article: The team has made the most precise map of the "oldest light" in the cosmos. Earlier observations of this radiation had suggested the first generation of stars were bursting into life by about 420 million years after the Big Bang. Planck's data indicates this great ignition was well established by some 560 million years after it all began. "This difference of 140 million years might not seem that significant in the context of the 13.8-billion-year history of the cosmos, but proportionately it's actually a very big change in our understanding of how certain key events progressed at the earliest epochs," said Prof George Efstathiou, one of the leaders of the Planck Science Collaboration. The assessment is based on studies of the "afterglow" of the Big Bang, the ancient light called the Cosmic Microwave Background (CMB), which still washes over the Earth today. The European Space Agency's (Esa) Planck satellite mapped this "fossil" between 2009 and 2013. It contains a wealth of information about early conditions in the Universe, and can even be used to work out its age, shape and do an inventory of its contents. Scientists can also probe it for very subtle "distortions" that tell them about any interactions the CMB has had on its way to us. One of these would have been imprinted when the infant cosmos underwent a major environmental change known as re-ionisation. It is when the cooling neutral hydrogen gas that dominated the Universe in the aftermath of the Big Bang was then re-energised by the ignition of the first stars. These hot giants would have burnt brilliant but brief lives, producing the very first heavy elements. But they would also have "fried" the neutral gas around them - ripping electrons off the hydrogen protons. And it is the passage of the CMB through this maze of electrons and protons that would have resulted in it picking up a subtle polarisation. The Planck team has now analysed this polarisation in fine detail and determined it to have been generated at 560 million years after the Big Bang. The American satellite WMAP, which operated in the 2000s, made the previous best estimate for the peak of re-ionisation at 420 million years. The problem with that number was that it sat at odds with Hubble Space Telescope observations of the early Universe. Hubble could not find stars and galaxies in sufficient numbers to deliver the scale of environmental change at the time when WMAP suggested it was occurring. Planck's new timing "effectively solves the conflict," commented Prof Richard McMahon from Cambridge University, UK. "We had two groups of astronomers who were basically working on different sides of the problem. The Planck people came at it from the Big Bang side, while those of us who work on galaxies came at it from the 'now side'. "It's like a bridge being built over a river. The two sides do now join where previously we had a gap," he told BBC News. That gap had prompted scientists to invoke complicated scenarios to initiate re-ionisation, including the possibility that there
summary: Scientists working on Europe's Planck satellite say the first stars lit up the Universe later than previously thought.