Isn't space expansion logically required?

[Gerinski]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

We know that spacetime is one single entity, there is no space AND time, but just spacetime.

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?

 

[phinds]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

We know that spacetime is one single entity, there is no space AND time, but just spacetime.

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?

Since space is just geometry, what's your point? Space-time grows and things on a cosmological scale get farther apart but that's just geometry and since space isn't "something", it's just distance, there's no need to "create" more of it.

 

[Gerinski]

Since space is just geometry, what's your point? Space-time grows and things on a cosmological scale get farther apart but that's just geometry and since space isn't "something", it's just distance, there's no need to "create" more of it.

Space is more than just geometry, space is where fields reside. Are you saying that the space fields occupied 5 billion years ago had the same dimensional extension as the space which fields occupy at the present?

 

[phinds]

Space is more than just geometry, space is where fields reside. Are you saying that the space fields occupied 5 billion years ago had the same dimensional extension as the space which fields occupy at the present?

No, I'm saying space is just the framework in which the fields and matter exist. Space isn't a "thing", it's just dimensions. Stuff, on a cosmological scale, gets father apart but that doesn't mean more space is "created" in between the stuff ... it's just stuff changing position in the framework

EDIT: I see that I'm just repeating myself, so I guess I'm not explaining it in a way that is helpful to you. I'd suggest a forum search for something like "space time geometry" and/or "metric expansion" and check out the links at the bottom of this page.

 

[Jimster41]

I'm a bit taken aback by the statement space-time is just dimensions, like coordinates we wrote on stuff, and so, not really a thing.

Doesn't GR show that really geometry is the fundamental thing, in terms of our experience. I mean to say the traveling twin just had some run-ins with some of his or someone else's, "notions of geometry"... doesn't help his now older brother. And just because our approach of writing coordinates on real stuff to figure out what the principles of how stuff behaves are, ends up causing fits of confusion doesn't means it's just our coordinate writing that is confused. It actually means the geometry of our world, is a thing, and it is weird.

 

[Gerinski]

Stuff, on a cosmological scale, gets father apart but that doesn't mean more space is "created" in between the stuff

Well I guess you are one the few ones believing that space does not expand. If you say that stuff gets farther apart but space does not expand (no new space gets created), then you believe that space does truly stretch. At any rate, 1 Km eventually becomes 2 Km, no matter which way you think of it. I can agree that spacetime is only a set of relationships governing how spacetime events can relate to each other, but the properties of these relationships hold true, call it whatever you like. The magnitude of spacetime dimensions grow larger with time, you like it or not.

 

[phinds]

Well I guess you are one the few ones believing that space does not expand. If you say that stuff gets farther apart but space does not expand (no new space gets created), then you believe that space does truly stretch. At any rate, 1 Km eventually becomes 2 Km, no matter which way you think of it. I can agree that spacetime is only a set of relationships governing how spacetime events can relate to each other, but the properties of these relationships hold true, call it whatever you like. The magnitude of spacetime dimensions grow larger with time, you like it or not.

Yes, the magnitude of the dimensions get larger, as I have said all along, but no space is "created", things just get farther apart in the framework of space and in that sense there is definitely an expansion but this "fabric" and "stretching" stuff is crap propagated by pop-science.

 

[rootone]

I think the problem is in thinking of space as a substance.
Space is not any kind of substance, so it can't be 'created'.
Space can contain things which are substantial, ie things which have a mass.
Space can also contain various kinds of fields, (energy essentially).
Space itself though is not a form of matter or energy, It is Phinds pointed out, a 'framework' we use to describe things like distances, velocity, position and so on.

 

[PeroK]

I think the problem is in thinking of space as a substance.
Space is not any kind of substance, so it can't be 'created'.
Space can contain things which are substantial, ie things which have a mass.
Space can also contain various kinds of fields, (energy essentially).
Space itself though is not a form of matter or energy, It is Phinds pointed out, a 'framework' we use to describe things like distances, velocity, position and so on.

One can create confusion, yet confusion isn't a substance.

