I often get asked what an "expanding universe" really means.
It's confusing, and for very good reasons. So, if you are perplexed by this, don't feel bad. We all are, although cosmologists — physicists that work on the properties of the universe — have figured out ways to make sense of it. In what follows, I'll try to explain how to picture this.
In the next few weeks, we will address other bizarre cosmic questions, such as the meaning of the Big Bang and the future and material composition of the universe.
Here is the problem: From our everyday perspective, when we see something expanding, we immediately also see what it's expanding into. An inflating balloon grows outwards into the space surrounding it. We can easily picture this because we are seeing things from the outside. We see the balloon and its surface growing as air is pumped into it. This is the privileged observer's view, one where we have a detached and complete grasp of what's going on, a view "from the outside."
It is very, very hard for us to get over this balloon-expanding image, no less because we use it all the time to explain the expansion of the universe! But the universe is no balloon.
What really complicates things is that we are trapped inside. We can't step outside the universe to see galaxies moving away from one another. When we measure the cosmic expansion, we must do it from the inside, sort of like a fish that wants to describe the ocean as a whole.
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