7. Why does the author refer to “inflating a balloon with small coins attached to its surface”?
- To help explain how the universe can expand while the galaxies remain the same size
- To imply that the universe must eventually stop expanding
- To support the statement that most stars are found in disk-shaped galaxies
- To help explain how the universe began as a single point of dense matter
Determining the age of the universe has been more complicated. Most stars in the universe are clustered into enormous disk-like galaxies. The distance between our galaxy, known as the Milky Way, and all others is increasing. In fact, all galaxies are moving away from one another, evidence that the universe is expanding. It is not the galaxies themselves that are expanding but the space between them. What is happening is analogous to inflating a balloon with small coins attached to its surface. The coins behave like galaxies: although they do not expand, the space between them does. Before the galaxies formed, matter that they contain was concentrated with infinite density at a single point from which it exploded in an event called the big bang. Even after it assembled into galaxies, matter continued to spread in all directions from the site of the big bang.