice-covered continent.

His last expedition was in December, when his team helped install a new camera on a remarkable instrument known as the South Pole Telescope. The telescope’s 10-metre-diameter dish gathers ancient light from the universe’s fiery youth and may illuminate some of the great mysteries of the cosmos – its structure, its evolution and, especially, the nature of the mysterious entity known as dark energy.

December, of course, is “summer” at the South Pole – at least you’ve got the sun above the horizon – though the temperature doesn’t climb much above -30˚C. And that’s without the wind. “We tend not to count the wind chill,” Dobbs says. “It makes it too depressing.” The telescope is located at the mundsen-Scott South Pole Station, which rests on a high, flat plateau, nearly three kilometres above sea level. The altitude means there’s less atmosphere to hamper the telescope’s view. The air is also extraordinarily dry, allowing for greater transparency at the critical millimetre wavelengths that the scope will be gathering. The combination of “high and dry,” Dobbs explains, makes the South Pole ideal for this particular kind of observing.

The South Pole Telescope will scrutinize the cosmic microwave background (CMB), the so-called echo of the big bang. The CMB is radiation that was emitted when matter and radiation first went their separate ways, about 400,000 years after that initial explosion. Its discovery, in the mid-1960s, revolutionized cosmology. But that was just the beginning. In the 1980s and ’90s, astronomers used radio telescopes to map that radiation, which covers the entire sky, in fine detail. Then, in the late ’90s, came a shock. Astronomers measuring the properties of supernovae, or exploding stars, in distant galaxies found that the universe is not merely expanding, but also accelerating. Something is acting against gravity, pushing all the matter in the universe apart. Astronomers and physicists call that something “dark energy.” But what is it exactly and what role has it played in cosmic evolution?

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Matt Dobbs, a CIFAR Fellow and a physicist and cosmologist at McGill University, arrives at the Amundsen-Scott South Pole Station. PHOTO BY KEITH VANDERLINDE

Cosmology – the attempt to understand the origin and evolution of the universe itself – has ancient roots, though in its modern guise it may be said to have begun a century ago, when astronomers first deduced that we live in an expanding universe. At that time, optical telescopes on mountaintops were the only tools available. Today, hundreds of experiments are directly probing our universe’s rich past. “The experiments have become more and more ambitious, accompanied by a huge flood of data,” says Dick Bond of the University of Toronto, who has served as the head of CIFAR’s Cosmology & Gravity program since 2002, and has spearheaded the program’s growth over the last decade. Many cosmology experiments, by their very nature, have been expensive multinational collaborations – and yet Canada has been a major player, according to Bond, who is also a past director of the Canadian Institute for Theoretical Astrophysics. “It’s recognized that Canadian astrophysics – particularly as a result of the CIFAR program – is probably the highest ranked science enterprise in Canada, in terms of its international impact,” he says. Known as one of the world’s leading authorities on the CMB, Bond is also a core member of the Planck Surveyor team. Like the South Pole Telescope, the orbiting Planck satellite, launched in 2009, is being used to map the CMB in exquisite detail.

The South Pole Telescope will examine dark energy’s role back when the universe was about half its present age. At that time, dark energy may have interacted strongly with galaxies and clusters of galaxies. The South Pole Telescope won’t be imaging those galaxy clusters directly, but it will be looking at their effect on the CMB. “Sometimes you see a ‘hole’ in the CMB,” explains Dobbs. When that happens, “that’s a sign that there’s a cluster of galaxies in front,” he says.

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A timeline of the evolution of the universe, produced by the science team at NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). The team, which includes CIFAR Advisor Lyman Page(Princeton) and Fellow Gary Hinshaw (University of British Columbia), won the 2012 Gruber Cosmology Prize for its contributions to cosmology. IMAGE, COURTESY OF NASA

Those “holes” preserved in the CMB offer a clue as to what the universe looked like billions of years ago. They should also help scientists understand the changing role that dark energy has played in cosmic history. The strength of the dark energy itself is believed to be constant, but matter and energy are being continuously diluted by the expansion of the universe. This means that as the universe ages, dark energy becomes more and more important. “It appears that very recently, dark energy took over as the dominant constituent of the universe,” Dobbs says. “One mystery is ‘Why now? Why not much earlier or much later?’ We don’t know the answer to this.”

