Key points are not available for this paper at this time.
If there is a light Abelian gauge boson ^' in the hidden sector its kinetic mixing with the photon can produce a hidden cosmic microwave background (HCMB). For meV masses, resonant oscillations ^' happen after big bang nucleosynthesis (BBN) but before CMB decoupling, increasing the effective number of neutrinos N_^eff 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- data, N_^eff>3 is preferred. It is tempting to attribute this effect to the HCMB. The interesting parameter range will be tested in upcoming laboratory experiments.
Jaeckel et al. (Fri,) studied this question.