As overwhelming as the light from the night sky can be, the universe may actually contain a great deal more matter that you can’t see.
This invisible matter is called dark matter, which does not interact with or reflect electromagnetic radiation, so only its gravitational effects can be observed. There may be as much as five times more dark matter than normal matter in the universe.
Dr. Hank Hoekstra of the Canadian Institute for Theoretical Astrophysics set out to determine exactly where this dark matter is to be found in the universe. He conducted his research with Dr. Howard Yee of U of T’s Department of Astronomy and Astrophysics and grad student Michael Gladders (now at the Observatories of the Carnegie Institution of Washington in Pasadena, California).
“Most of the matter in the universe is dark matter, it’s invisible, so then the question is…whether the light distribution [of regular matter] traces the underlying dark matter,” said Hoekstra. “You could have a situation in the universe where you see a lot of light and there’s a lot of dark matter, but there could be places where you see very little light but there could be significant amounts of dark matter,” he explains. An analogy would be trying to determine where people on Earth live by the light distribution around the planet seen from above at night. There are regions, like Asia, where there is a disproportionate amount of darkness compared to the number of people.
Hoekstra and his team used a new method known as weak gravitational lensing. This technique studies the way in which light from a distant object is bent when passing by a large body. This allowed them to determine that dark matter haloes have about five times more mass than their host galaxy. Weak gravitational lensing also allowed them to determine the shape of these haloes. The lensing signature is different above and below the plane of a galaxy as compared to along the edges. “This is consistent with ‘cold dark matter’ theories, which predict a flattened halo,” explains Hoekstra. “Every halo forms from the merging of smaller clumps [of dark matter], and as a result of this merging you get this particular flattening.”
Cold dark matter, formed of slow moving, massive particles, pulls together forming bigger clumps, remaining associated with the host galaxy. Hot dark matter consists of particles like neutrinos, which have zero or near-zero mass and move so fast that they hardly interact with each other or with any galaxies.
Dark matter may account for 25 per cent of the mass of the universe, with normal matter making up only five per cent. “Because there’s so much of it, and it’s gravity that really counts, [dark matter] basically determines what the large scale structure [of the universe] will look like…once we understand what the dark matter is doing, we can also hopefully understand how the galaxies formed.”
Weak gravitational lensing can also be used to study the large-scale structure of the universe, showing it to consist of thin filaments and dense knots of matter, between which there are vast light-years of emptiness. It turns out that on the large scale, in terms of over a few million light years, the distribution of dark matter and regular matter in fact trace each other well.
As strange as the concept of dark matter might sound, astrophysicists are now invoking dark energy to account for the observed acceleration of the expansion of the universe. Dark energy could be responsible for the remaining 70 per cent of the universe’s mass.
