Let me conclude by looking forward into the distant cosmic future. As
the universe
continues to expand, the scale of clustering will grow. In a simple
model with 
= 1
and scale independent initial perturbations, the typical mass scale of
clustering rises in direct proportion to the age of the universe. In
this particular model, the virial
velocities and temperatures of typical clusters are independent of
time. The typical
number of galaxies in a rich cluster is now about 103. At a
much later time, this number will, instead, be
The typical number of galaxies in the mean correlation scale is now only about
10. It is the smallness of this number which makes it hard to study
clusters; for
instance, we cannot map out their shapes, isodensity contours, etc.,
with any precision
because of discreteness effects. But at later times this number will also have
risen in proportion to the size of rich clusters. Moreover, the
characteristic cooling
time for the gas in a cluster increases as t3, even
if
In a cosmic perspective and time-frame we may be observing clusters at a
confusing
stage in their development, but if we view astronomical advances on a human
timescale we can feel very encouraged. Progress in delineating
largescale structures,
streaming velocities, and the optical and X-ray properties of individual
clusters, not
only locally but far enough away to see evolution, is rapid and
encouraging. Now is therefore the best of times for a conference like this.
NOTE The topics mentioned in this introductory survey are
all treated more fully
in later chapters. For this reason, and to avoid duplication, literature
references are not given here.
gas does not decrease.
Therefore, one will not need to worry about the complications of cooling
flows in the remote future.