High School Understanding the Expansion of Space: Galaxies Moving Away and Proving Expansion

[Richard lanning]

How does knowing that galaxies are all moving away from each other and the farthest ones are moving more quickly prove that space is itself expanding and not just that the galaxies are moving through it?

 

[kimbyd]

How does knowing that galaxies are all moving away from each other and the farthest ones are moving more quickly prove that space is itself expanding and not just that the galaxies are moving through it?

Galaxies are, on average, moving further away from one another at a rate proportional to their distance. That statement is what is meant by, "Space is expanding." The two descriptions are one and the same.

Edit:
One way to think of it is by looking at the Einstein Field Equations which describe General Relativity. They can be written as:

$$G_{\mu\nu} = {8\pi G \over c^4}T_{\mu\nu}$$

I'm sure this looks like a bunch of incomprehensible characters, but the basic premise is simple: on the left is what is known as the "Einstein tensor" which describes space-time. When people say, "space is expanding," they are talking about the behavior of the Einstein tensor.

On the right hand side is what is known as the "stress-energy tensor". This object describes the matter content of the universe. When people say, "galaxies are moving away from one another," they're describing the behavior of the stress-energy tensor.

The two terms are equal, so they're just different ways of looking at the exact same thing.

 

[Richard lanning]

Thank you for the reply...another question if I may...If we happened to be at the "center" of the big bang or the "center of the universe" whatever that means, would we not see the same movement of the galaxies and their speeds that we see now??

 

[Orodruin]

If we happened to be at the "center" of the big bang or the "center of the universe" whatever that means

It does not mean anything, there is no such thing. The expansion would look the same wherever you are. In fact, it is one of the underlying assumptions behind the standard model of cosmology.

 

[Richard lanning]

It does not mean anything, there is no such thing. The expansion would look the same wherever you are. In fact, it is one of the underlying assumptions behind the standard model of cosmology.

It does not mean anything, there is no such thing. The expansion would look the same wherever you are. In fact, it is one of the underlying assumptions behind the standard model of cosmology.

Thanks for the reply...

 

[mfb]

The redshift/distance relation we see is incompatible with the idea of things moving through space.

 

[Orodruin]

The redshift/distance relation we see is incompatible with the idea of things moving through space.

Depends on how far away you go. If you do not go too far away, it can be interpreted exactly as things moving through space, which was Hubble's original interpretation.

 

[rede96]

The redshift/distance relation we see is incompatible with the idea of things moving through space.

From a relativity point of view isn't the statement 'things moving through space' just as wrong as saying 'things static in space'? All we can talk about is how things move relative to some frame of reference.

As regards expansion I always got confused over questions like 'Is 'space' expanding' and never really got a satisfactory answer as opinions seem to differ.

For me is a much better way of thinking of expansion is from our point of view, in simple terms, everything is moving away from us and the further away something is the faster it is moving away. But that same can be said from any frame of reference we chose in the universe.

 

[Orodruin]

From a relativity point of view isn't the statement 'things moving through space' just as wrong as saying 'things static in space'? All we can talk about is how things move relative to some frame of reference.

In cosmology, there is a preferred reference frame. The geometry of space-time does give a particular frame preference in much the same way that there would be preferred directions on the surface of an ellipsoid. When one talks about "moving in space" it typically refers to motion relative to that frame (called the "comoving frame"). This is also the CMB rest frame so you can actually measure your motion relative to it. Most of the dipole contribution to the CMB temperature comes from our motion relative to the CMB rest frame.

 

[kimbyd]

In cosmology, there is a preferred reference frame.

I think it's very misleading to call this a "preferred reference frame". There is a convenient reference frame. You can do all the math in a different frame and get the same result. It's just that the math is easier if we choose a reference frame that matches the symmetry provided by the expansion.

 

[mfb]

Depends on how far away you go. If you do not go too far away, it can be interpreted exactly as things moving through space, which was Hubble's original interpretation.

Well, but we do go far away.

As regards expansion I always got confused over questions like 'Is 'space' expanding' and never really got a satisfactory answer as opinions seem to differ.

Space is expanding, and the (educated) opinions don't differ there.

For me is a much better way of thinking of expansion is from our point of view, in simple terms, everything is moving away from us and the further away something is the faster it is moving away.

Don't do it. This leads to various misconceptions.

 

[Orodruin]

I think it's very misleading to call this a "preferred reference frame". There is a convenient reference frame. You can do all the math in a different frame and get the same result. It's just that the math is easier if we choose a reference frame that matches the symmetry provided by the expansion.

Fine. I will sign off on that. Clearly you can use any coordinates you like. Let's call it a "singled out" or "natural" frame.

 

[Orodruin]

Well, but we do go far away.

We do, but Hubble didn't, which is something I think is important to understand in order to get the connection between modern cosmology and the historical context. In other words, the first thing people usually hear is about "things moving apart" and then later we tell them that things are not moving (relative to the comoving frame), but space is expanding. Locally, it is the same thing, just different simultaneity conventions.

