Constraints on millicharged particles from Planck
  We revisit cosmic microwave background (CMB) constraints on the abundance of
millicharged particles based on the Planck data. The stringent limit
Omega_{mcp}h^2 < 0.001 (95% CL) may be set using the CMB 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 LCDM shifts prefered scalar spectral index of
primordial perturbations to somewhat larger values as compared to minimal
model, even approaching Harrison-Zeldovich spectrum under some assumptions.