Scanning for these holes in the CMB is the first part of the South Pole Telescope’s mission. The second part is even more ambitious. With its new camera, the telescope will have even greater sensitivity – enough to measure the “polarization” of the CMB radiation. Light waves can be oriented in different directions; if you’ve ever used a polarizing filter on your camera or worn Polaroid sunglasses, you’ve seen this effect directly. The microwave radiation that makes up the CMB is believed to be polarized too, in part as a result of gravitation effects in the very early universe, back when the cosmos underwent its first, explosive growth spurt – the brief period that cosmologists refer to as cosmic “inflation.”

“Encoded in that microwave background radiation is information from a much earlier time – a fraction of a second after the big bang,” Dobbs says. At that time, known as the inflation era, the universe was awash in “gravity waves.” Predicted by Einstein’s theory of gravity, these are ripples in the fabric of space, released when anything with mass undergoes an acceleration. Gravity waves from the inflation era would have become stretched out as the universe expanded. That subtle effect – the interaction between gravity waves and the CMB photons – survives as a barely perceptible polarization effect in the CMB (amounting to differences of just 100 parts per billion). “It’s only today that we have the technology necessary to measure the orientation of these photons at the level that’s necessary to see this effect,” says Dobbs.

The South Pole Telescope isn’t the only attempt to measure CMB polarization; Planck is also searching for it, along with a dozen other experiments around the world. “We’re all in stiff competition trying to be the first ones to see it,” says Dobbs.

Established in 1986, CIFAR’s Cosmology & Gravity program now boasts 15 Fellows, along with nearly 20international Associates, and another10 Scholars and Junior Fellows. Some, like Dobbs, are experimentalists while others focus on theory; and they’re involved in every branch of cosmology, astrophysics and particle physics. One of the group’s great strengths is its ability to bring together researchers from a diverse array of specializations– everything from gravity to physical cosmology to numerical relativity. The interactions between those researchers– many of them are serendipitous, springing from the group’s annual meeting and other encounters – have fostered a steady stream of research and discovery. Largely due to the work of CIFAR scientists, Canada is now seen as a major player in all of those fields.

While many cutting-edge cosmology experiments are U.S.- or European-led efforts – not surprising, given the cost of such projects – some are uniquely Canadian. Take the CHIME project, for example. Short for “Canadian Hydrogen Intensity Mapping Experiment,”CHIME, like the South Pole Telescope, is designed to probe the structure and evolution of the cosmos. It’s a little easier to get to, however, being sited near Penticton, B.C., home of the Dominion Radio Astrophysical Observatory (DRAO). And while the South Pole Telescope is a U.S.-led project(with McGill as the sole Canadian partner), CHIME is a wholly Canadian experiment. It’s a joint venture of McGill, the University of British Columbia and the University of Toronto, and is hosted by DRAO. It is also a project with significant CIFAR participation– alongside Dobbs and Bond are Gary Hinshaw of UBC, Keith Vanderlinde of McGill and Ue-Li Pen of U of T, who are also key contributors. The project, says Dobbs, represents “a coming of age for Canadian observational cosmology.”

CHIME will consist of a giant array of radio antennas that will scan the entire sky simultaneously. The antennas are stationary – they cannot be “aimed” – rather, their total output will be digitized and compiled by a special computer, designed and built at McGill; the data will then make its way to a computer cluster at U of T, developed by Ue-Li Pen, another CIFAR member. The end result will be a map of the sky, just as though the array had been aimed in specific directions.

The technology isn’t exactly new, Dobbs points out, but because it will utilize machinery similar to that used by the cellphone industry, the components have fallen sharply in price. “For the first time, we’re able to assemble a telescope that’s able to do that processing entirely with commercially available components,” he says.

CHIME is designed to map the distribution of matter in the universe at the very largest scales, collecting radiation from a time when the universe was about half its presentage. The key question is how that distribution changed over time. Once again, dark energy is the real target. As the universe evolved, gravity and dark energy were engaged in a kind of tug-of-war over the destiny of the matter that makes up the cosmos, with gravity pulling the galaxy clusters together, and dark energy pushing them apart.