 

[kimbyd]

We do, but Hubble didn't, which is something I think is important to understand in order to get the connection between modern cosmology and the historical context. In other words, the first thing people usually hear is about "things moving apart" and then later we tell them that things are not moving (relative to the comoving frame), but space is expanding. Locally, it is the same thing, just different simultaneity conventions.

"Distances between objects are increasing on average" is in some ways less misleading than "space is expanding".

They both describe the same thing, but "space is expanding" can lead to some misconceptions about the nature of space-time.

 

[Orodruin]

"Distances between objects are increasing on average" is in some ways less misleading than "space is expanding".

Both are to some extent misleading and convention dependent. Distances increasing comes with its own set of problems.

They both describe the same thing, but "space is expanding" can lead to some misconceptions about the nature of space-time.

So does ”distances increase on average” so I don’t really see a clear preference for one or the other from this point of view. I have been here long enough to see both multiple times.

 

[Ken Ucarp]

Galaxies are, on average, moving further away from one another at a rate proportional to their distance. That statement is what is meant by, "Space is expanding." The two descriptions are one and the same.

Edit:
One way to think of it is by looking at the Einstein Field Equations which describe General Relativity. They can be written as:

$$G_{\mu\nu} = {8\pi G \over c^4}T_{\mu\nu}$$

I'm sure this looks like a bunch of incomprehensible characters, but the basic premise is simple: on the left is what is known as the "Einstein tensor" which describes space-time. When people say, "space is expanding," they are talking about the behavior of the Einstein tensor.

On the right hand side is what is known as the "stress-energy tensor". This object describes the matter content of the universe. When people say, "galaxies are moving away from one another," they're describing the behavior of the stress-energy tensor.

The two terms are equal, so they're just different ways of looking at the exact same thing.

So if I'm understanding you, the left side is an abstraction which manifests itself concretely by measurements of the physical universe that fit the right side. And the right side could have been otherwise, but just so happens to be as noted. I'm also guessing that there are some solutions to this whereby the left side would be 0, i.e. no expansion. It's just that that's not what is observed.

 

[Orodruin]

I'm also guessing that there are some solutions to this whereby the left side would be 0, i.e. no expansion. It's just that that's not what is observed.

If the left-hand side is zero, then so is the right-hand side. This corresponds to a Universe empty of content and indeed such solutions exist. The simplest one is just the Minkowski space of special relativity.

 

[PeterDonis]

the left side would be 0, i.e. no expansion

The left side of the Einstein Field Equation does not mean "no expansion"; it means "no stress-energy", i.e., as @Orodruin said, no content. Such solutions are usually called "vacuum" solutions, to emphasize the fact that there is no matter, radiation, etc. present.

For a spacetime to describe "no expansion", it needs to have a property which is called being "stationary". There is no simple way to express this criterion in terms of what you would see on the left side or the right side of the Einstein Field Equation; at least, I'm not aware of one. This property is neither equivalent to nor disjoint from the property of the left side of the EFE being zero: there are solutions which have both properties (vacuum and stationary), solutions which have only one (vacuum or stationary, but not both), and there are solutions which have neither property. The solution that describes our actual universe is of the latter sort (neither vacuum nor stationary).

 

[Orodruin]

For a spacetime to describe "no expansion", it needs to have a property which is called being "stationary".

Could you specify what you mean by "no expansion" here? As we discussed a few days ago, Minkowski space (or rather, the interior of the future light cone of any event in it) can be described by a FLRW-type metric with a linearly growing scale factor, yet it is certainly a stationary space-time.

 

[PeterDonis]

Could you specify what you mean by "no expansion" here?

I mean that the spacetime is stationary, in the technical sense of having a timelike Killing vector field. Yes, I know Minkowski spacetime, strictly speaking, doesn't fit this, because it has a timelike KVF but also can be described as "expanding" using an FLRW-type metric. For a "B" level thread I think it's better to put that aside as an edge case than to try to elucidate it.

 

[Orodruin]

For a "B" level thread I think it's better to put that aside as an edge case than to try to elucidate it.

Agreed, I was only asking out of interest in if I was missing something.

 

[kimbyd]

I'm also guessing that there are some solutions to this whereby the left side would be 0, i.e. no expansion. It's just that that's not what is observed.

Only if the universe is completely empty. If there is any matter/energy in the universe, then a solution which is neither expanding nor contracting is unstable, meaning it will eventually transition to either expanding or contracting.

 

[PeterDonis]

If there is any matter/energy in the universe, then a solution which is neither expanding nor contracting is unstable, meaning it will eventually transition to either expanding or contracting.

Note that this is true given the underlying assumptions of the FRW models, i.e., that spacetime is homogeneous and isotropic. There are other non-vacuum solutions of the EFE which are stationary (for example, a solution describing a spherically symmetric star surrounded by empty space), and are not unstable in the way you describe.

 

[ObjectivelyRational]

Just following up - with the goal of extracting an accurate conception of reality implicit in the statement "space is expanding" or "the universe is expanding" which is accessible to a layperson:

How would you answer an 8 year old who wanted to an accurate picture of reality and asked the following questions:

Is there 'more' "space" (in the universe) now, than there was, say, a million years ago, and was there 'more' "space" then (in the universe) than there was, say, a billion years ago? If yes, in what sense, is there "more space"?

 

[PeterDonis]

How would you answer an 8 year old who wanted to an accurate picture of reality

I would tell him that I can't give him an "accurate picture of reality" in the sense he means it. The best I can do is to describe our best current models.

Is there 'more' "space" (in the universe) now, than there was, say, a million years ago, and was there 'more' "space" then (in the universe) than there was, say, a billion years ago?

Mu. The question presupposes that the question "how much space is there in the universe?" has a well-defined answer. It doesn't.

Some people might prefer to sugar coat the above by trying to find a way to answer the question. However, any such answer would be moving away from your stated goal, which is an accurate picture of reality.

 

[ObjectivelyRational]

I would tell him that I can't give him an "accurate picture of reality" in the sense he means it. The best I can do is to describe our best current models.
Mu. The question presupposes that the question "how much space is there in the universe?" has a well-defined answer. It doesn't.

Some people might prefer to sugar coat the above by trying to find a way to answer the question. However, any such answer would be moving away from your stated goal, which is an accurate picture of reality.

Second Follow up (addressed to everyone also):

How would you answer an 8 year old who, wishing to decide how to take and/or trust what scientists tell him, i.e. how to take what a statement made by a scientist means, asks:

When a scientist says "space is expanding" or the "the universe is expanding" should I believe him? If yes, in what sense should I believe what he is saying about space or the universe is accurate?

 

[PeterDonis]

When a scientist says "space is expanding" or the "the universe is expanding" should I believe him?

Not unless you understand, in precise terms, what he means by "space is expanding" or "the universe is expanding". Scientists use everyday words with meanings that are different from their everyday meanings; so when a scientist says "space is expanding" or "the universe is expanding", it doesn't mean what those words would mean in ordinary conversation. What the scientist actually means is not something that can be explained in a sentence or two.

in what sense should I believe what he is saying about space or the universe is accurate?

It's accurate in the sense that, once you understand the precise technical meanings of the words the scientist is using (i.e., what they actually refer to in the mathematical model the scientist uses to make predictions), you will agree that the statement is true with those meanings given to the words.

 

[ObjectivelyRational]

Not unless you understand, in precise terms, what he means by "space is expanding" or "the universe is expanding". Scientists use everyday words with meanings that are different from their everyday meanings; so when a scientist says "space is expanding" or "the universe is expanding", it doesn't mean what those words would mean in ordinary conversation. What the scientist actually means is not something that can be explained in a sentence or two.
It's accurate in the sense that, once you understand the precise technical meanings of the words the scientist is using (i.e., what they actually refer to in the mathematical model the scientist uses to make predictions), you will agree that the statement is true with those meanings given to the words.

Suppose the 8 year old is in fact your son whom you wish (I will assume) to honestly inform. Are the above your final answers to him as he stares at you with inquisitive eyes? And in your opinion is there any other alternative statement(s) in everyday language which would be more accurate for your 8 year old son to hear?

This may or may not be true, or accurate:
e.g. Son, the galaxies and other large parts of the universe are flying apart from each other, and faster the farther they are from each other AND from what we know, it looks like that from any point in space. There is no Nothing, which these parts of the universe are flying into, but since the distances between all of these large structures is continually increasing, we refer to this as an expanding universe. Space is not a thing which is being created by the expansion of the universe to fill in the "Nothingness", as I said there is no Nothing, but the magnitude of the relationship "distance" between each and every thing (on a large scale) is indeed increasing.

 

[AdrianDW]

Hi Peter,
In my idea, all of this is easy to explain...even to an eight year old.
From my perspective you are too 'close' and too schooled to see something big and obvious.
I am not expecting a reply, just saying.

 

[PeterDonis]

Suppose the 8 year old is in fact your son whom you wish (I will assume) to honestly inform. Are the above your final answers to him as he stares at you with inquisitive eyes?

If he doesn't ask any follow-up questions, yes. In my experience of 8 year olds, however, I would expect plenty of follow-up questions.

Notice that the question as you phrased it does not ask for my explanation. It just asks if the 8 year old should believe what "scientists" are saying.

And in your opinion is there any other alternative statement(s) in everyday language which would be more accurate for your 8 year old son to hear?

Statements of what? Of whether he should believe scientists? I don't think you should believe anyone about anything without doing some sanity checking.

This may or may not be true, or accurate

It's not a bad start at an explanation at the 8 year old level. The main follow-up question I would expect is what "there is no Nothing" means.

Here is another possible way of describing our best current model to an 8 year old in a few sentences:

When we observe distant galaxies, we see that they appear to be moving away from us, and the farther away they are, the faster they appear to be moving away. Scientists describe this by saying that the universe is expanding; but that does not mean (as it would imply in ordinary usage) that there is some pre-existing space that the universe is expanding into. It is just the best word we can find in ordinary language to describe the detailed model that scientists have built to explain what we observe.