B Understanding the Expansion of Space: Galaxies Moving Away and Proving Expansion

[PeterDonis]

In my idea, all of this is easy to explain

If you think it is easy to explain, I think you don't understand it well enough. Any "easy" explanation is going to leave out a lot of complications. But the complications don't go away or become less important because you leave them out.

 

[ObjectivelyRational]

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

Thank you (finally) for your answer. I think the 8 year old would be satisfied, and more importantly, properly informed.

 

[kimbyd]

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?

You might have guessed from the other responses here, but it turns out to be remarkably difficult to describe the behavior of the universe in words and get it right. The fundamental problem is that the language of the universe is mathematics, and words are simply not precise enough to capture things. The best you can get with words is a glimpse of the underlying mathematical description. It's sometimes possible to make a statement that is correct, but it's generally going to incomplete.

But I agree with PeterDonis: it's accurate to say that distances between galaxies are increasing. Most everything is getting further away from most everything else.

One complication that can be added to the above is that not everything is moving further away from everything else. We aren't moving further from the Earth, and the Earth isn't moving further from the Sun (at least, not by much and not because of the expansion of the universe). Some things are orbiting other things. The Andromeda galaxy, the nearest big galaxy to our Milky Way, is moving towards us and will collide with the Milky Way in about 4 billion years. But far-away things are always moving away from one another. And the further they are, the faster the distance between them is increasing.

 

[Metmann]

The central number to mention should be $H = h\cdot 100\, \frac{\text{km}}{\text{s}}/\text{Mpc}$ with $h \approx 0.67$. In our vicinity, gravitational interactions can be strong enough to erase the effect of cosmic expansion and may lead to attraction. On large enough scales cosmic expansion dominates. One should keep in mind, that standard (isotropic) cosmology describes the Universe on scales above $\approx 100\, \text{Mpc}$. Sure, expansion (i.e. in fact redshift) can be observed on smaller scales than this, but one should never confuse the local evolution of the Universe with the global one.

 

[Graeme M]

I've always struggled to understand what is meant by "space is expanding" but have never really tried to grapple with it. This thread is a bit over my head, but it might be clearing up some of my conceptual confusion. This is sort of what I get from what has been said above.

The material universe we observe contains objects (galaxies?) which are "moving" apart from each other. That movement apart exists in all possible directions - that is, over time the (true? actual?) distance between any two objects relative to the CMB will increase, even if those two objects have no velocity relative to the CMB (ie are stationary). Thus, the "expansion of space" just means that true distances between galaxies are increasing (so galaxies are not actually "moving" in this context, in the common sense of the term "moving").

Questions.

Often this idea talks of "space" as some kind of thing, yet isn't that just a term for distance? I mean, if it is the case that the matter in the observable universe causes all physical phenomena such as gravity, time, movement etc, then if we were to absent the material universe from space there would be nothing at all? I mean by this that there would still be an infinite "space" which has no properties other than the potential for such quantities as "distance". In effect, that would mean that objects are not moving "through" space, they are simply increasing/decreasing distance between themselves. In fact, if there were no other objects to measure motion against, one couldn't say that a moving object is even moving.

If "space" without matter is nothing at all, does that mean that it is this nothing at all that always exists (if one can even say such a thing) and that our physical universe emerged into this nothing and thereby caused time and gravity and galaxies and so on to come into being (which is what the Big Bang is all about)?

 

[jbriggs444]

I mean, if it is the case that the matter in the observable universe causes all physical phenomena such as gravity, time, movement etc, then if we were to absent the material universe from space there would be nothing at all?

In the absence of any possible experiment, the scientific answer is "we do not know". That answer is not the same as "nothing at all". However, we can extrapolate from what we do know...

