Direct millicharged dark matter cannot explain EDGES

Abstract

Heat transfer between baryons and millicharged dark matter has been invoked as a possible explanation for the anomalous 21-cm absorption signal seen by EDGES. Prior work has shown that the solution requires that millicharged particles make up only a fraction $(m_{\chi }/\mathrm{MeV})0.0115\%\lesssim f\lesssim 0.4\%$ of the dark matter and that their mass $m_{\chi }$ and charge $q_{\chi }$ have values $0.1\lesssim (m_{\chi }/\mathrm{MeV})\lesssim 10$ and $10^{-6}\lesssim (q_{\chi }/e)\lesssim 10^{-4}$. Here we show that such particles come into chemical equilibrium before recombination, and so are subject to a constraint on the effective number $N_{\mathrm{eff}}$ of relativistic degrees of freedom, which we update using Planck 2018 data. We moreover determine the precise relic abundance $f$ that results for a given mass $m_{\chi }$ and charge $q_{\chi }$ and incorporate this abundance into the constraints on the millicharged-dark-matter solution to EDGES. With these two results, the solution is ruled out if the relic abundance is set by freeze-out.

7 pages, 7 figures

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