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Description
The unexpectedly strong 21cm absorption signal detected by the EDGES
experiment suggests that the baryonic gas at the end of the dark ages
was colder than predicted within the standard cosmological scenario. I
will review possible ways to lower the baryon temperature after
recombination. In the second part of the talk I will focus on one
specific model with asymmetric milli-charge dark matter. The model
includes a stable, negatively-charged particle with a significant
cosmological abundance, such that the universe remains charge-neutral
but the electron and proton numbers are no longer equal. The deficit
of electrons after recombination results in an earlier decoupling of
the baryon and CMB temperatures, and thus in a colder gas at the
cosmic dawn. I will discuss the phenomenological constraints and
possible smoking guns of this scenario.
