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

[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.

 

[timmdeeg]

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

If an object is in free fall its trajectory is a 'geodesic'. In this case the object doesn't feel a force acting on it. This is true for Galaxies, planets orbiting around a star and apples falling down to Earth.

 

[PeterDonis]

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.)

As @timdeeg has pointed out, this does not fit with observation because your model predicts that galaxies should feel a force, and they don't.

The reason the concept of "space expanding" doesn't make sense is that "space" is frame-dependent, and the laws of physics are frame-independent. So whatever is going on in the universe, "space expanding" can't be a good description of it.

 

[rede96]

If an object is in free fall its trajectory is a 'geodesic'. In this case the object doesn't feel a force acting on it. This is true for Galaxies, planets orbiting around a star and apples falling down to Earth.

If I understood you correctly your saying that expansion is due to space time curvature. Correct?

 

[rede96]

As @timdeeg has pointed out, this does not fit with observation because your model predicts that galaxies should feel a force, and they don't

I was using ‘force’ in very loose terms just to indicate there was something going on to cause objects to move apart. I wasn’t implying they felt any force from acceleration. The way I thought about it wouldn’t be any different.

 

[PeterDonis]

I was using ‘force’ in very loose terms just to indicate there was something going on to cause objects to move apart.

Then you shouldn't use the word "force", because it will only lead to confusion. Not just for others, but for you. It leads you to think that "there was something going on to cause objects to move apart". There isn't. There is just the geometry of spacetime.

To illustrate what I mean, consider tidal gravity: two objects free-falling radially above a gravitating mass (like the Earth), starting from slightly different altitudes. These objects will move apart as they fall. Is there "something going on" that causes this? If so, what is this "something"? If not, how is this case different from the expansion of the universe?

 

[timmdeeg]

If I understood you correctly your saying that expansion is due to space time curvature. Correct?

Well, to know what geodesic means is not the clue to have a notion of what expansion means. But you can imagine two neighboring geodesics describing the trajectories of two objects. If the spacetime is curved then their geodesics accelerate relative to each other, in the case of an expanding universe they accelerate away from each other. Whereas if the spacetime is flat their relative acceleration is zero (which doesn't exclude of course that these particles move relative to each other with constant speed).

 

[PeterDonis]

If the spacetime is curved then their geodesics accelerate relative to each other, in the case of an expanding universe they accelerate away from each other.

This is not quite right. In a matter-dominated universe the expansion is decelerating, and the geodesics in question (the worldlines of comoving objects) are converging, not diverging. But the universe is still expanding.

The correct definition of "expanding" for the universe is that the congruence of worldlines of comoving objects has a positive expansion scalar. Unfortunately, that's already getting beyond the "B" level of this thread. But you can find more information here:

https://en.wikipedia.org/wiki/Congr...atical_decomposition_of_a_timelike_congruence

 

[PeterDonis]

your saying that expansion is due to space time curvature.

The expansion is due to the positive expansion scalar of the congruence of comoving worldlines (see my previous post). But the fact that such a congruence exists and has the properties it has (not just positive expansion, but every comoving observer sees the universe as homogeneous and isotropic) is due to the particular geometry of the spacetime in question (FRW spacetime).

 

[timmdeeg]

This is not quite right. In a matter-dominated universe the expansion is decelerating, and the geodesics in question (the worldlines of comoving objects) are converging, not diverging. But the universe is still expanding.

Ah yes, it follows from the second Friedmann equation that the second derivative of the scale factor is negative in this case (what I didn’t take into account). Thanks for correcting and for the link.

 

[Dan White]

This is an old thread but my understanding is that there is an idea from quantum physics that says that empty space may be teeming with activity at a quantum level. I thought this might be of interest to the OP if he is still around.