Expansion of the universe diagram
Figure 26.20: This is a simplified diagram of the expansion of the universe over time. Note
that the distance between galaxies gets bigger as you go forward in time, but the size of each
galaxy stays about the same. ( 17 )
An inflating balloon is not exactly like the expanding universe. For one thing, the surface
of a balloon has only two dimensions, while space has three dimensions. But it is true that
space itself is stretching out between galaxies like the rubber stretches when a balloon is
inflated. This stretching of space, which causes the distance between galaxies to increase, is
what astronomers mean by the expansion of the universe.
One other difference between the universe and our balloon model involves the actual size
of the galaxies. On the inflating balloon, the dots you made will become larger in size as
you inflate it. In our universe, however, the galaxies stay the same size; it is just the space
between the galaxies that increases as the universe expands.
Formation of the Universe
The discovery that the universe is expanding also told astronomers something about how
the universe might have formed. Before this discovery, there were many ideas about the
universe, most of them thinking of the universe as constant. Once scientists learned that
the universe is expanding, the next logical thought is that at one time it had to have been
smaller.