Chapter summary

Galaxies have a very irregular distribution on the sky. The highest concentrations are called clusters. We think that the clusters are gravitation-ally bound. However, the galaxies that we see in a cluster do not appear to have enough mass to hold the cluster together, even if we allow for the dark matter that we know to be in the galaxies. This means that there must be dark matter in the clusters that is not associated with the individual galaxies.

Virtually all of the galaxies that we see are moving away from us. The more distant galaxies are moving away faster. There is a simple relationship between how far away a galaxy is and how fast it is moving away. This relationship is called Hubble's law. This simple relationship results from the expansion of the universe. We can also use Hubble's law to tell us the distance to galaxies whose redshift we can measure.

The quantity that currently tells us the actual distance to a galaxy with a given redshift is called the Hubble constant. It is constant in that it has the same value everywhere in the universe; how ever, it can change with time. Measuring the Hubble constant is very difficult. We need measurements of the distances to galaxies that don't involve using the redshift. These are more accurate for nearby galaxies. However, it is only for the distant galaxies that the effect of the expansion of the universe is much greater than the effect of the random motions of the galaxies. The most likely values of the Hubble constant are between 50 and 100 km/s/Mpc, with much recent work suggesting a value of 70 km/s/Mpc.

On larger scales, the clusters are gathered into superclusters, which are tens of megaparsecs in extent. Of comparable size are large volumes with no galaxies, called voids. We learn about this large-scale structure from redshift surveys, in which we measure the redshifts of thousands of galaxies. (These are done both in the visible and radio parts of the spectrum, depending on the types of galaxies we are looking at.)

Efforts to understand all of this structure are just getting under way. An important problem is that we don't know how much dark matter there is on the largest scales, and we don't know the nature of that dark matter. Some have even tried to turn the problem around, using the distribution of galaxies to tell us something about the

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