21cm absorption signal from charge sequestration
  The unexpectedly strong 21cm absorption signal detected by the EDGES
experiment suggests that the baryonic gas was colder at redshift $z\sim 17$
than predicted in the standard scenario. We discuss a mechanism to lower the
baryon temperature after recombination. We introduce a stable,
negatively-charged particle with a non-negligible cosmological abundance, such
that the universe remains charge-neutral but the electron and proton numbers
are no longer equal. The deficit of electrons during recombination results in
an earlier decoupling of the baryon gas temperature from that of the CMB. This
implies a smaller ratio of the gas and CMB temperature at $z\sim 17$. The
parameter space of the mechanism where the 21 cm absorption signal is
significantly enhanced is probed by the CMB spectrum, cooling of stars and
supernovae, and colliders. Nevertheless, we find viable regions corresponding
to sub-eV or MeV-scale milli-charged particles, or to TeV-scale multi-charged
particles.
