If there is a hidden photon – i.e .
a light abelian gauge boson \gamma ^ { \prime } in the hidden sector – its kinetic mixing with the standard photon can produce a hidden cosmic microwave background ( hCMB ) .
For meV masses , resonant oscillations \gamma \leftrightarrow \gamma ^ { \prime } happen after nucleosynthesis ( BBN ) but before CMB decoupling , increasing the effective number of neutrinos ( N _ { \nu } ^ { \mathrm { eff } } ) but also the baryon to photon ratio at decoupling .
The current agreement between BBN and CMB data provides new constraints on the kinetic mixing .
However , if one includes Lyman- \alpha data , N _ { \nu } ^ { \mathrm { eff } } > 3 is preferred .
It is tempting to interpret this effect in terms of the hCMB .
Interestingly , the required parameters will be tested in the near future by laboratory experiments .