Is space stretching or is new space being created?

[revnice]

Do we even know? My understanding of dark energy is that particles come into existence, exert and outward force, then vanish. My problem with that is how, of course, then how does dark energy know to push everything in the same direction? The pressure exerted would be in all directions, even if the space was moving so why isn't stuff pushed all over the sky?

Thanks - rev

 

[Dale]

Do we even know?

What does it mean? I don’t know the difference between space stretching or new space being created

 

[revnice]

I've heard expansion explained both ways. New space coming into existence or existing space stretching, no new space added.

 

[Dale]

Ok, so what is the difference? I mean, if the difference is just some alternate words that some people use, then pick the words that you prefer. If there is an actual measurable difference, what is it?

 

[PeroK]

I've heard expansion explained both ways. New space coming into existence or existing space stretching, no new space added.

First, the universe cannot be static. It must either be expanding or contracting. This is analagous to a ball being thrown up. It's either moving up or falling back down. It cannot hover at a constant height. The equations that govern spacetime (Einstein's Field Equations of General Relativity) have no static solution. It doesn't require dark energy for this. Dark energy is responsible for the accelerated expansion, but is not needed for expansion itself. If there were no dark energy, the universe would still be expanding, but the expansion would be slowing down.

Second, space itself has no physicality, so that "stretching space" makes no sense to me. How could you identify a region of "stretched space" from "unstretched space"? It's all just the same vacuum. Likewise, thinking of the universe expansion as some sort space creation is also problematic.

What you can measure is the curvature of spacetime. That leaves you with a more agnostic, mathematical approach that says that if we measure time in a certain way (technically using comoving coordinates), then the measured distance between distant objects generally increases over time.

In general, modern physics demands that you learn to see things differently and develop a new way of understanding things, rather than trying to map everything back to some basic notions. This in a way exemplifies the difference between physics as an academic subject and physics as a popular science. Popular science attempts to explain everything in terms of things you already understand, so you don't have to expand your intellect. A textbook on Cosmology or General Relativity will try to get you to expand your way of thinking, so you don't have to rely on thinking about stretched rubber sheets to understand spacetime geometry.

 

[Ibix]

Another point: relativity models spacetime, not space. You can imagine slicing 4d spacetime into a "stack" of 3d slices, each one being "space at one instant", and with galaxies being further apart in each subsequent slice. But nothing is being stretched. Each instant of time is a different part of spacetime.

 

[PeroK]

Another point: relativity models spacetime, not space. You can imagine slicing 4d spacetime into a "stack" of 3d slices, each one being "space at one instant", and with galaxies being further apart in each subsequent slice. But nothing is being stretched. Each instant of time is a different part of spacetime.

This exemplfies the "new way of thinking". Being able to move on from our natural understanding of "3D space + time" and develop an understanding of "spacetime" as four-dimensional, where space and time and no longer separate.

People like to underplay the importance of mathematics (and popular science authors avoid it all costs), but it's only by developing mathematical thinking (in a broad sense) that you can understand modern physics.

 

[Demystifier]

I've heard expansion explained both ways. New space coming into existence or existing space stretching, no new space added.

Those are just analogies, which help to intuitively understand it in terms of everyday concepts, but neither is exactly true. Here is a third analogy, which I find quite appealing. The universe expansion is like changing the resolution of the computer screen. When you increase the resolution, the size of the screen and the number of pixels remain the same. But the screen appears bigger because the icons become smaller so you create space for putting more icons on the screen.

 

[DaveC426913]

My problem with that is how, of course, then how does dark energy know to push everything in the same direction? The pressure exerted would be in all directions, even if the space was moving

It would not be pushing in "the same direction"; it would be pushing in all directions uniformly, causing space to expand in all directions.

so why isn't stuff pushed all over the sky?

Because the expansion is uniformly distributed.

 

[revnice]

Dale:
>Ok, so what is the difference?
Beats me, but one thing seems pretty certain, when objects move away from each other, there's more distance and more space between them. Since we're not expanding into any kind of container or boundary, new space must be coming from somewhere or getting stretched.

