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tokens: Acknowledgments We thank Stevie Bergman, Jeffrey Filippini, William Jones, Lyman Page and George Efstathiou for useful discussions. SMF was partially supported by Science and Technology Facilities Council (STFC) in the United Kingdom. The Center for Computational Astrophysics is supported by the Simons Foundation. HVP was partially supported by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) ERC GA no. 306478-CosmicDawn. LV acknowledges support by Spanish Ministerio de Economía, Industria y Competitividad (MINECO) under projects AYA2014-58747-P and MDM-2014-0369 of Institut de Ciències del Cosmos, Universitat de Barcelona (ICCUB) (Unidad de Excelencia Maria de Maeztu). JEG acknowledges support by Katherine Freese through a grant from the Swedish Research Council (Contract No. 638-2013-8993). The work of JE was performed within the LabEx Institut Lagrange de Paris (ILP) (reference ANR-10-LABX-63) part of the Idex Sorbonne Universités à Paris pour l'Enseignement et la Recherche (SUPER) and was supported by the Agence Nationale de la Recherche (ANR), as part of the programme Investissements d'avenir under the reference ANR-11-IDEX-0004-02.