Ultralight axions which couple sufficiently strongly to photons can leave imprints on the sky at diverse frequencies by mixing with cosmic light in the presence of background magnetic fields. I describe such direction dependent grey-body distortions of the CMB spectrum, enhanced by resonant conditions in the IGM plasma. I also find that if such axions are produced in the early universe and represent a subdominant dark radiation component today, they could convert into X-rays in supervoids, and brighten them at X-ray frequencies.
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Département de Physique Théorique
Université de Genève
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