The equations for the theory of general relativity have what are known as "vacuum solutions". These are descriptions of space-time within a universe that is devoid of matter and energy. https://en.wikipedia.org/wiki/Vacuum_solution_(general_relativity)

A number of vacuum solutions are known. One of those is the de Sitter Universe. https://en.wikipedia.org/wiki/De_Sitter_universe

 

[ObjectivelyRational]

In the absence of any possible experiment, the scientific answer is "we do not know". That answer is not the same as "nothing at all". However, we can extrapolate from what we do know...

The equations for the theory of general relativity have what are known as "vacuum solutions". These are descriptions of space-time within a universe that is devoid of matter and energy. https://en.wikipedia.org/wiki/Vacuum_solution_(general_relativity)

A number of vacuum solutions are known. One of those is the de Sitter Universe. https://en.wikipedia.org/wiki/De_Sitter_universe

I think Graeme M is wondering about space as such... i.e. in absence of anything else whatever, and that would include energy, fields, probabilities of every kind. Is it a thing in itself. If there were no things whatever to relate by distances or specify by position, i.e. no As, or Bs to say A is distant from B, or A has a magnitude of A1 at point X or B has a probability of B2 at point Y, no As changing into B from time T1 to T2, absolutely nothing, what could the concept of space-time be used for by scientists? If it is only for relating things and events it would be inapplicable to a completely "empty" universe. If however, it were something in and of itself, one could not dismiss it, after all, it would still "be there".

IMHO the hypothetical universe Graeme is raising does not include anything capable of being described as a something about space-time or something at or within space-time or related by space-time, his hypothetical is what if there were literally nothing but space-time itself...

Your answer (we don't know but) "not the same as nothing at all" is supported with "vacuum solutions" which on a reading by a layperson (translated) means either

1. the answer is identically zero. No matter no energy nothing to cause gravitation, and what looks like, no space-time "tensor". OR
2. IF a universe had dark energy (cosmological constant) but no "normal" matter flat space is a solution.

Of course this is a fiction and he is really just trying to get at what space-time actually is, is it math or something real, out there, independent of things in or at (or when) it?. Is it a relationship between things and a parameterization of "at" and "when" things interact or exhibit properties etc.or is it something in and of itself quite real and independent of everything else.

 

[jbriggs444]

IMHO the hypothetical universe Graeme is raising does not include anything capable of being described as a something about space-time or something at or within space-time or related by space-time, his hypothetical is what if there were literally nothing but space-time itself...

In other words, "tree falling in a forest". Pure philosophy. Unacceptable subject matter here.

My response was aimed at drawing the conversation back to something with a scientific basis.

Edit:

1. the answer is identically zero. No matter no energy nothing to cause gravitation, and what looks like, no space-time "tensor".

That would be a mistaken interpretation.

2. IF a universe had dark energy (cosmological constant) but no "normal" matter flat space is a solution.

It is not the only such solution.

 

[ObjectivelyRational]

In other words, "tree falling in a forest". Pure philosophy. Unacceptable subject matter here.

My response was aimed at drawing the conversation back to something with a scientific basis.

Edit:
That would be a mistaken interpretation.

It is not the only such solution.

OK so the hypothetical is invalid, it is an exercise in unreality, the universe is not in fact a nothing, and asking a WHAT IF it were what it is not is of no use here.

Agreed.

Back to something with a scientific basis. The question for which Graeme wants an answer is a perfectly valid one (and need not invoke the philosophical hypothetical we both agree is inappropriate to science)

On the basis of all of the experimental, perceptual, empirical evidence of reality gathered by physicists to date, what do physicists know (or claim) about the status of space-time itself. It is a thing in itself or only a relationship/parameterization relevant to other things and their properties and interactions? I'm curious too.

 

[jbriggs444]

It is a thing in itself or only a relationship/parameterization relevant to other things

What difference does it make?

What experiment can you run whose result will depend on the answer to that question? If there is none then it's not a scientific question.

 

[ObjectivelyRational]

Every field, every particle, every bit of energy, all their complicated interactions and systems, molecules, things, everything in the universe has causal interactions with something else. Understanding this, the nature of things, all of it, is what knowledge and science and experiment is for. It is crucial to understand the nature of everything, and hence, if space-time is another thing that could have a nature, its something we need to investigate and understand.

To have within our equations a concept, "space-time" whose referent "might be" one of those very things about which its nature and effects and possible applications would be crucial to know VERSUS simply a background parameter for relating and describing the real things in the universe, is incredibly important distinction relevant to our knowledge.

The very question of whether something is one of the things we are studying and gaining knowledge about versus simply one of our tools to help us understand all other things (the real things) is not trivial nor unscientific. It is fundamental to the exercise and understanding of science.

If it were true that space-time in fact was something, then it would be possible to design an experiment which could show in some way the nature of space-time and its interaction with other things, for example could it be balled up and stored in a smaller space, could it be converted into energy or matter or anything else, can it be used to cause something or interact with other things. The number of possible experiments and indeed applications and technologies is endless... certainly that is if it IS a thing in itself.

 

[PeterDonis]

IF a universe had dark energy (cosmological constant) but no "normal" matter flat space is a solution.

It isn't.

It is not the only such solution.

With a nonzero cosmological constant and zero stress-energy tensor, flat spacetime is not a solution at all. The solution is de Sitter spacetime.

 

[jbriggs444]

Every field, every particle, every bit of energy, all their complicated interactions and systems, molecules, things, everything in the universe has causal interactions with something else. Understanding this, the nature of things, all of it, is what knowledge and science and experiment is for. It is crucial to understand the nature of everything, and hence, if space-time is another thing that could have a nature, its something we need to investigate and understand.

To have within our equations a concept, "space-time" whose referent "might be" one of those very things about which its nature and effects and possible applications would be crucial to know VERSUS simply a background parameter for relating and describing the real things in the universe, is incredibly important distinction relevant to our knowledge.

The very question of whether something is one of the things we are studying and gaining knowledge about versus simply one of our tools to help us understand all other things (the real things) is not trivial nor unscientific. It is fundamental to the exercise and understanding of science.

If it were true that space-time in fact was something, then it would be possible to design an experiment which could show in some way the nature of space-time and its interaction with other things, for example could it be balled up and stored in a smaller space, could it be converted into energy or matter or anything else, can it be used to cause something or interact with other things. The number of possible experiments and indeed applications and technologies is endless... certainly that is if it IS a thing in itself.

That's a lot of words. But nothing that describes an actual experiment to distinguish between "thing" and "not a thing"

 

[PeterDonis]

"vacuum solutions" which on a reading by a layperson (translated) means either

Neither of your "translations" are correct. "Vacuum solution" means a solution with a zero stress-energy tensor. Pretty much any GR textbook will tell you this.

 

[PeterDonis]

what do physicists know (or claim) about the status of space-time itself

GR models spacetime as a manifold with metric whose geometry is determined by the stress-energy tensor via the Einstein Field Equation.

There are various attempts to go beyond that (and beyond GR), but all of them are speculative at this point and we have no way to test them experimentally.

 

[PeterDonis]

Every field, every particle, every bit of energy, all their complicated interactions and systems, molecules, things, everything in the universe has causal interactions with something else.

The causal interaction of spacetime with "everything else", in GR, is captured by the Einstein Field Equation.

 

[ObjectivelyRational]

The causal interaction of spacetime with "everything else", in GR, is captured by the Einstein Field Equation.

I thought the Einstein Field Equation and the stress energy tensor provide a prediction of the causal consequences which would happen to a thing or things (at particular places and times .. i.e. if they were "there" "then") due to distributions of matter and energy i.e. other things. Is this incorrect?

 

[timmdeeg]

OK so the hypothetical is invalid, it is an exercise in unreality, the universe is not in fact a nothing, and asking a WHAT IF it were what it is not is of no use here.

