A lighthouse galaxy shines unexpectedly through the fog of the cosmic dawn

Modern observational cosmology provides a snapshot of the Universe just after its formation in the hot, dense pinprick of the Big Bang, which occurred around 13.8 billion years ago. Measurements of the cosmic microwave background radiation, emitted about 300,000 years after that event, depict a cosmos filled with hot hydrogen and helium gas with almost uniform density and temperature¹¹. But the next 300 million or so years of cosmic history remain shrouded in mystery. This ‘cosmic dawn’ is a crucial period in which, as the Universe expanded and cooled, stars, galaxies and black holes started emerging and growing from the nearly homogeneous gas. Writing in Nature, Witstok et al.² report the observation of ultraviolet light emitted from a galaxy when the Universe was just 330 million years old — a finding that could require cosmologists to revisit some aspects of early cosmic history.