Dark Matter, the Missing Link of the Universe

A substance scientists think might be key to the evolution of the universe has never been seen or touched, but an indirect method of observation is providing clues about where it is and what it does.

We know that the world is round, not flat, but many basic facets of the universe remain elusive to even the brightest among us. How did the universe begin, for instance, and what is it made of? Consider the substance called dark matter, which cosmologists agree constitutes a remarkable 80 – 90 per cent of all matter in existence. Scientists believe that it literally shapes the universe and has more impact on us than the sun, the moon and the stars all combined — yet dark matter has never been seen and we have no idea what it is made of.

“It could be a new type of particle – something yet to be discovered,” says UBC cosmologist Ludovic Van Waerbeke, who is trying to improve our understanding of dark matter not by studying it directly, but by observing its effect on starlight.

Dark Matter literally shapes the universe and has more impact on us than the sun, the moon and the stars all combined.

A tall man with a gentle manner and beguiling French accent, Van Waerbeke is an associate professor in UBC’s Physics and Astronomy department. He has made significant strides in the study of dark matter in collaboration with Catherine Heymans – a UBC postdoctoral fellow from 2005-2008 now teaching at the University of Edinburgh. Heymans and Van Waerbeke sparked interest around the globe, recently, by creating a giant 3-D map of this elusive substance, the first-ever multi-dimensional observation of dark matter at large scale. This vast cosmic map, which was unveiled at the annual meeting of the American Astronomical Society held this January in Austin, Texas, reveals a universe comprised of an intricate web of dark matter and galaxies.

The observations show that dark matter in the Universe is distributed as a network of gigantic dense (white) and empty (dark) regions, where the largest white regions are about the size of several Earth moons on the sky. Credit: Van Waerbeke, Heymans, and CFHTLens collaboration.

“Without dark matter,” says Van Waerbeke, “we probably wouldn’t exist.” He is convinced that dark matter is the link, not yet fully understood, that connects the universe and fundamental physics, and holds the key to many puzzling cosmic mysteries including the evolution of the universe. It could actually contain new physics, new understanding of how nature works,” he says.

“Without dark matter,” says Van Waerbeke, “we probably wouldn’t exist.”

The story of dark matter originated just 80 years ago with Swiss astronomer Fritz Zwicky. He observed that galaxies were moving much faster than could be accounted for by the gravitational pull of stars and planets and other visible entities of the universe. There had to be more mass in the universe, Zwicky reasoned, than could possibly be accounted for by ordinary matter, so he invented the concept of dark matter. Scientists began building models incorporating dark matter, says Van Waerbeke, and study after study supported the hypothesis. By the late 1970s, scientists had become convinced that the quantity of dark matter in the universe meant it played a pivotal role in the formation of galaxies and other cosmic structures.

Still, to this day dark matter has never been captured, and the only way to ‘see’ it is to examine how it affects the gravitational field around itself.

Van Waerbeke’s mapping relied on a process known as weak gravitational lensing, which presumes that light speeding towards Earth from distant stars follows a bent path owing to distortions in spacetime. According to Einstein’s theory of relativity, the dimensions of space and time form a unified continuum – spacetime – which is warped due to gravity.  The path of light coming from afar will be bent, therefore, in much the same way as a marble’s path would bend if it was rolled across a trampoline with someone standing on it.

To measure dark matter in specific regions of the cosmos, scientists measure the curvature in spacetime. Then they use Newton’s law of gravity to deduce the mass that caused the gravitational force in the first place. “It’s an indirect type of detection,” Van Waerbeke says, “but a very robust one.”

Together with Heymans, Van Waerbeke headed up an international team of some 20 scientists. They used data extracted from approximately 4250 images of light, captured over a period of five years by the Canada-France-Hawaii Telescope (CFHT) located on the summit of Mauna Kea in Hawaii. The light they analysed was emitted by no less than 10 million galaxies, which themselves spanned a distance of a billion light years. These galaxies are six billion light-years away from us, meaning that the light observed by the Mauna Kea telescope began its journey to Earth when the source galaxies were only about half the present age of the universe.

The light observed by the Mauna Kea telescope began its journey to Earth when the source galaxies were only about half the present age of the universe.

“By measuring how much the images from the survey deviated from the galaxies’ shapes, we knew how much gravity there was,” says Jonathan Benjamin, a PhD student at UBC who was part of the CFHT lens surveying team. The team subtracted the gravitational effect of ordinary matter known to be present, such as planets and stars, and this enabled them to work out the mass of the dark matter. While this basic measurement is very simple, the degree of precision is extremely high, says Benjamin.

Dark matter itself is a relatively new concept and, while its composition remains a mystery, many other things about it are well understood. “Dark matter, we know, has a mass,” Benjamin says. “We see lots of signatures of it in the universe. We really understand it in a lot of ways. It’s never been directly detected, so a particle physicist has never seen it, and they don’t know its fundamental physical properties in a physical sense, but as far as observational astronomy goes, it’s actually pretty well characterized.” When Benjamin was an MSc student he happened to enroll in Van Waerbeke’s cosmology class, and became so fascinated by this field of study that he asked Van Waerbeke to take over as his thesis supervisor.

Van Waerbeke works in a simple office, furnished with little more than a couple of chairs, a blackboard littered with equations, and the requisite wooden desk. An electric guitar rests on a small, dark blue sofa-chair which offers the only hint of colour in the room, aside from Van Waerbeke’s thick mop of curly red hair. In his spare time, he plays rock and blues with a local band.

“I was interested in sky things from the age of six or seven,” he says. “When I was a teenager I was an amateur astronomer, and built a telescope. I had interest in music as well, but I always had a very strong interest in and attraction for physics.”

His office window opens onto the drab white wall of the north section of Hennings Building, barely meters away. It’s ironic that this man, whose passion in life is to study the distant universe, works in a place that doesn’t even have a view. But perhaps it’s not inappropriate for the study of dark matter, since some of the researchers helping him map the cosmos have achieved their results not by peering at the sky through a telescope, nor even by viewing saved images of the cosmos, but by crunching numbers from catalogues of data.

It’s ironic that this man, whose passion in life is to study the distant universe, works in a place that doesn’t even have a view.

PhD student Jessica Ford is one such researcher. She came to UBC from the United States, specifically to work with Van Waerbeke. “When it comes to wanting to study the universe as a whole,” she says, “dark matter is the biggest mass component, so that’s the most important thing to study.”

Not everyone shares their passion for dark matter; the theory has attracted its share of skeptics. But are there credible scientists who favour a different theory for the unexplained pull of gravity in outer space? “Yes and no,” says Van Waerbeke. “It could be that our understanding of gravity at large scale needs revision.”

If gravitational fields in outer space are subject to different constraints from those known to Einstein and Newton, then there may not be as much matter out there as scientists have come to believe. “General relativity has never been tested, really tested, at the scale of galaxies and clusters of galaxies,” Van Waerbeke says, “so we are just assuming that gravity works the same way as it works here on earth or in the solar system or in nearby stars.”

Three diagrams on his blackboard illustrate the point: Ptolemy’s earth-centered universe, Copernicus’ sun-centered system and, halfway between the two, an “epicentric” model, in which the earth revolves around the moon and, in that manner, both revolve about the sun. We could be like the astronomers who espoused the epicentric view, he says – approaching the truth, but not quite there.

With that in mind, Van Waerbeke and his colleagues are already hard at work on their next project. When done, they will have created an even larger and more detailed cosmic map that reflects not only the curvature but also the colour and brightness of light from ancient galaxies. And they will have taken us yet another step closer to solving the dark matter mystery.

Photo: Canada France Hawaii Telescope