If there is a light Abelian gauge boson γ^' in the hidden sector its kinetic mixing with the photon can produce a hidden cosmic microwave background (HCMB). For meV masses, resonant oscillations γ↔γ^' happen after big bang nucleosynthesis (BBN) but before CMB decoupling, increasing the effective number of neutrinos N_νᵉᶠᶠ and the baryon to photon ratio, and distorting the CMB blackbody spectrum. The agreement between BBN and CMB data provides new constraints. However, including Lyman-α data, N_νᵉᶠᶠ>3 is preferred. It is tempting to attribute this effect to the HCMB. The interesting parameter range will be tested in upcoming laboratory experiments.
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Jaeckel et al. (2008) studied this question.
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