  With the imminent promise of constraints on the epoch of reionization from
observations of microwave background anisotropies, the question of whether or
not the standard Cold Dark Matter (CDM) model permits early reionization has
been subjected to detailed investigation by various authors, with the
conclusion that reionization may occur at quite high redshift. However, it is
widely accepted that this model is excluded, as when normalised to the COBE
observations it possesses excessive galaxy clustering on scales below tens of
megaparsecs. We examine the trends of observations, first in a fairly model
independent way, and second by considering variants on the standard CDM model
introduced to resolve the observational conflicts. We conclude that the epoch
of reionization favoured by the observational data is typically considerably
later than the standard CDM model suggests, and amongst models which may fit
the observational data only the introduction of a cosmological constant leads
to a reionization redshift close to that of standard CDM.

Astrophysics