Cosmology: Hydrogen was not ionized abruptly

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When and how the first stars and galaxies ionized the primordial hydrogen atoms that filled the early Universe is not known. Observations with a single radio antenna are opening a new window on the process. See Letter p.796


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  1. Jonathan Pritchard and Abraham Loeb are in the Institute for Theory and Computation, Harvard University, Cambridge, Massachusetts 02138, USA.;

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