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How Do We Know the Universe is Expanding?

One the fundamental characteristics of our universe is one of the simplest to state but terribly difficult, at least at first, to understand. That's the idea that our universe is expanding. Huh? What could that mean? If the universe is everything and it is expanding then what is it expanding into?

Let's ignore all those tricky questions for a bit and just look at the universe. When we look out into the sky we see a huge number of stars and an equally large number of galaxies.

When we observe carefully, we are able to deduce the distance to many of these galaxies and an interesting correlation was noticed. This was first done by Edwin Hubble, so we call it Hubble's law. It says that object will be seen to be moving away from us at a speed proportional to the distance to that object. Or in an equation:


We also see that at a large enough scale the universe is extremely uniform. This is far from obvious because the scale required is HUGE. Clearly the distribution of matter in the Solar System far from uniform. The same is true of scales the size of our galaxy or even galaxy clusters. But, at truly astronomical scales, the universe looks the same, at every place, and in every direction. This can be summarized by saying the universe is homogeneous (the same everywhere) and isotropic (the same in every direction).

This is what we would expect to see if we were at the center of a massive explosion but this explosion would have to be a very special one. Imagine the universe were a grid of galaxies like this:

If things moved according to Hubble's law after a little while it would look like this:

Putting these two images on top of each other we can see the effect very easily.

But watch what happens when we move these images relative to each other:

As the point where the two images overlap moves we are seeing what the pattern looks like from that point. It looks the same. This is certainly NOT like any real explosion. It also shows that the expansion of the universe doesn't have us at the center. Or at least our location isn't special. Every point in space seems to be at the center.

So what would account for this observation. Imagine that instead of the galaxies moving through space, say that space is expanding and taking the galaxies with it. Imagine a spring and you place a bead at the bottom of every loop. If you stretch the spring all of the beads get further apart even though they aren't moving relative to the spring. This is what galaxies are doing in our expanding universe. This is also exactly what was predicted by Einstein's General Theory of Relativity.