 

[rootone]

Sure, the term 'created' can be applied to abstract entities, but I think the OP is envisiging that expansion of space implies that something physical is being created.

 

[PeroK]

Sure, the term 'created' can be applied to abstract entities, but I think the OP is envisiging that expansion of space implies that something physical is being created.

One could argue that distance is physical rather than abstract.

 

[rootone]

I won't disagree with that.
Yes you can say that expansion means distance is being created, it's a valid statement, though it sounds like odd way of saying 'distances are increasing'
I had the impression though I could be wrong, that the OP envisages 'Stuff; that is either a form of matter or energy coming newly into existence..

 

[Jimster41]

Spacetime is just a container. It's geometry labels, It's measure merely abstract... Unbelievable.

By what thing do you move and age? Something else, something real?

Quit calculating for a second, and look around...it's not just puzzles, this is the stuff the world is made of.

 

[rootone]

Yes spacetime is a geometric framework, (well actually there a number of geometric frameworks that can describe space.)
Yes I am moving and ageing within the context of the framework, How is that unbelievable?

 

[Jimster41]

OMG.

Honestly though I love nerds. I aspire to be a real one.

 

[phinds]

Spacetime is just a container. It's geometry labels, It's measure merely abstract... Unbelievable.

By what thing do you move and age? Something else, something real?

Quit calculating for a second, and look around...it's not just puzzles, this is the stuff the world is made of.

No, it is the framework in which exists "the stuff the world is made of". 99+% of atoms are space but that doesn't make space "stuff", it's just distance. Quarks and electrons and photons, etc, are stuff.

 

[Chalnoth]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

Nope. There is no such requirement. It is perfectly possible for space to either expand or contract. Doing neither is only possible in perfectly empty space with no cosmological constant (if there's any matter or other energy such as photons, then a universe that is neither expanding nor contracting is unstable: it will very soon start doing one or the other).

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

This isn't an accurate representation of spacetime in General Relativity. In General Relativity, the entire past and future is part of the same manifold. This is more or less required by the fact that different observers disagree on what the definition of "now" is.

If I'm standing here on Earth in the Milky way, and I have a clock that reads 12:01 today, and another person is standing on some other planet off in the Andromeda galaxy but that clock reads 12:03 on the same day according to one observer, then another observer could look at both clocks and see the same time, or see that my clock is faster than than the clock of the person on Andromeda. There's no way to say who has the right answer.

To bring it all together, if we can't agree on "now", then we can't agree on what times at different locations are in the future. And if it's not possible to agree upon which times at different locations are in the future, it makes no sense to say that the future is newly-created.

 

[Jimster41]

No, it is the framework in which exists "the stuff the world is made of". 99+% of atoms are space but that doesn't make space "stuff", it's just distance. Quarks and electrons and photons, etc, are stuff.

Not so fast friend. I know of what you speak but I'm far enough into Susskind's QM for dummies to know that just whether or not matter and energy, or geometry comes first, is not a matter entirely settled.

 

[phinds]

Not so fast friend. I know of what you speak but I'm far enough into Susskind's QM for dummies to know that just whether or not matter and energy, or geometry comes first, is not a matter entirely settled.

I'm not following you. What do you mean "comes first" and why does it matter? The discussion is about whether or not space has a material existence.

 

[Jimster41]

Nope. There is no such requirement. It is perfectly possible for space to either expand or contract. Doing neither is only possible in perfectly empty space with no cosmological constant (if there's any matter or other energy such as photons, then a universe that is neither expanding nor contracting is unstable: it will very soon start doing one or the other).This isn't an accurate representation of spacetime in General Relativity. In General Relativity, the entire past and future is part of the same manifold. This is more or less required by the fact that different observers disagree on what the definition of "now" is.

If I'm standing here on Earth in the Milky way, and I have a clock that reads 12:01 today, and another person is standing on some other planet off in the Andromeda galaxy but that clock reads 12:03 on the same day according to one observer, then another observer could look at both clocks and see the same time, or see that my clock is faster than than the clock of the person on Andromeda. There's no way to say who has the right answer.

