Speed limit found for sluggish dark matter
By Marcus Woo STEP aside, speed of light. Now we know the speed of dark. If particles of dark matter had never formed the clumps they are in today, they would scurry around space at no more than a sluggish 54 metres per second. The finding is one of the few firm values for a characteristic of “cold dark matter”, thought to be the most common type in the universe. Dark matter makes up about 80 per cent of the universe’s matter. Astronomers think most of it should be cold, meaning it is made of particles that move well below the speed of light. Although we can see dark matter’s gravitational effects on stars and galaxies, it does not otherwise interact with ordinary matter, and we know frustratingly little about its properties. For the first time, Cristian Armendáriz-Picón and Jayanth Neelakanta of Syracuse University in New York have calculated how fast the particles would zip around randomly in space. When dark matter coalesced in the early universe, it also pulled together gas and dust to make galaxies. The speed of the dark particles would have influenced how easily they could bunch up. So the team looked at snapshots of how matter was distributed at different points in cosmic history. They then calculated how fast dark matter must have been moving to create the observed structure and how fast it would be today if it wasn’t in clumps (arxiv.org/abs/1309.6971). Future work can use the speed to test models of particles that make up cold dark matter, which should bring us closer to identifying the elusive substance. This article appeared in print under the headline “The speed of dark: how fast do you think it is?