  In this paper we critically examine predictions of the Lya forest within the
standard cold dark matter (SCDM) model, paying particular attention to the low
end of the column-density distribution. We show in particular that the width of
these lines, typically measured by the b-parameter of a Voigt profile, is
sensitive to spatial resolution in numerical simulations and has previously
been overestimated. The new result, which predicts a distribution with a median
b of around 20-22 km/s at z=3, is substantially below that observed. We examine
a number of possible causes of this discrepancy and argue that it is unlikely
to be rectified by an increase in the thermal broadening of the absorbing gas,
but is instead telling us something about the distribution of matter on these
scales. Although the median differs, the shape of the b-parameter distribution
agrees quite well with that observed, and the high-end tail is naturally
produced by the filamentary nature of gravitational collapse in these models.
In particular, we demonstrate that lines of sight which obliquely intersect a
filament or sheet tend to produce absorption lines with larger b parameters. We
also examine the physical nature of the gas which is responsible for the
forest, showing that for lines with neutral column densities below N_HI ~ 10^14
cm^-2 (for this model at z=3), the peculiar infall velocity is actually slower
than the Hubble flow, while larger lines have, on average, turned around and
are collapsing.
