18.1. Discuss the following statement: 'For Hubble's law to be true, we must be at the center of the universe.'

18.2. Draw a diagram, like Fig. 18.11, showing that the argument presented works even if the three galaxies are in a triangle, rather than in a straight line.

18.3. Why do we call the Hubble time an 'upper limit' or a 'lower limit' to the age of the universe?

18.4. What are the main problems in the accurate determination of the Hubble constant?

18.6. Can we use Hubble's law to determine distances within our own galaxy?

18.7. Of the distance measurement techniques listed in Table 18.1, which ones depend on knowing the intrinsic luminosity of some type of object?

18.8. What is the value of supernovae as distance indicators?

18.9. When we look at a distant galaxy, we see it as it was a long time ago. How does this make the use of distance indicators more difficult?

18.10. What is the evidence for the existence of clusters and superclusters of galaxies?

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