  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. We explore such direction dependent
grey-body distortions of the CMB spectrum, enhanced by resonant conditions in
the IGM plasma. We 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.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena;General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology;High Energy Physics - Theory