Signatures of a hidden cosmic microwave background

TL;DR

This paper explores how a hidden photon could create a hidden cosmic microwave background, affecting early universe parameters and offering testable predictions for future laboratory experiments.

Contribution

It introduces the concept of a hidden CMB generated by photon-hidden photon oscillations and derives new constraints from cosmological data.

Findings

Resonant photon-hidden photon oscillations occur after BBN but before CMB decoupling.

Including Lyman-alpha data suggests a higher effective number of neutrinos.

Future laboratory experiments can test the predicted parameter space.

Abstract

If there is a hidden photon -- i.e. a light abelian gauge boson in the hidden sector -- its kinetic mixing with the standard photon can produce a hidden cosmic microwave background (hCMB). For meV masses, resonant photon-hidden photon oscillations happen after nucleosynthesis (BBN) but before CMB decoupling, increasing the effective number of neutrinos but also the baryon to photon ratio. The current agreement between BBN and CMB data provides new constraints on the kinetic mixing. However, if one includes Lyman-alpha data, an effective number of neutrinos higher than 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.