“Dark energy is the great unknown,”Dobbs says. “We want to measure its parameters very carefully. We know what the universe looked like before dark energy was there, before it was important, and we know what the universe ended up looking like today. But no one has really traced it through that period where it started to take effect.”

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LEFT: An artist’s interpretation of some of ACT’s observations. As CMB radiation leftover from the big bang moves through the universe, it interacts with astrophysical objects along the way – for example, becoming slightly redder and cooler if it passes through a galaxy cluster moving away from Earth and bluer and hotter if it passes through a cluster moving toward Earth. These changes to the primordial CMB anisotropies are known as “secondary anisotropies.” The resolution and sensitivity of ACT make it a relevant tool for studying these small-scale effects in addition to characterizing the primordial signal.

SO WHAT, EXACTLY, IS THIS MYSTERIOUS DARK ENERGY? One possibility is that the dark energy is the “cosmological constant” – an energy associated with empty space introduced by Einstein almost a century ago. At that time, Einstein added the cosmological constant to his equations for gravity as a kind of fudge factor to keep the universe stable. A few years later, when astronomers found that the universe really was expanding, it seemed like the cosmological constant was no longer needed (and Einstein referred to the fudge as the“greatest blunder” of his career). With the discovery of cosmic acceleration, however, the cosmological constant may be making a comeback – if it is indeed the culprit behind dark energy.“

A cosmological constant seems like the simplest possibility right now,” says Rob Myers, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo and also a CIFAR Fellow. “But it’s a fair question to ask: Can we make an observation that really nails it down? Is this really a cosmological constant or not?”

Unfortunately, when particle physicists calculate how strong the cosmological constant ought to be, they come up with a number vastly larger than the observed value. “It’s not what you’d expect at all,” Dobbs says. That has left physicists wondering if they may have missed something, if there could be an alternative explanation. “Is there something more fundamental – some symmetry, some physics that we don’t understand, that we haven’t thought of yet, which would be a simpler and more elegant explanation [ for dark energy]?” Dobbs asks. “The general feeling in the community is that we haven’t understood this effect yet.”

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A number of telescopes are currently studying the universe at microwave and millimetre wavelengths, looking for clues about its fiery birth. Leading the pack is the Planck satellite – a project led by the European Space Agency (ESA) – with crucial contributions from several CIFAR members, including Dick Bond, head of the Institute’s Cosmology & Gravity program. This all-sky Planck image, released in July 2010, shows not only the Milky Way and its companion galaxies, known as the Magellanic Clouds, but also a number of exotic radio sources such as quasar sand the nuclei of active galaxies. Seven “veils” in total are superimposed on the pristine radiation generated after the big bang, in which clues about the early universe are encoded. Planck is just one part of a larger effort. Together with data from the South Pole Telescope and the Atacama Cosmology Telescope, CIFAR members and their ESA, NASA and Canadian Space Agency (CSA) colleagues are using the data to remove the veils and reveal a more complete picture of the early universe than any image that these telescopes could produce on their own. IMAGE, ⓒ ESA, HFI AND LFI CONSORTIA

If things go well, experiments like the South Pole Telescope, the Atacama Cosmology Telescope and CHIME may reveal what makes dark energy work, whether it’s Einstein’s cosmological constant or perhaps something even more esoteric. Of course, more surprises could be in store. The Large Hadron Collider, now smashing protons together at incredibly high energies at CERN – the giant particle accelerator on the Franco-Swiss border, near Geneva – could reveal limitations in the current model of particle physics. And equally ambitious experiments are poised to directly measure gravity waves for the first time – perhaps within a decade. New discoveries, as well as new knowledge, loom on the horizon.

For Dick Bond – and for everyone in the field – it means that the euphoria that began in the 1990s shows no signs of letting up. Back then, cosmologists spoke of a “golden age” in their field. “And it wasn’t hype,” Bond notes, reflecting on the continuing advances, many of them brought about by CIFAR’s Cosmology & Gravity group, whose members bring an unparalleled depth of expertise to the field.

Cosmology’s golden age, it seems, is now in its third decade.