Pierre Sikivie, physicist at the University of Florida in Gainsville, USA, says that rings of invisible matter are spreading out from the centre of our Galaxy like ripples on a pond. The last of these passed the Sun about 900 million years ago, and the next is due in 350 million years. "The rings could shake up the comet cloud around the Sun, causing cataclysmic events on the Earth," he points out. Most of the matter in the Universe is dark (it cannot radiate energy as light). As the gravity of our Galaxy pulls in matter from surrounding space, dark matter would suffer a different fate from visible matter. "Collisions between particles of ordinary matter allow it to convert its energy of motion into heat and settle into a thin disc at the centre of the Galaxy," he explains. "But dark matter cannot shed this energy and flies back out again like a pendulum" (New Scientist, Vol 159, No 2141).
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