REIONIZATION IN AN OPEN COLD DARK-MATTER UNIVERSE - IMPLICATIONS FOR COSMIC MICROWAVE BACKGROUND FLUCTUATIONS

We generalize previous work on early photoionization to cold dark matt er (CDM) models with Omega < 1. Such models have received recent inter est because the excess power in the large-scale galaxy distribution is phenomenologically fit if the ''shape parameter'' Gamma = h Omega(0) approximate to 0.25. It has been argued that such models may require e arly reionization to suppress small-scale anisotropies in order to be consistent with experimental data. We find that if the cosmological co nstant lambda = 0, the extent of this suppression is quite insensitive to Omega(0). Given a sigma(8)-normalization today, the loss of small- scale power associated with a lower Omega(0) is partially canceled by higher optical depth from longer look-back times and by structures for ming at higher redshifts before the universe becomes curvature dominat ed. The maximum angular scale on which fluctuations are suppressed dec reases when Omega(0) is lowered, but this effect is also rather weak a nd unlikely to be measurable in the near future. For flat models, on t he other hand, where lambda(0) = 1 - Omega(0), the negative effects of lowering Omega(0) dominate, and early reionization is not likely to p lay a significant role if Omega(0) much less than 1. The same applies to CDM models where the effective Gamma is lowered by increasing the n umber of relativistic particle species.