To bring it all together, if we can't agree on "now", then we can't agree on what times at different locations are in the future. And if it's not possible to agree upon which times at different locations are in the future, it makes no sense to say that the future is newly-created.

And yet the overall a(t) of the universe, it's expansive shape, has to also be accounted for when considering the relativity of simultaneity right?

 

[Jimster41]

I'm not following you. What do you mean "comes first" and why does it matter? The discussion is about whether or not space has a material existence.

Well if matter and energy are emergent properties of space-time geometry (there are papers over in BTSM) then the question of which is a more capable primary description is open. It can be argued, I believe, that geometry may be "primary". In other words the primary "material" is quanta of space-time: "Vacuum state" properties, and algorithms.

Clearly that's 'out there' thinking, but it's not mine, I'm not making it up.
Aren't 'strings' an example? Aren't they a fundamental geometric object, from which matter and energy, and vacuum are made?

Bottom line, to my thinking re the material aspect of raw space-time. One twin gets old, one doesn't. They are both made of matter, but we describe what happened to them in terms of the geometry of space-time their paths took. And what happens to them is pretty material if you ask me. So what's the difference between that and the case of expansion? In neither case do we really know what is going on QM wise, between matter and changing geometry. The temperature or energy density of the CMB decreased because someone forgot to move our temperature framework labels, or because "space grew"? Seems fair to say "space grew", and mean it.

 

[rootone]

In neither case do we really know what is going on QM wise, between matter and changing geometry

You don't need QM to understand that if you have an expanding volume of 'stuff', then less 'stuff' exists in one cubic metre of it.
Less stuff means less energy, so not surprisingly the temperature is less.

 

[Jimster41]

You don't need QM to understand that if you have an expanding volume of 'stuff', then less 'stuff' exists in one cubic metre of it.
Less stuff means less energy, so not surprisingly the temperature is less.

And yet if you ask "how" and then how that, then how that... it's turtles all the way down. And so I would contend that, we actually do need QM to truly understand any thing...fully.

Not that we can't make iPhones and cars and the internet and rockets and pizzas and gravity boots and blow ourselves up without truly understanding everything fully. Though we probably will need QM for the gravity boots...

 

[phinds]

Well if matter and energy are emergent properties of space-time geometry (there are papers over in BTSM) then the question of which is a more capable primary description is open. It can be argued, I believe, that geometry may be "primary". In other words the primary "material" is quanta of space-time: "Vacuum state" properties, and algorithms.

Clearly that's 'out there' thinking, but it's not mine, I'm not making it up.
Aren't 'strings' an example? Aren't they a fundamental geometric object, from which matter and energy, and vacuum are made?

Bottom line, to my thinking re the material aspect of raw space-time. One twin gets old, one doesn't. They are both made of matter, but we describe what happened to them in terms of the geometry of space-time their paths took. And what happens to them is pretty material if you ask me. So what's the difference between that and the case of expansion? In neither case do we really know what is going on QM wise, between matter and changing geometry. The temperature or energy density of the CMB decreased because someone forgot to move our temperature framework labels, or because "space grew"? Seems fair to say "space grew", and mean it.

I don't follow your logic at all, but I do get that you seem to believe space "grows" in the same sense that flowers grow and I completely disagree. I think things just get farther apart and I don't see how any of the above contravenes that.

 

[rootone]

And yet if you ask "how" and then how that, then how that... it's turtles all the way down.

Unless string theory or something like that proves to be a real 'theory of everything', then you are right.
Science is in the meanwhile a method of uncovering the layers of turtles and discovering what the nature of the turtles are in each layer.
This sometimes produces useful new technologies.
(I would definitely consider anti gravity boots to be useful technology, but the operating manual might be hard to understand)

 

[tom aaron]

If spacetime is a single entity, shouldn't it be simply logical that space expands?

We know that spacetime is one single entity, there is no space AND time, but just spacetime.

As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?

What exists has to exist. It exists because it is the rational way that matter and energy act. Not that we understand how it all works. What you are describing are observations (some debatable). Everything is the way it is because that is the reality of existence.

