In March astronomers in the BICEP2 collaboration announced they had found gravitational waves from the Big Bang. But now the evidence is being questioned by other scientists. Dr Lucie Green reports on the debate and asks if scientists can ever know what happened billions of years ago when the universe was formed. Image: The BICEP2 telescope at twilight, which occurs only twice a year at the South Pole. The MAPO observatory (home of the Keck Array telescope) and the South Pole station can be seen in the background. Image copyright: Steffen Richter, Harvard University.