1. To test your understanding of active galaxies, do Interactive Exercise 17-1 on the Web. You can print out your answers, if required. 2. Which force in nature is believed to have formed second? a....

1. To test your understanding of active galaxies, do Interactive Exercise 17-1 on the Web. You can print out your answers, if required.

2. Which force in nature is believed to have formed second? a. gravity b. electromagnetic force c. weak force d. strong force e. All formed at the same time.

3. What does it mean when astronomers say that we live in an expanding universe?