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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?

https://www.youtube.com/watch?v=veQtF3_2tOE

Randy Zeitman
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  • Welcome! Related: https://physics.stackexchange.com/q/136860/32426 – peterh Jun 10 '19 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 '19 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 '19 at 19:35
  • What is the second part? A point expanding from '0D' to 3D? – Randy Zeitman Jun 11 '19 at 19:37
  • Space, like matter or energy, is an object (contained within the microscopic or submicroscopic parts of material objects), whose "vacuum energy" has been seen in many simple experiments. In English, what you (and many writers) have coloquially described as "space" can be accurately described as "nothing" or "nothingness", although, because of mass/energy equivalence, there may be limits on the perception of it, typically described, hypothetically, as requiring magnification energies which would result in the collapse, into a black hole, of whatever objects a volume of it might contain. – Edouard Apr 28 '23 at 19:17

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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:

enter image description here

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.

Sir Cumference
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  • "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 '19 at 20:05
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    @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" ... – hmakholm left over Monica Jun 12 '19 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. – hmakholm left over Monica Jun 12 '19 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 '19 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. – hmakholm left over Monica Jun 12 '19 at 21:32
  • @HenningMakholm Are you saying, as I do, that there's no such "thing" as space and it's really a synonym for distance? – Randy Zeitman Jun 13 '19 at 01:39
  • @RandyZeitman I like the idea of “space” as a proxy for distance, but then someone can ask “what is distance?” and dispute that it can expand since it’s not made of anything. In the end, what we’re talking about is the portion of the universe defined by a set of coordinates (which might not be specified), and if there’s a low matter density it’s convenient to call it “space”. – Chappo Hasn't Forgotten Jun 13 '19 at 05:35
  • @Chappo Distance is the contrast between physical distinctions." For example the distance between two balls. Each ball is a physical "thing", a distinction, because it has an edge. It's where it 'ends'. Outside of that edge there is no more ball. You had ball, now you have none. That's contrast. On/Off. It's there, it's not. Then you have another edge for the other ball. All balls are contrasts. The degree of contrast can be various (conventional) identifiers such as size and distance. These identifiers are not literally real ... they are ideas, models, to express our sensory inputs. – Randy Zeitman Jun 13 '19 at 16:50
  • Davis, of the Davis & Lineweaver team whose numerous essays (all of them with titles containing the phrase "Expanding Confusion") have warned against the assumption that material objects might be carried away by the observed expansion of space, which they feel to have resulted from a confusion of Special Relativity with GR, although I lack the mathematical knowledge required to follow their argument. Nobel winner John Mather has described the expansion as "the Universe expanding into itself". In pop. sci., I've seen that effect depicted as phys. distortions of shapes in different localities. – Edouard Apr 28 '23 at 19:48
  • The causal separation between localities may be a factor in the increasing acceptance of multiversal, rather than single-universal, cosmological models. – Edouard Apr 28 '23 at 19:54