# 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

To answer the first part of your question, it is best to first describe the nature of the universe in the moments immediately before the Big Bang. According to the book, "The Five Ages of the Universe," the entirety of the universe was contained into an almost infinitesimally small point. Molecules were unable to exist because particles were unable to bond due to the intense heat. Because of this, matter accounted for only a small minority of the energy, with the majority of energy being contained in the background field radiation (which now exists as the Cosmic Microwave Background).

Now, let's move on to the beginning of the expansion. The majority of matter that did exist consisted of quarks, which are elementary subatomic particles. These quarks were made up of matter and antimatter, with an excess of matter. At some point before the Big Bang, matter and antimatter pairs (as they commonly do) began to annihilate each other, leaving only the aforementioned excess of matter (this "excess of matter" would go on to form the entire universe as we know it). As the so-called quarks and antiquarks destroy one another, the leftover quarks "condense into protons and neutrons," and eventually no free quarks are left. Because the Universe consisted almost exclusively of radiation, it continued to expand, essentially at superluminal speed, and the existing matter eventually became the entire universe as we know it.

So what's the difference between what I just described and an explosion? Well, an explosion needs an area to explode into. The Big Bang was the entire universe, it did not expand into empty space as is commonly thought -- it WAS the empty space, and the occupied space, and everything in between. Second, an explosion undergoes a chemical change. Take the example of an atomic bomb. In the atomic bomb, the uranium underwent a nuclear reaction, permanently altering the chemical makeup of the uranium. However, the universe today contains the same matter and energy that existed in the Big Bang. It would be impossible for an explosion to have occurred because no chemical change occurred. Finally, we must look at the very thing that causes an explosion. An explosion is destructive because of a difference of pressures inside the area of the explosion and outside the area of the explosion. Once again, take the atomic bomb. Around 1% of the Uranium underwent nuclear fission, but the pressure inside the bomb was so much greater than the pressure outside the bomb that it was enough to level a city. In the Big Bang, there was no outside, the entirety of the universe was contained inside a single point. Therefore, no explosion could have taken place because it would have been impossible for a difference in pressures.

Finally, to answer the second part of your question. From what I can gather, you are asking about the center of the universe. Well, everywhere is the center of the Universe. Why? Because if you were to focus on any given point in the universe, the rest of the universe would appear to be moving from that point. For example, if I focus on the Milky Way, the rest of the universe would move relative to our galaxy. However, if I focus on another point, the Milky Way (and the rest of the universe) would move relative to the other point.

TLDR: It would have been impossible for an explosion to have taken place because the entirety of the universe was contained "inside" the Big Bang. The Universe pretty much expands because things are getting farther apart, not getting created (hence the balloon analogy.) Everywhere is the center of the Universe because everywhere is expanding.

• Cosmology has taken on the aspects of a religion,& when science starts to look like a religion that's a sure sign something is wrong. My reserves of blind faith ran out many decades ago,which is why I'm an atheist. Andromeda is moving toward us,by the way. – Michael Walsby Jun 12 at 9:03
• I'm not entirely sure why you just brought up religion, none of this post comes anywhere near a religion. Further, there is no blind faith involved in this -- I provided a citation and can give more. As for the Milky Way vs Andromeda, I was merely making a point. Yes, they are moving towards one another; however, the point at which the Andromeda Galaxy WAS at is expanding. I'll add this to the answer. – N. Bar Jun 12 at 14:23
• This contains so many errors of science, it's not worth trying to counter each of them. Can I simply recommend you have a look at Wikipedia's explanation of the quark epoch and later for a basic primer on what most scientists broadly accept. [The earlier periods are also worth reading, but there's much less agreement.] It makes no sense at all to say "the moments immediately before the Big Bang", since time starts at the Big Bang. – Chappo Jun 12 at 16:03
• A comment is a perfect place to counter each one of my "mistakes." Doing otherwise is just criticism for the sake of criticism. Also, I will admit that I made a mistake in referring to the quark epoch as "immediately before the Big Bang;" however, it simplifies already fuzzy timeline of the Big Bang and further elucidating would just be pedantic for this context. To be more exact, it would be closer to the period of superluminal expansion. Finally, there is no agreed-upon answer to the nature of time before the Big Bang. – N. Bar Jun 12 at 20:59