How are galaxies distributed in space ?

Galaxies are not distributed evenly through space.

As we have seen, a big galaxy may have satellite galaxies. On a larger scale, galaxies come in clusters. For example there is a small ``Local Cluster'' containing our Galaxy and the Andromeda galaxy plus several others (mostly small). An example of a big cluster of galaxies is the Virgo Cluster. One can look on a larger scale. Margaret Geller of Harvard has done the following.

Choose a thin slice of sky.
The idea is to examine everything in that slice out to a certain distance.
Viewed from above, the region examined then looks like a slice from a very thin pie.
Measure the angles to all the galaxies in that slice.
``Measure'' the distances to the galaxies by measuring the redshifts and assuming the Hubble relation.
Plot the points.

Here is my impression of the results. The real data is in Fig26-23 of your book. Unfortunately, I couldn't find an electronic version.

The thing one notices is not so much the clusters of galaxies as the voids. The distribution of galaxies appears to be something like the distribution of soap molecules in a pile of soap bubbles.

An important task of astronomy in the coming decades is to try to explain this.

It would help if we knew how much dark matter there is and what it is made of.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu