I honestly don’t have a decent analogy for you to explain how the universe is expanding without a center and without an edge. It just does, whether we can wrap our minds around it or not. But I CAN give you a way to think about it.
If I were to ask you, where is the center of the Earth, you would have no problem pointing that out. It’s down there, in the middle of everything, where molten core is. Duh. Okay, great.
No let me ask the question slightly differently. Where is the surface of the SURFACE of the Earth? Not the whole three-dimensional sphere. Just the two-dimensional surface. In other words, give me the latitude and longitude coordinates of the center of the surface.
Is it the North Pole? The South? Is it latitude zero, longitude zero? But all those coordinates are arbitrary. We decided where the line of zero longitude is by pure convention – it’s the line that passes through the Greenwich Observatory. And we decided that the equator based on where the Sun is at noon during the equinoxes. Yes, we have good reasons for those particular definitions, but we could have just as easily picked other conventions.
I could, if I felt like it, define my current position as the center of the Earth’s surface. And so could you. And so could anybody. We all have a valid claim to being at the center – which is, no claim at all.
The entire universe is like the surface of the Earth. No center.
Now imagine that the Earth is expanding. Cities getting farther apart with time. Continents and oceans growing larger. It’s easy to imagine blasting off into space and looking down at the globe, seeing the Earth expanding. But that’s cheating!
There is no outside perspective to the universe (otherwise it would just be a part of the universe). There’s no additional dimension that we can access to give us that outside, orbital view of the expanding cosmos. We’re in it, and that’s it.
Living in the universe is like living here on the surface of the Earth. We can figure out if the Earth is expanding: it takes longer to travel between points, images of distant cities appear to be redshifted, and so on. We could confidently say that the Earth is expanding without EVER leaving the surface. All we get is an expanding two-dimensional world, and that’s it.
But just like living on the Earth, there is a limit to what we can see. On our planet, the horizon is defined by the curvature (and oh yes, curvature is going to come back in a big way in just a little bit). We can’t see the entire Earth because it curves down and away from our view.
The universe has a horizon too. We can only see a small portion of it.
Take away the expansion of the universe for a second. Just assume that the universe exists, and it’s static, and much less complicated. If it has a finite age, then there’s a limit to what we can see. Light only travels so fast and the universe is only so old: this means there’s a bubble containing all the light that has reached us since the beginning of the universe. It’s like shining a flashlight in a dark forest: there’s the illuminated portion around us, surrounded by the unknown darkness beyond.
Now if the universe wasn’t expanding, then the radius of this bubble would simply be the age of the universe times the speed of light, which comes out to around 13.77 billion light-years. And in cosmology we give this radius a special name: the Hubble radius or Hubble distance, named after, of course, Edwin Hubble, the guy who discovered the expansion of the universe, and also because it would be awkward if it were named after James Hubble or Melinda Hubble.
Plus, the more distant objects appear to be younger. Light takes time to travel, so by the time light reaches us, we don’t get to see what a distant galaxy is like RIGHT NOW. We only get to see it as it was when the light was released. We don’t see the Sun right now; we see the Sun as it was eight minutes ago. We see the Andromeda Galaxy as it was 2.5 million years ago, and on and on. So the innermost portions of our observable bubble are more contemporary, more modern, more in-our-time. As we work our way outwards we see images of the younger universe. The outermost shell of our observable bubble, right at the very limit of what we can see, is an image of the universe just after the Big Bang.
So the observable bubble isn’t just a limit in space, representing a physical distance. It’s also a limit in time. We can’t see anything earlier than the Big Bang – because there was no such thing as earlier, but that’s a different article) – and so the maximum extent that we see in space is also the deepest we can see into the past.
But like I said, different article.