DARK matter may have made hydrogen gas around the universe's first stars colder than expected, it was suggested last month. Now new work indicates it is black holes that are to blame, through raising the background temperature and making the gas simply appear ultracold in comparison. That is what Aaron Ewall-Wice at NASA's Jet Propulsion Laboratory in Pasadena, California, and his team found by modelling the radiation of the first black holes. Black holes emit lots of radio waves and the first ones were probably developing around 180 million years after the big bang, about the same time as the first stars.
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