A huge effort to map dark matter across the cosmos has released its first data. Dark matter is the invisible “web” that holds galaxies together; by watching how clumps of it shift over time, scientists hope eventually to quantify dark energy – the even more mysterious force that is pushing the cosmos apart. The map will eventually span one-eighth of the sky; this first glimpse covers just 0.4%, but in unprecedented detail. It shows fibres of dark matter, studded with galaxies, and voids in between. The international collaboration, known as the Dark Energy Survey (DES), will present its preliminary findings at a meeting of the American Physical Society. The survey involves more than 300 scientists from six countries and uses images taken by one of the best digital cameras in the world: a 570-megapixel gadget mounted on the Victor Blanco telescope at the Cerro Tololo Inter-American Observatory, high in the Chilean Andes.
“Our goal all this time has been to see the invisible – to see dark matter,” said Sarah Bridle, an astrophysics professor at the University of Manchester and co-chair of the DES weak lensing group, which produced the map. “To be able to look at a map and say, ‘That part of the sky’s got more dark matter in it, that bit’s empty,’ is the dream that we’ve had all this time,” Prof Bridle told the BBC. The survey commenced more than two years ago and will run for another three. This preliminary map was made using data from the camera’s very first test images. The business of mapmakingis complicated when the stuff being mapped is invisible and millions of light years away. To spot dark matter, astrophysicists must pick out distortions – caused by dark matter’s gravitational “lensing” of passing light – within very accurate images. The distortions are much smaller than the warping of light by our own atmosphere, and even the irregularities added by the telescope itself. So those quantities first have to be subtracted.
In the last two years, she and her many DES colleagues have measured the shapes of no less than two million galaxies within the first crop of images. And they’ve measured those dimensions to an accuracy of within 0.1%, which has required countless hours of heavy-duty computing. It paints an impressively clear picture, with no big surprises. Bigger surprises may come when the survey reaches its ultimate goal: testing the idea of dark energy.
Once the DES team has finished its map of dark matter, spreading its massive tendrils across the cosmos, they will be in a position to measure just how fast those tendrils, along with all the matter we can see, are flying apart. This expansion of the universe is happening at an increasing rate, and dark energy is the force physicists have proposed to account for that increase. So by comparing dark matter “clumpiness” in different ages of the universe – which we can see as different distances – scientists will be able to pinpoint the rate of expansion. And that will point to the nature of dark energy. It might simply yield a precise measurement of this mysterious force.
By Jonathan Webb
Science reporter, BBC News, 14 April, 2015.