Originally posted to 13.7 Cosmos & Culture on 5/24/2017
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.
Current measurements indicate that the universe has a flat (or nearly flat) geometry. For cosmology, a flat geometry means that rays of light actually travel on a straight line across space. (In a curved geometry, the rays would trace a curved path, like when you run your finger over the meridian of a globe.) Also, and very importantly, a flat geometry means that the universe is probably infinite. If you'd start moving on a straight line, you'd never come back to where you started. (In a spherical geometry, if you move, say, along the equator, you'd get back to your starting point.)
How can something infinitely-large be expanding? The usual follow-up question (inspired by the expanding balloon image) is: "Expanding into what?" Good questions. The straight answer is that the universe doesn't expand into anything. There is no space "out there" for it to expand into. What the cosmic expansion does is stretch space itself, as if space were made of some kind of stretchy rubber material. There is no physical border out here, only stretching space.
To picture this, we need to build a "view from the inside," as human observers inside the cosmos. Here is one way: Imagine a checkerboard. A checkerboard is a two-dimensional space — you can move in two directions. Real space is three-dimensional, as we can also move vertically up and down. But picturing spaces in 2D is much easier for us, so the checkerboard will do. Now imagine you put a penny at each vertex of the checkerboard, the point where two lines meet. The checkerboard is our flat universe (it extends to infinity in all directions), and the coins are galaxies. Of course, the real universe is not this orderly, but the idea is the same.
If the universe were static — that is, not expanding or contracting — the coins (galaxies) would just stay there. In an expanding universe, the squares would stretch and grow equally in all directions and the coins would be carried along, like corks floating on a river. If you were a creature sitting in one coin ( or galaxy), you'd see all your neighbors move away from you. Proudly, you'd think you are the center of the expansion. But your pride is an illusion. Every observer in every coin would see the exact same thing, its neighbors moving away. No coin (or point in the geometry) is more important than any other point. When it comes to the cosmic expansion, we have complete space democracy. In other words, the universe has no center. It's incorrect to picture the expansion as a bomb that exploded far in the past. (We will get to this wrong picture of the exploding universe and the Big Bang another week.)
Back to the real universe, every point is a center of the expansion, as observers measure their neighbors moving away, carried by the stretching geometry. This is what the cosmic expansion means. In 1929, American astronomer Edwin Hubble measured the expansion and showed that it obeys a very simple law, where the galaxies move away from one another with velocities that grow in proportion to their distances. So, an observer sees a galaxy that is twice as far away from a closer one move away (or recede) twice as fast.
A slight, but very interesting, complication is that since gravity is an attractive force, when two or more galaxies are sufficiently close, in their movements they can detach from the general cosmic expansion as they drift toward one another. We say that they acquire "peculiar velocities." Peculiar velocities are very important in astronomy and cosmology because they allow us to map how large concentrations of mass are distributed across space in big galaxy clusters, some containing thousands or more galaxies. Our own galaxy, the Milky Way, is part of what we call the "local group."
The mapping of these large concentrations of galaxies — and of all galaxies — is the grand quest of extragalactic astronomy. There is a story here, a story that began when matter first gathered into clumps way back when the universe was very young, and how these clumps grew to become galaxies distributed in large groups. To uncover this story, we need to go back to the universe's early history (as close to the Big Bang as we can), and to know what the cosmic ingredients are — that is, the different kinds of matter and stuff that fill space.
We will get to these two issues during the next few weeks to complete our cosmic trilogy.