Possible interaction between baryons and dark-matter particles revealed by the first stars


The cosmic radio-frequency spectrum is expected to show a strong absorption signal corresponding to the 21-centimetre-wavelength transition of atomic hydrogen around redshift 20, which arises from Lyman-α radiation from some of the earliest stars1,2,3,4. By observing this 21-centimetre signal—either its sky-averaged spectrum5 or maps of its fluctuations, obtained using radio interferometers6,7—we can obtain information about cosmic dawn, the era when the first astrophysical sources of light were formed. The recent detection of the global 21-centimetre spectrum5 reveals a stronger absorption than the maximum predicted by existing models, at a confidence level of 3.8 standard deviations. Here we report that this absorption can be explained by the combination of radiation from the first stars and excess cooling of the cosmic gas induced by its interaction with dark matter8,9,10. Our analysis indicates that the spatial fluctuations of the 21-centimetre signal at cosmic dawn could be an order of magnitude larger than previously expected and that the dark-matter particle is no heavier than several proton masses, well below the commonly predicted mass of weakly interacting massive particles. Our analysis also confirms that dark matter is highly non-relativistic and at least moderately cold, and primordial velocities predicted by models of warm dark matter are potentially detectable. These results indicate that 21-centimetre cosmology can be used as a dark-matter probe.

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Figure 1: Simulated 21-cm intensity using a model with baryon–dark matter scattering.
Figure 2: Global 21-cm signal in models with baryon–dark matter scattering.
Figure 3: Constraints on dark-matter properties using cosmic dawn observations.


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I am grateful to J. Bowman for alerting me to possible indications of very deep absorption in the EDGES results, which inspired this work. This project was made possible through the support of a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the author and do not necessarily reflect the views of the John Templeton Foundation.

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Barkana, R. Possible interaction between baryons and dark-matter particles revealed by the first stars. Nature 555, 71–74 (2018). https://doi.org/10.1038/nature25791

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