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Many lines of evidence suggest that nonbaryonic dark matter constitutesroughly 30% of the critical closure density, but the composition of this darkmatter is unknown. One class of candidates for the dark matter is compactobjects formed in the early universe, with typical masses M between 0.1 and 1solar masses to correspond to the mass scale of objects found with microlensingobserving projects. Specific candidates of this type include black holes formedat the epoch of the QCD phase transition, quark stars, and boson stars. Here weshow that accretion onto these objects produces substantial ionization in theearly universe, with an optical depth to Thomson scattering out to z=1100 ofapproximately tau=2-4 [f_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where\epsilon_{-1} is the accretion efficiency \epsilon\equiv L/{\dot M}c^2 dividedby 0.1 and f_CO is the fraction of matter in the compact objects. The currentupper limit to the scattering optical depth, based on the anisotropy of themicrowave background, is approximately 0.4. Therefore, if accretion onto theseobjects is relatively efficient, they cannot be the main component ofnonbaryonic dark matter.Comment: 12 pages including one figure, uses aaspp4, submitted to Ap

Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter