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But what is the Universe expanding into? [Q&A]


It is probably the most common question astronomers encounter: “Fine, you say that the Universe is expanding. OK, I buy that. But what is it expanding into?”

There is really no good way to answer this because the correct answer, that it is not expanding into anything, that there is no outside to the Universe, simply doesn’t make any sense to the questioner, and just sets up a recursive mystification: if there is no outside, what is it expanding into?!

The problem is, that is not how we look at it, and there is a disconnect in how the mathematics is set up and how it gets translated to plain English.

To begin with, people have an intuitive, common sense idea of what expansion means. When you see something expanding, that is, literally see it with your eyes, what you see is a change in the size of an object which has a well-defined boundary that is gobbling up more and more of whatever medium it is located in. The key words here are size, boundary, and medium.

But the Universe is everything that there is. It has no spatial boundary, or edge, where it stops being the Universe and starts being something else. So what is really going on?

It is probably fair to blame Einstein for this. His genius lay in realizing that it is not possible to describe the structure of the Universe in totality, but it is perfectly possible to describe it in locality. You can be very precise in saying how two nearby points in space-time behave. That is what the General Theory of Relativity is: a description of the metric of space-time over small distances. All cosmologists do is take this metric and extrapolate it as much as they can given what we have seen of the Universe. So when they say that the Universe is expanding, what they really mean is that the metric is changing.

Think of it this way. At any given instant, it is easy to imagine the Universe as everything there is. Now imagine you can draw a grid of lines on it to define a spatial, 3D coordinate system. The lines are just the paths along which light would travel. Now pick a couple of points in this grid and measure the spatial distance between them. Now blink, and look at the Universe in the next instant. It looks pretty much the same, but if you measure the distance between the two points you had picked out before, it will be slightly different, slightly stretched. Locally there is more space that light needs to traverse to get from here to there. But even in this new instant, the Universe is still everything that there is. You see how that works?

The bottom line is that this is not a material expansion we are talking about, i.e., not an expansion of stuff, but a change in the geometric properties of space-time.