# What does it mean for space to expand?

In this YT video Michelle Thaller says The Big Bang wasn’t an explosion but an expansion and as such there's no empty center where the explosion would've been.

To explain expansion she uses the analogies of surface of a balloon being blown up and stretching a rubber sheet.

But isn't it true that any shape, be it having an edge (the sheet) or a balloon/sphere, would have a point that isn't expanding? ... the center of an expanding 2D rectangle, a sphere or cube, etc?

Is it more accurate to say that it's not like a sheet or balloon but that either every point in space is expanding three dimensionally, or, that space is actually not expanding, because it doesn't exist, and that what we call 'space expanding' is actually cosmic entities traveling in uncoordinated directions?

• Welcome! Related: physics.stackexchange.com/q/136860/32426 – peterh Jun 10 at 23:58
• I do not follow the second part but the first one is a nice observation. Somehow infinity is required to use the rubber analogy. Which makes me wonder about tger the fact that in flat space there were always disconnected regions. I have tried twice related Qs on Phys SE but I wasn't even understood except by one user who have reformulated the Q more mathematically and again got no answer. My question isn't identical to yours, but related for sure. – Alchimista Jun 11 at 12:13
• What is the second part? A point not expanding? (or perhaps it's a point that's expanding more slowly. Imagine stretching a square, rubber, sheet in all (four) 2D directions ... don't points at the edges expand the most and at the center the least? How can space be 'disconnected' when it's really just a proxy for distance between things? – Randy Zeitman Jun 11 at 19:35
• What is the second part? A point expanding from '0D' to 3D? – Randy Zeitman Jun 11 at 19:37

When we talk about the expansion of the Universe, we're really saying that space is being created between all matter. Let me explain.

Imagine setting up a grid that keeps track of all points in space. "Expansion" just means that the distances between objects on the grid are getting larger. In essence, more space is being created between the objects. Below is a gif I've made to demonstrate this:

A more useful way to describe this is to say the grid is expanding — that space itself, as a coordinate system, is growing. As an analogy, imagine are walking your dog. Suddenly, the ground begins expanding between you. You and your dog will separated and continue receding away from each other. So the same thing is happening with our universe.

The grid is in fact growing, and objects are being swept away with it. A consequence of this is that they can recede away from each other faster than light; while objects are indeed limited in how fast they can move through space, there is no limit as to how fast space can be created between them.

Now that we've gotten the core concepts down, I'll introduce one more bit of terminology. The "scale factor of the Universe" refers to how much the Universe has expanded, compared to now. For example, if in a billion years the scale factor is 3, that means that every object in the Universe is 3 times farther from each other compared to now. If the scale factor 700 million years ago was 0.8, then everything was closer by a factor of 0.8 at that time. By definition, the scale factor is 1 right now.

So, if the Universe is expanding now, we'd expect it to be smaller as we look further back in time — i.e. the scale factor would be less. General relativity predicts the scale factor to be zero at 13.8 billion years ago. This would mean that every object would be zero times its current distance from us — in other words, there would be no space.

If you think a Universe without space is impossible, you're correct. We apparently have a contradiction. In GR, you can't have a spacetime with zero space.

Our modern physical theories work fine up a few fractions of a second after the moment of contradiction, and our observations do agree with the idea of an extremely dense early universe. However, our theories break down as we try to model the Universe at earlier and earlier times, until they no longer prove accurate, preventing us from explaining the most interesting moment.

This is why the moment of the Big Bang is one of the biggest mysteries in cosmology. Theories like quantum gravity have arisen to try to explain the conditions near the Big Bang, but none are sufficient as of now.

• "In essence, more space is being created between the objects." How can space be created? ... from what?... it has no atoms so how can be a "thing" (that gets created). Would it be more true to say that the distance between the objects increases but the 'amount' of space does not ... "space does not increase" ... just because distance increases doesn't mean that "space" increases. In other words, is space different or the same as distance? – Randy Zeitman Jun 12 at 20:05
• @RandyZeitman: Instead of "space is created", which seems to imply someone doing the creating, one should perhaps say "more space comes into being". But even that is not an exact description. Like the balloon analogy, it is just an attempt to describe one feature of a particular precise mathematical description (namely, the Robertson-Walker metric) in sort-of everyday terms, without trying to show the actual mathematics. It is often preferred to give such than analogy rather than brushing honestly curious people off with "go learn a semester of differential geometry and then we can talk" ... – Henning Makholm Jun 12 at 20:37
• ... but one shouldn't take it for more than it is. Trying to draw conclusions from the analogy alone is pretty hopeless -- sometimes they're right, at other times they don't correspond to anything that happens in the actual math. And unfortunately the only way to tell one from the other is to double down and learn the math. – Henning Makholm Jun 12 at 20:38
• @HenningMakholm Thank you and yes there's no conventional word (created) that will do it seems ... nor existential (being). That said shouldn't the conclusion be that 'space is a proxy for distance'? Space does not expand ... distance increases ... and objects are not 'in' anything, "space" is a proxy for distance - it does NOT exist without conventional items to define it (as the absence of conventional objects). – Randy Zeitman Jun 12 at 21:14
• @RandyZeitman: If you can form a mental picture for yourself what works just by "distances increase", then by all means go for that! It's closer to the mathematics than speaking about space as a "thing" of itself is. – Henning Makholm Jun 12 at 21:32