For example, if two particles of dust are drifting away from each other in my living room, they're drifting apart in the same amount of space, nothing new is added because my living room is not expanding. But the universe is, and if anything gets bigger, it's my outrageous claim that something's been added, stretched, increased or changed. (Changed as in water to ice, the same amount of water has an increased volume when frozen).

Dave:
>Because the expansion is uniformly distributed.
Got it.

Demystifier:
>like changing the resolution of the computer screen
I like that one too, as long as I don't think about it too much.

 

[DaveC426913]

Dale:
>Ok, so what is the difference?
Beats me, but one thing seems pretty certain, when objects move away from each other, there's more distance and more space between them. Since we're not expanding into any kind of container or boundary, new space must be coming from somewhere or getting stretched.

Well "space" isn't a thing to be stretched.
Well space isn't a thing to be "stretched".

Well, space isn't a "thing" to be stretched.

Consider simply that objects are getting farther apart due to some unknown action.

That does not in-and-of-itself require anything to happen to the space between them.

 

[Dale]

Beats me

Then it doesn’t matter what the answer is.

it's my outrageous claim that something's been added, stretched, increased or changed

Hardly seems outrageous. Seems like the difference between two possible slogans for some advertising campaign

Frankly, I think you are getting caught up in words. There doesn’t appear to be any physics at issue, so just use the words you prefer.

 

[revnice]

Dave:
>That does not in-and-of-itself require anything to happen to the space between them.
Can't say I agree with you there. If everything is moving away from everything else, there must necessarily be more space between everything.

Dale:
Not knowing doesn't mean the answer doesn't matter, it matters because it's not known.

 

[weirdoguy]

Can't say I agree with you there. If everything is moving away from everything else, there must necessarily be more space between everything.

If you'd look at the math, then you'll see that there is no necessity for such thing. And if you are not able to "read" the math yourself, all there is left is to believe people that are. You have no basis to disagree.

 

[PeroK]

Dale:
>Ok, so what is the difference?
Beats me, but one thing seems pretty certain, when objects move away from each other, there's more distance and more space between them. Since we're not expanding into any kind of container or boundary, new space must be coming from somewhere or getting stretched.

This is exactly the constrained thinking I was talking about. You want to understand the cosmos in the same terms you understand your living room. You are not able to conceive of physics beyond what you already know. That's the opposite of learning. Learning is understanding something new or in a new way. Expanding your knowldege. What you are doing is essentially denying there can be anything beyond your current living-room experience. That the cosmos can't be like that because your living room is not like that.

We are here to help you expand your knowledge. Not to help you deny anything beyond what you already know.

 

[Dale]

Not knowing doesn't mean the answer doesn't matter, it matters because it's not known.

The issue isn’t that the answer is unknown. The issue is that the question is unphysical. You are asking “Is space stretching or is new space being created?” but you don’t have any physical meaning behind those words.

So the question that you are asking is just a choice of words and therefore the answer doesn’t matter. There is no physics content to the question, so the answer can be simply whatever words you prefer.

 

[DaveC426913]

Dave:

That does not in-and-of-itself require anything to happen to the space between them.

Can't say I agree with you there. If everything is moving away from everything else, there must necessarily be more space between everything.

Answer me this:

I am currently moving away from my kitchen table, propelled by an unspecified action. Does it necessarily follow that space is being "created" or "stretched" between me and the table?

Yes we use the term "create space" in a figurative way, but surely you acknowledge it's just an abstract concept of distance between things.

 

[PeroK]

Answer me this:

I am currently moving away from my kitchen table, propelled by an unspecified action. Does it necessarily follow that space is being "created" or "stretched" between me and the table?

Yes we use the term "create space" in a figurative way, but surely you acknowledge it's just an abstract concept of distance between things.

There is a fundamental difference between the universe expansion and an expanding kitchen, say.

 

[revnice]

[Mentor’s note: although this post violates the forum rule against using AI as a source - and indeed is a good example of why we have that rule - we are leaving this post up as it has attracted replies]

This from Google:

Yes, space is being created as the universe expands. However, this expansion isn't about new matter appearing, but rather the fabric of space-time itself stretching. Dark energy, a mysterious force, seems to be causing this expansion and creating new space

 

[revnice]

Dave:
"created" or "stretched" between me and the table?
No, because your kitchen is not expanding, it's contained by the walls. If you had no walls and the kitchen expanded to double the size, you'd have double the space between everything.

 

[Ibix]

This from Google:

Yes, space is being created as the universe expands. However, this expansion isn't about new matter appearing, but rather the fabric of space-time itself stretching. Dark energy, a mysterious force, seems to be causing this expansion and creating new space

I assume that's an AI-generated summary. You can tell by the way it's clearly and confidently written and completely wrong. LLMs are hilariously bad at physics, I'm afraid.

 

[Dale]

This from Google:

OK, so you have Google’s AI preferred words. What are yours?

 

[revnice]

It could well be AI generated but I've heard celebrity physicists express expansion in precisely those terms. Namely, that the fabric of spacetime can ripple, stretch and vibrate. It's how we measured the merging of two neutron stars a few years ago with the gravitational wave detector. The AI answer seems to be saying both stretching and creating are going on but it's as uncertain as I am 'seems to be causing this expansion.'

 

[revnice]

Dale:
My words are essentially the same only phrased as a question. Which is true, creating or stretching? The AI answer doesn't answer that for me because it's vague, could be either, could be both.

 

[Nugatory]

Dale:
My words are essentially the same only phrased as a question. Which is true, creating or stretching?

Neither or both, which is why the AI answer is not so much wrong as it is misleading. The underlying problem is that space is not something that can be “stretched” or ”created” as these words are generally understood; we only see that terminology used in non-technical presentations that are trying to explain a mathematical concept to a non-mathematical audience. When the pros talk about cosmic expansion they use the language of differential geometry and the unhelpful notions of “fabric”, “stretch”, “creation” never come up.

 

[PeroK]

This from Google:

Yes, space is being created as the universe expands. However, this expansion isn't about new matter appearing, but rather the fabric of space-time itself stretching. Dark energy, a mysterious force, seems to be causing this expansion and creating new space

Rubbish!

 

[Nugatory]

but I've heard celebrity physicists express expansion in precisely those terms

They will use exactly those words because those are the best that natural language offers, but when they do they’re speaking metaphorically, not precisely. “Fabric of spacetime” and all the things that a fabric can do - ripple, stretch, vibrate - are metaphors, not precise descriptions.

There’s nothing wrong with these descriptions as long as you understand how little of the real thing you’re getting. But the difference between reading these descriptions and doing the work to understand the real thing is the difference between looking at a photograph of a delicious meal in a magazine and actually cooking, tasting, and eating the real thing.

 

[Ibix]

Namely, that the fabric of spacetime can ripple, stretch and vibrate.

And, as Nugatory has just pointed out, it's at best misleading. Spacetime isn't fabric, for starters (one of our German speakers suggested that Einstein once gave a lecture on the "stoff" of spacetime, which could mean "fabric" or "structure" of spacetime - and he meant the geometric structure and someone translated it wrong...). And although I see why people (even people who know better) fall back on calling gravitational waves "vibrations" they really aren't - nothing's changing if you take a spacetime view, and you're describing different parts of spacetime if you take a spatial view.

This really doesn't fit well into non-mathematical language. That's why popsci is misleading at best and LLM summaries of it are terrible.

Dark energy, a mysterious force, seems to be causing this expansion and creating new space

And that is flat wrong, no "maybe" or "I see why people say that" about that.

 

[Dale]

Which is true, creating or stretching?

And, as I asked you before, what is the measurable difference between creating space and stretching it?

If there is no measurable difference then it is a question of words not of physics.

You cannot get a physics answer until you have a physics question. And physics doesn’t have a means of determining what is true other than through measurements.

 

[revnice]

Well gentlemen (and I use the word loosely) it appears that I'll never have the slightest clue on this question because the explanation in layman's terms is completely wrong. My math is limited to 2 + 2 can equal 5 if the value of two is large enough.

Accept it from those who know? The last time I did that I lived with a lie for 65 years (as have all of you) but that's off topic. I'll call this one closed because it's dead-ended for me. Thanks for trying.