Narrowing the window for millicharged particles by CMB anisotropy

We calculate the cosmic microwave background (CMB) anisotropy spectrum in models with millicharged particles of electric charge $q\sim 10^{-6}-10^{-1}$ in units of electron charge. We find that a large region of the parameter space for the millicharged particles exists where their effect on the CMB spectrum is similar to the effect of baryons. Using WMAP data on the CMB anisotropy and assuming Big Bang nucleosynthesis value for the baryon abundance we find that only a small fraction of cold dark matter, $\Omega_{mcp}h_0^2 < 0.007$ (at $95\%$ CL), may consists of millicharged particles with the parameters (charge and mass) from this region. This bound significantly narrows the allowed range of the parameters of millicharged particles. In models without paraphoton millicharged particles are now excluded as a dark matter candidate. We also speculate that recent observation of $511$ keV $\gamma$-rays from the Galactic bulge may be an indication that a (small) fraction of cold dark matter is comprised of the millicharged particles.