What’s up with this expanding universe anyway?

So, you’ve heard about the expansion of the universe. If you haven’t, well…surprise! The universe is expanding. But what does this mean? Well, in a previous post, I explained a bit about how space and time are not rigid, they can be bent. You might like to think of the universe as being made of rubber (I do…sort of). So what physicists really mean when they say the universe is expanding is that the rubbery spacetime in which we live, is being stretched.

Misconception #1: Expansion = Motion

Some people may confuse the idea of an expanding universe with the idea that all of the galaxies are moving away from each other. This is not a great way of thinking about it. What’s better is to think of this rubber sheet and draw some dots on it that represent galaxies. Now, if you start stretching the rubber sheet it will look like the dots are all moving away from each other, but actually, with respect to the rubber, they aren’t moving at all (very zen).

Misconception #2: Special Relativity is Violated

You may have heard that some galaxies appear to move away from us at speeds greater than the speed of light. Being the knowledgeable person that you are, you worry about Einstein’s special relativity being violated, since nothing can travel faster than the speed of light. Actually, there is no problem. Since expansion does not equal motion, the apparent speeds of these galaxies are not due to movement, but to the expansion of spacetime, so Special Relativity is not violated.

Misconception #3: All matter is expanding away from the center of the universe.

This may tie your brain in a knot at first, but I’ll say it anyway; there is no center of the universe. Remember that the universe is like a piece of rubber. If we were riding on one of those dots drawn on the surface while the rubber was being stretched, what would we see. Well, we would actually look around and see all of the dots moving away from us. This is because since the universe is being stretched the same amount in all directions, the distances between all dots are increasing at the same rate. It doesn’t matter which dot (galaxy) we choose to ride on, it will still look like all of the other dots (galaxies) are moving away from us. So since what we see doesn’t depend on from where we are looking, there can’t be any center.

Now consider this: if the very space in which we exist is expanding, then why aren’t the atoms in our body expanding away from each other? Why aren’t we just falling apart? Well, on very small distance scales, this effect is tiny… very tiny. The atoms in our body hold on to each other by electromagnetic forces. On larger scales (like a galaxy), gravity is the dominant force holding things together. That’s why galaxies don’t disintegrate because of spacetime expansion, gravity holds the stars together.