We revisit cosmic microwave background constraints on the abundance of millicharged particles based on the Planck data. The stringent limit ${\Omega }_{\mathrm{mcp}}{h}^2<0.001$ (95% CL) may be set using the cosmic microwave background data alone if millicharged particles participate in the acoustic oscillations of baryon-photon plasma at the recombination epoch. The latter condition is valid for a wide region of charges and masses of the particles. Adding the millicharged component to $\Lambda \mathrm{CDM}$ shifts the preferred scalar spectral index of primordial perturbations to somewhat larger values as compared to the minimal model, even approaching the Harrison-Zeldovich spectrum under some assumptions.

• Received 11 October 2013

DOI:https://doi.org/10.1103/PhysRevD.88.117701