Abstract
The existence of millicharged dark matter (mDM) can leave a measurable imprint on 21-cm cosmology through mDM-baryon scattering. However, the minimal scenario is severely constrained by existing cosmological bounds on both the fraction of dark matter that can be millicharged and the mass of mDM particles. We point out that introducing a long-range force between a millicharged subcomponent of dark matter and the dominant cold dark matter (CDM) component leads to efficient cooling of baryons in the early universe, while also significantly extending the range of viable mDM masses. Such a scenario can explain the anomalous absorption signal in the sky-averaged 21-cm spectrum observed by EDGES, and leads to a number of testable predictions for the properties of the dark sector. The mDM mass can then lie between 10 MeV and a few hundreds of GeVs, and its scattering cross section with baryons lies within an unconstrained window of parameter space above direct detection limits and below current bounds from colliders. In this allowed region, mDM can make up as little as $10^{-8}$ of the total dark matter energy density. The CDM mass ranges from 10 MeV to a few GeVs, and has an interaction cross section with the Standard Model that is induced by a loop of mDM particles. This cross section is generically within reach of near-future low-threshold direct detection experiments.
Highlights
Dark matter (DM)-baryon interactions can affect the thermal history after recombination [1,2], possibly leading to observable deviations in the 21-cm global signal compared to the ΛCDM prediction [3]
We present a new setup where millicharged dark matter (mDM)-baryon scattering produces large effects that can be observed in 21-cm cosmology, without being constrained by any current cosmological, terrestrial, or astrophysical probe
The cooling capability of the mDM is enhanced by introducing a long-range interaction between the cold dark matter (CDM) component and the mDM fraction, thereby allowing the CDM component to act as a heat sink while only interacting weakly with the Standard Model (SM)
Summary
Dark matter (DM)-baryon interactions can affect the thermal history after recombination [1,2], possibly leading to observable deviations in the 21-cm global signal compared to the ΛCDM (cold dark matter) prediction [3]. We show that if mDM has an additional long-range interaction with the rest of the dark matter (which forms a cold dark matter bath), it may remain cold for longer, thereby greatly increasing the effective heat capacity of the hidden sector This substantially improves the efficiency of baryon cooling, extending the region of parameter space in which we expect large. The setup, which we depict, can explain the recent sky-averaged 21-cm spectrum measurement presented by the EDGES Collaboration [21], while opening up the higher mass and smaller density regions of the mDM parameter space This can be accomplished while ensuring that the momentum transfer between mDM and CDM remains small enough to circumvent the stringent CMB constraints.
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