Direct millicharged dark matter cannot explain EDGES. (arXiv:1903.09154v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Creque_Sarbinowski_C/0/1/0/all/0/1">Cyril Creque-Sarbinowski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ji_L/0/1/0/all/0/1">Lingyuan Ji</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kovetz_E/0/1/0/all/0/1">Ely D. Kovetz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kamionkowski_M/0/1/0/all/0/1">Marc Kamionkowski</a>
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 $(m_chi/{rm MeV}) 0.0115% lesssim f
lesssim 0.4%$ of the dark matter and that their mass $m_chi$ and charge
$q_chi$ have values $0.1 lesssim (m_chi/{rm MeV})lesssim 10$ 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 $N_{rm eff}$ 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.
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 $(m_chi/{rm MeV}) 0.0115% lesssim f
lesssim 0.4%$ of the dark matter and that their mass $m_chi$ and charge
$q_chi$ have values $0.1 lesssim (m_chi/{rm MeV})lesssim 10$ 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 $N_{rm eff}$ 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.
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