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

We generalize previous work on early photoionization to cold dark matter (CDM) models with Omega < 1. Such models have received recent interest 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 early reionization to suppress small-scale anisotropies in order to be consistent with experimental data. We find that if the cosmological constant 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 forming at higher redshifts before the universe becomes curvature dominated. The maximum angular scale on which fluctuations are suppressed decreases when Omega(0) is lowered, but this effect is also rather weak and unlikely to be measurable in the near future. For flat models, on the other hand, where lambda(0) = 1 - Omega(0), the negative effects of lowering Omega(0) dominate, and early reionization is not likely to play 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 number of relativistic particle species.