It depends on what you mean saying "nothing". If you mean the absence of matter, radiation and $\Lambda$ and thus zero energy density then this model is known as 'empty universe' which is expanding linearly.

http://www.astro.ucla.edu/~wright/cosmo_02.htm see the spacetime diagrams
http://www.dark-cosmology.dk/~tamarad/papers/thesis_complete.pdf Chapter 4 The empty universe

 

[PeterDonis]

this model is known as 'empty universe' which is expanding linearly

Actually, this spacetime is flat Minkowski spacetime, just in unusual coordinates. (More precisely, it's the future light cone of the origin of Minkowski spacetime in unusual coordinates.)

 

[PeterDonis]

I thought the Einstein Field Equation and the stress energy tensor provide a prediction of the causal consequences which would happen to a thing or things (at particular places and times .. i.e. if they were "there" "then") due to distributions of matter and energy i.e. other things.

The Einstein Field Equation is just what I said: it expresses the causal relationship between stress-energy and the geometry of spacetime.

Given the geometry of spacetime, yes, you can predict the trajectories of objects; but the Einstein Field Equation does not do that, it just tells you the geometry of spacetime.

 

[ObjectivelyRational]

The Einstein Field Equation is just what I said: it expresses the causal relationship between stress-energy and the geometry of spacetime.

Given the geometry of spacetime, yes, you can predict the trajectories of objects; but the Einstein Field Equation does not do that, it just tells you the geometry of spacetime.

Are there any observable effects caused by "the geometry of spacetime" besides causal effects on things (other than spacetime itself)?
Are there any observable effects on "the geometry of spacetime" caused by anything other than the energy mass distribution of things (other than spacetime itself)?

 

[ObjectivelyRational]

Graeme, I think you can take it from here.

Just a final parting thought:

If everything observable about the nature of "the geometry of spacetime" consists entirely in the direct linking of the relationship between causes, (specifically, mass-energy distribution) and their effects, (specifically, effects upon observable things in the universe), then "the geometry of spacetime" (using ocam's razor) is best thought of as only a mathematical device describing the relationship of cause and effect between real objects, which objects in the real world have a direct cause and effect relationship, there being no need to assume a reified spacetime also existing in and of itself.

Good luck Graeme!

 

[PeterDonis]

Are there any observable effects caused by "the geometry of spacetime" besides causal effects on things (other than spacetime itself)?

What does this even mean? What observable effects would you expect to see that aren't causal effects?

Are there any observable effects on "the geometry of spacetime" caused by anything other than the energy mass distribution of things (other than spacetime itself)?

I'm not sure what this means either. If you mean, is the stress-energy tensor the only source of gravity, per the Einstein Field Equation, then yes.

 

[Graeme M]

Thanks... I think! I will read the various linked references and see if that helps. Of course the fact that I don't know anything of relativity (beyond the usual basic grasp) or of advanced mathematics means I am up against it!

Some of the replies talk of "space-time", again in terms that suggest it is a thing. So I think a part of my conceptual difficulty is grasping what this means in a physical sense. I thought space-time referred to how the trajectories of phycial entities is affected over time by gravity (that is, for example, curved space describes how objects trajectories "curve" due to gravitational forces), rather than referring to anything through which objects move. The Einsteinian Field Effect equations referred to earlier seems to my naive eye to represent that physical fact by way of a mathematical abstraction that can be used to predict this effect for different physical scenarios (naive eye, I said - don't let the fact that I used the words Einsteinian Field Effect equation suggest I have any idea what I am talking about!).

If that were the case, then wouldn't we be describing not a place or a location but a relative motion that depends entirely for its force upon physical entities? And if space is not a physical entity, which to my mind it isn't, how can the equation, or the terms in the equation, describe anything about it?

