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We calculate the cosmic microwave background (CMB) anisotropy spectrum inmodels with millicharged particles of electric charge q\sim 10^{-6}-10^{-1} inunits of electron charge. We find that a large region of the parameter spacefor the millicharged particles exists where their effect on the CMB spectrum issimilar to the effect of baryons. Using WMAP data on the CMB anisotropy andassuming Big Bang nucleosynthesis value for the baryon abundance we find thatonly 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 andmass) from this region. This bound significantly narrows the allowed range ofthe parameters of millicharged particles. In models without paraphotonmillicharged particles are now excluded as a dark matter candidate. We alsospeculate that recent observation of 511 keV gamma-rays from the Galactic bulgemay be an indication that a (small) fraction of CDM is comprised of themillicharged particles.Comment: 10 pages, 3 figures; v2: journal version, references adde

Narrowing the window for millicharged particles by CMB anisotropy