We don't actually 'know' much of anything other than models of space time that sort of act the way they do. Until we get some handle on what gravity is, what dark energy is, some relationship between QM and General Relativity...we don't actually know much of anything.

Someone mention Leonard Susskind...perhaps it's all some Holographic matrix.

 

[Jimster41]

Unless string theory or something like that proves to be a real 'theory of everything', then you are right.
Science is in the meanwhile a method of uncovering the layers of turtles and discovering what the nature of the turtles are in each layer.
This sometimes produces useful new technologies.
(I would definitely consider anti gravity boots to be useful technology, but the operating manual might be hard to understand)

Hey man you want to start a gravity boots thread, I'm there.

 

[rootone]

Hey man you want to start a gravity boots thread, I'm there.

Well anti/gravity boots was your idea, but I am sure that if either of us started such a thread it would consumed by the local black hole within an instant.

 

[Gerinski]

This is going to places I did not intend (although surely interesting!). I think what I asked was pretty simple.

For my question, it doesn't matter whether spacetime is 'physical' or purely informational, whether it is 'material' or not. It doesn't matter if the future exists already or not. Neither space not time needs to 'be created'. All that is rather irrelevant to my basic question.

I just said, spacetime is believed to be one single entity. At the event of 'me now', there is more time extension since the Big Bang than there was at the event when the solar system formed, and even more than at the event when the first galaxies formed. I don't care whether also the future 'already exists'. The extension of the time dimension is larger 'in my now' than in my past, and smaller now than in the future.

Similarly, the spatial extension of spacetime is larger in my here and now than it was in the past, and it will be even larger in the future. It doesn't matter whether you think it gets created or it has always been there. I just say that its dimensional magnitude extends in both the time and the spatial dimensions. As it gets older it gets bigger.

So now the question was, is it not kind of logical, given that it is a single entity spacetime?

If you think not so, it means that you defend that space is completely arbitrary from time. As the extension of the time dimension gets larger, space can get smaller or larger or stay the same, there is no relationship whatsoever between space and time magnitudes. If so that's fine, I guess this is the orthodox viewpoint, but I was just wondering it there could be some relationship between time extension and spatial extension, since they are both aspects of the same entity.

 

[PeroK]

This is going to places I did not intend (although surely interesting!). I think what I asked was pretty simple.

For my question, it doesn't matter whether spacetime is 'physical' or purely informational, whether it is 'material' or not. It doesn't matter if the future exists already or not. Neither space not time needs to 'be created'. All that is rather irrelevant to my basic question.

I just said, spacetime is believed to be one single entity. At the event of 'me now', there is more time extension since the Big Bang than there was at the event when the solar system formed, and even more than at the event when the first galaxies formed. I don't care whether also the future 'already exists'. The extension of the time dimension is larger 'in my now' than in my past, and smaller now than in the future.

Similarly, the spatial extension of spacetime is larger in my here and now than it was in the past, and it will be even larger in the future. It doesn't matter whether you think it gets created or it has always been there. I just say that its dimensional magnitude extends in both the time and the spatial dimensions. As it gets older it gets bigger.

So now the question was, is it not kind of logical, given that it is a single entity spacetime?

If you think not so, it means that you defend that space is completely arbitrary from time. As the extension of the time dimension gets larger, space can get smaller or larger or stay the same, there is no relationship whatsoever between space and time magnitudes. If so that's fine, I guess this is the orthodox viewpoint, but I was just wondering it there could be some relationship between time extension and spatial extension, since they are both aspects of the same entity.

"Logical" means that one thing necessarily follows from another. There is no logical imperative that demands that space and time are equivalent in any or all respects.

If it were logical that space must somehow expand in the same way as time, then you would be able to take the conclusion further:

1) Each spatial dimension would "pass" in the way time passes.

2) Spatial dimensions would pass in one direction only: from the past to the future.

3) Or, time would be a spatial-like dimension, expanding in both directions - but not "passing".

4) It might also be logical that there be three time dimensions.

The expansion of space and the passing of time are very different phenomena. In fact, if you take the "flat" spacetime metric of special relativity, then the distance between two points in spacetime is given by:

$d^2 = (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 - c^2 (\Delta t)^2$

In any case, in the theory of spacetime, there is a very clear and fundamental distinction between the three spatial dimensions and the time dimension, as you can see from the spacetime metric above.

 

[Jimster41]

"Logical" means that one thing necessarily follows from another. There is no logical imperative that demands that space and time are equivalent in any or all respects.

If it were logical that space must somehow expand in the same way as time, then you would be able to take the conclusion further:

1) Each spatial dimension would "pass" in the way time passes.

2) Spatial dimensions would pass in one direction only: from the past to the future.

3) Or, time would be a spatial-like dimension, expanding in both directions - but not "passing".

4) It might also be logical that there be three time dimensions.

The expansion of space and the passing of time are very different phenomena. In fact, if you take the "flat" spacetime metric of special relativity, then the distance between two points in spacetime is given by:

$d^2 = (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 - c^2 (\Delta t)^2$

In any case, in the theory of spacetime, there is a very clear and fundamental distinction between the three spatial dimensions and the time dimension, as you can see from the spacetime metric above.

And yet doesn't that very metric describe a conservation law that constrains their relation to one another, in a real sense asserting they are "part of the same entity", whatever system it is that enforces that complicated symmetry?

Also, doesn't the standard FLRW model of the universe define distance as a function of time?

$-{ c }^{ 2 }d{ \tau }^{ 2 }=-cdt^{ 2 }+a{ (t) }^{ 2 }d{ \Sigma }^{ 2 }$

Marcus' threads here on that model and others like it are really good at giving one (me at least) a sort of shocked sensation about the connection between space and time, when describing the spatial universe as having a history we can know...

My sense of what the OP is noticing is that they are deeply connected, inextricably tied, an in a real sense aspects of the same entity. IMHO this is a deep and exciting revelation that should be noticed. It certainly defines a contrast between walking around looking at clocks and rulers, when you don't know anything about Relativity, and looking at them when you do.

I'm not trying to say they are "the same", just that getting a bit stupefied by their apparently deep connection, is a good place surely, from which to ask, what is the difference between them? But then I like that feeling of being stupefied and shocked by relationships in physics. It makes me remember them.

Not to imply you are stupefied @Gerinski. Sort of the opposite.

 

[Chalnoth]

And yet the overall a(t) of the universe, it's expansive shape, has to also be accounted for when considering the relativity of simultaneity right?

I don't understand what you're saying. You need to understand the geometry between observer A and observer B to translate one time at observer A with another time at observer B, but there is still no single unambiguous way to translate between two observers at different locations and/or traveling at different velocities.

 

[Jimster41]

I think I understand. But maybe not.
You have to do it. But there is no specific way that is more correct in terms of reference frame.

I think my point is that just because which way you run the labels doesn't matter doesn't mean the "framework" is invented. Rather I think it highlights just how physically real the thing the "framework" describes is. It is always required for perspective but agnostic as to how any observer defines it (coordinate label-wise). Which seems pretty... 'wow' to me. It seems as easy to be impressed by the universality of the requirement for some "metric" that connects space and time, as it is to be unimpressed somehow by that same fact because it doesn't care how the coordinates of the metric are chosen.

But isn't it correct to say that the notion and operation of that framework (a framework required, but agnostic with respect to coordinate labeling) is actually constrained... to look something like the GR field eq? In other words it has to represent exactly the constraint system mass places on space and time. Any smart alien would have to have discovered something that notices this. The important thing is not the flexibility of coordinate perspective, but rather the universality and specificity of the constraints it places on the relationship between things (like the traveling twins).

Question I've wondered about: Is it correct that the FRLW use that $t$ to mean "proper time of an observer comoving with the CMB". So it picked the CMB as the reference frame?

 

[Chalnoth]

Yes, the reference frame for FLRW is the CMB rest frame.

 

[SeanS6]

I presume that the OP meant that the state space of the universe is increasing. That is the set of possible configurations everything in universe could potentially be in is increasing. That seems like a reasonable idea.