Signatures of a hidden cosmic microwave background

TL;DR

The agreement between BBN and CMB data provides new constraints and, including Lyman-alpha data, Nnu(eff) > 3 is preferred and the interesting parameter range will be tested in upcoming laboratory experiments.

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.

Figures (2)

• Figure 1: Isocontours of $\tilde{\Gamma}/H=1$ for different $\gamma^{\prime}$ masses.
• Figure 2: Isocontours of $x\equiv\rho_{\gamma'}/\rho_\gamma$ at the CMB epoch in the mass-mixing plane. The region above $x=0.32$ is excluded by the agreement between the baryon to photon ratio inferred from BBN and CMB data, while $x>0.2$ is excluded by the upper limits on the cosmic radiation density at CMB, usually expressed in terms of the effective number of neutrinos $N_\nu^\mathrm{eff}=3.046+\Delta N_\nu^\mathrm{eff}$. Also shown are the exclusion bounds from Coulomb law tests Bartlett:1988yy, CAST (Sun) Redondo:2008aa and light-shining-through-walls (LSW) experiments Ahlers:2007qf. The near future prospects of the ALPS experiment are also shown ALPS. Sensitivity in the region labeled 'Cavity' can be obtained by an experiment using microwave cavities Jaeckel:2007ch. The remaining region at higher masses can be probed by solar hidden photon searches Gninenko.