Constraints on millicharged particles from Planck data

We revisit cosmic microwave background constraints on the abundance of millicharged particles based on the Planck data. The stringent limit Ωmcph²<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 ΛCDM 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.