For the first 300,000 years after the Big Bang the rapidly expanding universe was dark and filled with neutral hydrogen gas doing nothing much. But over the next half billion years the first stars and galaxies arrive through a process known as re-ionization – turning the lights on in the universe.

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Using an amazing Dark Energy Camera which is part of the -meter Blanco Telescope, at Cerro Tololo Inter-American Observatory (CTIO), in northern Chile, scientists have captured a picture of 23 of these young galaxies – the very dawn of visual time. Arizona State University astronomers Sangeeta Malhotra and James Rhoads, working with international teams in Chile and China, are now attempting to find when the very first light illuminated the universe. This dramatic moment, known as re-ionization, occurred sometime in the interval between 300 million years and one billion years after the Big Bang.

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Mr Malhotra said: “Before re-ionization, these galaxies were very hard to see, because their light is scattered by gas between galaxies, like a car's headlights in fog. “As enough galaxies turn on and 'burn off the fog' they become easier to see. By doing so, they help provide a diagnostic to see how much of the 'fog' remains at any time in the early universe. "Several years ago, we carried out a similar study using a 64-megapixel camera that covers the same amount of sky as the full moon. "The Dark Energy Camera (DECam) by comparison, is a 570-megapixel camera and covers 15 times the area of the full moon in a single image."

NASA Light show: How re-ionisation turned the lights on in the universe

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