1 00:00:01,000 --> 00:00:07,000 An international team of astronomers using the NASA/ESA Hubble Space Telescope has discovered a ghostly ring of dark matter 2 00:00:07,000 --> 00:00:12,000 formed long ago during a colossal collision between two galaxy clusters. 3 00:00:12,000 --> 00:00:19,000 This is the first time that dark matter has been found with a distribution that differs radically from the distribution of ordinary matter. 4 00:00:36,000 --> 00:00:38,000 This is the Hubblecast! 5 00:00:38,000 --> 00:00:48,000 News and Images from the NASA/ESA Hubble Space Telescope. Travelling through time and space with our host Doctor J a.k.a. Dr Joe Liske. 6 00:00:49,000 --> 00:00:51,000 Hello and welcome to the Hubblecast! 7 00:00:51,000 --> 00:01:02,000 Scientists reckon that most of the matter in the Universe is something called “dark matter”, an unknown type of matter that neither emits nor reflects light. 8 00:01:02,000 --> 00:01:12,000 But does dark matter really exist? Can scientists prove it? The NASA/ESA Hubble Space Telescope is helping to answer this question. 9 00:01:14,000 --> 00:01:21,000 In 2004 an international team of astronomers pointed Hubble towards the constellation of Pisces, the Fish, 10 00:01:21,000 --> 00:01:28,000 to observe the galaxy cluster that goes by the telephone number of CL0024+17 11 00:01:28,000 --> 00:01:32,000 and which is located 5 billion light-years away from Earth. 12 00:01:43,000 --> 00:01:49,000 Now Hubble’s Advanced Camera for Surveys produced a stunning image of this cluster. 13 00:01:49,000 --> 00:01:52,000 The galaxies in the cluster are here seen in yellow. 14 00:01:54,000 --> 00:02:01,000 Analysing the image over the last couple of years, the team discovered a ring of dark matter – seen here in blue - 15 00:02:01,000 --> 00:02:09,000 and realised that the position of this ring did not match at all the position of the hot gas and the galaxies in the cluster. 16 00:02:09,000 --> 00:02:12,000 The ring itself is 2.6 million light-years across. 17 00:02:12,000 --> 00:02:22,000 Now this is the first time that dark matter’s been found with a distribution that is so radically different from the distribution of the ordinary matter. 18 00:02:23,000 --> 00:02:29,000 This remarkable finding is attributed to the collision of the cluster with another cluster between 1 to 2 billion years ago. 19 00:02:29,000 --> 00:02:36,000 The team's computer simulations show – here seen from the side - that when the two clusters smashed together, 20 00:02:36,000 --> 00:02:42,000 the dark matter fell to the centre of the combined cluster and bounced back out. 21 00:02:43,000 --> 00:02:48,000 In reality the collision occurred along our line of sight, so that we have a head-on view of it. 22 00:02:48,000 --> 00:02:56,000 From this perspective, the dark-matter structure looks like a ring, just as the new analysis shows. 23 00:02:57,000 --> 00:03:00,000 So how did astronomers spot the ring of dark matter? 24 00:03:00,000 --> 00:03:04,000 Well, tracing dark matter is not an easy task. 25 00:03:04,000 --> 00:03:10,000 The reason is of course that dark matter does not emit or reflect any light. 26 00:03:10,000 --> 00:03:16,000 The most direct way of detecting its influence is to study the way its gravity deflects light. 27 00:03:16,000 --> 00:03:23,000 Now to do this, astronomers study the faint light from galaxies that lie behind the cluster 28 00:03:23,000 --> 00:03:31,000 and whose light gets distorted and smeared into arcs and streaks by the gravity of the dark matter in the foreground cluster. 29 00:03:31,000 --> 00:03:35,000 Now this powerful trick is called gravitational lensing. 30 00:03:36,000 --> 00:03:46,000 To illustrate this, imagine that I am the background object being lensed by a massive foreground cluster. 31 00:03:46,000 --> 00:03:56,000 So by mapping the distorted light, astronomers can deduce the mass of the cluster, and they can trace the distribution of the dark matter within the cluster. 32 00:03:56,000 --> 00:04:04,000 This amazing image shows us some spectacular examples of faint background galaxies that have had their light bent by the cluster’s strong gravitation 33 00:04:05,000 --> 00:04:10,000 field. One of them, located about two times farther away than the yellow cluster galaxies in the foreground, 34 00:04:10,000 --> 00:04:15,000 has been multiply imaged into five separate arc-shaped components. 35 00:04:15,000 --> 00:04:20,000 Hubble’s high resolution can even show the details within this background galaxy. 36 00:04:21,000 --> 00:04:29,000 The ring's discovery is among the strongest evidence that dark matter actually exists and it increases confidence in our current theory of gravity. 37 00:04:29,000 --> 00:04:32,000 This is Dr J signing off for the Hubblecast. 38 00:04:32,000 --> 00:04:36,000 Once again nature has surprised us beyond our wildest imagination … 39 00:04:38,000 --> 00:04:42,000 Hubblecast is produced by ESA/Hubble at the European Southern Observatory in Germany. 40 00:04:42,000 --> 00:04:47,000 The Hubble mission is a project of international cooperation between NASA and the European Space Agency.