I suppose I am not being clear here. I just am struggling to see what space, or space-time, actually IS. Or how its expansion has any effect on anything, again because to me, that can't happen if it doesn't have physical form (space is distance, not a physical and hence describable thing). I can totally understand that space and space-time, defined mathematically, can describe the behaviour of physical entities, but that's not space or space-time per se. When I think about this, it seems to me that objects, say galaxies, travel "through" fields (say gravity), but not through space. Their motions only exist relative to each other, which we can only describe in terms of some coordinate system that references the entities concerned. Are we really talking about a thing or rather relationships?

Sigh... what am I missing or is this just too hard for me?

 

[PeterDonis]

I thought space-time referred to how the trajectories of phycial entities is affected over time by gravity (that is, for example, curved space describes how objects trajectories "curve" due to gravitational forces), rather than referring to anything through which objects move.

You talk as if these are two different, distinct ways things could be. They're not. They're just two different ways of describing the same physics.

don't let the fact that I used the words Einsteinian Field Effect equation suggest I have any idea what I am talking about

Then rather than using the words, you should first learn what they actually mean, so you will know what you are talking about when you use them. Trying to ask the kinds of questions you are asking without that background knowledge is not a good strategy. When you have that background knowledge, many questions you think you have now will either answer themselves, or simply vanish when you understand that you were asking them only because you have the wrong conceptual foundation.

I just am struggling to see what space, or space-time, actually IS.

Physics doesn't tell you what anything "actually is". If you're worried about that, Dr. Tyree's philosophy class is right down the hall.

I can totally understand that space and space-time, defined mathematically, can describe the behaviour of physical entities, but that's not space or space-time per se.

Why not?

 

[rede96]

I just am struggling to see what space, or space-time, actually IS. Or how its expansion has any effect on anything, again because to me, that can't happen if it doesn't have physical form (space is distance, not a physical and hence describable thing).

Like you I am just an interested layman with no background in math or physics. And I struggled with the same questions. The conclusions I came to, and this is just my way of looking at it, is that space is just that... nothing. There is no experiment we can perform to measure it as a physical thing. It has no physical properties we can measure. And if space can't be a physical thing, then how can space-time? So personally, for all intents and purposes, I just ignore it when thinking about the physical aspects of the universe.

So as such, it is meaningless to speak of 'space' expanding. How I like to view the universe and expansion is that in general the universe is made up of a lot of fields (QFT) of which some are responsible for matter of course. We know through observation that distant objects are all moving away from each other (with the exception of those locally bound by gravity.) And as the rate at which they are moving away from each other increases with distance, there must be some 'force' causing this. It can't be just natural momentum left over from the big bang.

So how I like to think of it (based on some reading / cosmology lectures) is that maybe there is some field that interacts with the other fields, (e.g. like the fields responsible for matter) in such a way that as this field expands, it causes other fields / objects that aren't bound by some local force, to move apart. And this would lead to the more distant objects moving away at an accelerating rate.

Like I said, I am not asserting this is actually what is going on. But personally I found it a good way to conceptually think of expansion that fits with what we see in observation and has some basis in physics. (E.g. QFT, the FRW equations, inflation theory etc.) And when people talk about 'space' expanding this is how I like to view it conceptually.

 

[timmdeeg]

We know through observation that distant objects are all moving away from each other (with the exception of those locally bound by gravity.) And as the rate at which they are moving away from each other increases with distance, there must be some 'force' causing this. It can't be just natural momentum left over from the big bang.

Here I disagree. There is no force acting due to which the galaxies are moving away from each other, they are in free fall. Matter is moving away from each other since the big bang and what we see today is just the continuation of that.

 

[jbriggs444]

is that space is just that... nothing

We have a model. Experiment agrees with the model. The model features something called space-time. That's not "nothing".

Requiring that it be a "physical thing" is superfluous.

 

[rede96]

Here I disagree. There is no force acting due to which the galaxies are moving away from each other, they are in free fall.

Can you define what being in “free fall” means?

 

[rede96]

Requiring that it be a "physical thing" is superfluous.

. Yes I agree.