Signatures of a Hidden Cosmic Microwave Background

If there is a light Abelian gauge boson ${\gamma }^{\prime }$ in the hidden sector its kinetic mixing with the photon can produce a hidden cosmic microwave background (HCMB). For meV masses, resonant oscillations $\gamma \leftrightarrow {\gamma }^{\prime }$ happen after big bang nucleosynthesis (BBN) but before CMB decoupling, increasing the effective number of neutrinos $N_{\nu }^{\mathrm{eff}}$ 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-$\alpha$ data, $N_{\nu }^{\mathrm{eff}}>3$ is preferred. It is tempting to attribute this effect to the HCMB. The interesting parameter range will be tested in upcoming laboratory experiments.

Figure 1

Isocontours of $\tilde{\Gamma }/H=1$ for different ${\gamma }^{\prime }$ masses.

Figure 2

Isocontours of $x\equiv {\rho }_{{\gamma }^{\prime }}/{\rho }_{\gamma }$ 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 effective number of neutrinos $N_{\nu }^{\mathrm{eff}}=3.046+\Delta N_{\nu }^{\mathrm{eff}}$ at the CMB epoch. Future Planck data [10] could push the bound to $x\simeq 0.01$. Distortions of the CMB blackbody would be unacceptable in the region FIRAS. Also shown are the bounds from Coulomb law tests [22], CAST (Sun) [23], and light-shining-through-walls (LSW) experiments (current [17] and near future “ALPS” [18]). The region labeled “Cavity” can be explored with microwave cavities [20] and the remaining region at higher masses with further solar ${\gamma }^{\prime }$ searches [24].