I What is the Cause of Space Expansion?

[KurtLudwig]

Does space expand only at the outer edges of the universe? What is the mechanism that creates more space particles? Is the huge volume of outer space pulling our universe apart? Please enlighten or comment. Kurt Ludwig

 

[jbriggs444]

Does space expand only at the outer edges of the universe? What is the mechanism that creates more space particles? Is the huge volume of outer space pulling our universe apart? Please enlighten or comment. Kurt Ludwig

You should Google "metric expansion".

No, it does not expand only at the outer edges. There is no indication that the universe has any edges. There is no such thing as a "space particle". No, there is no tension from empty space sucking the universe apart and thereby creating more... empty space.

 

[rootone]

The whole Universe is the entirety of space and time, there is nothing outside which it expands in to.
Our observable part of the Universe is expanding.
Other parts of the whole Universe not observable to us may also be expanding, I think the jury is still out on that one.

 

[kimbyd]

Does space expand only at the outer edges of the universe? What is the mechanism that creates more space particles? Is the huge volume of outer space pulling our universe apart? Please enlighten or comment. Kurt Ludwig

To add a little to jbriggs444's response, the expansion of space means that the distances between objects in the universe are getting larger. Galaxies today are further from one another than they were billions of years ago.

 

[Delta2]

To add a little to jbriggs444's response, the expansion of space means that the distances between objects in the universe are getting larger. Galaxies today are further from one another than they were billions of years ago.

Galaxies are getting further from one another due to metric expansion of space, due to dark energy, or due to both (or both are the same thing?)

 

[windy miller]

The whole Universe is the entirety of space and time, there is nothing outside which it expands in to.
Our observable part of the Universe is expanding.
Other parts of the whole Universe not observable to us may also be expanding, I think the jury is still out on that one.

There seems to be something problematic about this answer. you have defined "the universe" as the entirety of space and time. So of course you get the answer the the universe isn't expanding into some other space/time. But this just seems like a tautology, only telling us about how words have been defined and not about any objective reality. One could define the universe in a different way and indeed cosmologists that speak of a multiverse must define it in a different terms if the multiverse is to make sense. of course there are different types of multiverses that have been considered, for example the multiverse of eternal inflation or the suggestions that our universe is embedded in higher dimensional membrane. Who knows maybe our space time is embedded in some other object that we have not even thought of yet. i think its fine to say we don't require that the universe be expanding into anything else, but that's not the same as saying with confidence that it isn't.

 

[PeroK]

There seems to be something problematic about this answer. you have defined "the universe" as the entirety of space and time. So of course you get the answer the the universe isn't expanding into some other space/time. But this just seems like a tautology, only telling us about how words have been defined and not about any objective reality. One could define the universe in a different way and indeed cosmologists that speak of a multiverse must define it in a different terms if the multiverse is to make sense. of course there are different types of multiverses that have been considered, for example the multiverse of eternal inflation or the suggestions that our universe is embedded in higher dimensional membrane. Who knows maybe our space time is embedded in some other object that we have not even thought of yet. i think its fine to say we don't require that the universe be expanding into anything else, but that's not the same as saying with confidence that it isn't.

You're misunderstanding the concept of metric expansion. As time passes, the distance between points in space increases. There are no new points in space. There is no need to introduce a higher dimensional universe which contains our universe - although mathematically, of course, you could do that. But, there is no need to do that to completely explain and model the expanding universe.

Whether the universe is part of a greater multiverse is a different issue.

 

[kimbyd]

Galaxies are getting further from one another due to metric expansion of space, due to dark energy, or due to both (or both are the same thing?)

Metric expansion. Dark energy impacts how the expansion changes over time, but isn't a root cause of the expansion in any reasonable sense of the word.

 

[PeterDonis]

you have defined "the universe" as the entirety of space and time

No, he's defining "the universe" as what our current best-fit cosmological model describes. In that model, the universe is not expanding into any other pre-existing space, and there is nothing else besides the spacetime that appears in the model.

That model in itself does not prove that there is nothing else besides the spacetime that appears in the model; but we have no evidence for any such thing. Our current best-fit model is that because it best fits all the available evidence we have. If we ever get evidence that indicates something else is there, we can re-evaluate our model. Unless and until that happens, all the talk about multiverses and everything else is speculation--interesting speculation, but still speculation.

i think its fine to say we don't require that the universe be expanding into anything else, but that's not the same as saying with confidence that it isn't.

You have it backwards. We don't speculate that there might be something else and then demand that someone prove it wrong. We build the model that best fits the evidence we have. Then we continue to collect evidence and see what happens. Our current evidence gives us no indication that there is anything else, so that's what our current best-fit model says.

 

[windy miller]

Im sorry but I don't agree with you. The evidential demand is on the person making the claim not on the person who refuses to accept it. I am not claiming there is something beyond our familiar space-time. The multiverse or M theory may or may not be true,. But if you claim there isn't then then you need to show the evidence Saying the models doesn't require it isn't the same as saying as was a said "there is nothing outside which it expands in to.". That is a positive claim that requires evidence to support it. There is no evidence for life on Europa but that doesn't make the statement " there is no life on Eurpoa" a justifiable one.

 

[kimbyd]

Im sorry but I don't agree with you. The evidential demand is on the person making the claim not on the person who refuses to accept it. I am not claiming there is something beyond our familiar space-time. The multiverse or M theory may or may not be true,. But if you claim there isn't then then you need to show the evidence Saying the models doesn't require it isn't the same as saying as was a said "there is nothing outside which it expands in to.". That is a positive claim that requires evidence to support it. There is no evidence for life on Europa but that doesn't make the statement " there is no life on Eurpoa" a justifiable one.

What you're proposing is much more than the multiverse or M theory.

When you make the statement that space is expanding into something else, you're making a particular assumption: that there's a notion of absolute space, and that our universe is sort of a pocket within that absolute space. It turns out that mathematically, that doesn't make any sense at all.

General Relativity's key insight into the geometry of space-time is that you can describe curved space-time without reference to anything external.

Think of it this way: imagine the surface of a sphere. This is a curved, two-dimensional surface. We visualize it, however, as a structure in three dimensions. The key insight that General Relativity provides us is that you can fully and completely describe the shape of the sphere (including its curvature) in two dimensions alone. The fact that the spherical surface can be visualized in three dimensions is interesting, but not necessary.

So, this leaves the question: is the curvature described by General Relativity that explains the nature of space-time that we observe only 4-dimensional (3 space dimensions, one time), or is it embedded in a higher-dimensional structure like the sphere's surface can be embedded in three dimensions? Working through this carefully leads to the simple conclusion that you simply can't make the picture of curved 4-d space-time embedded in higher-dimensional space-time work in a sensible manner (caveat: this doesn't mean that higher dimensions are impossible, but the large higher dimensions required for this tend to leave observable effects).

Examining the math carefully, we are forced to conclude that space-time is fundamentally a dynamical system that isn't embedded in any sort of "super space-time".

 

[PeterDonis]

if you claim there isn't then then you need to show the evidence

I didn't claim "there isn't". I said there is no evidence for it. Which is precisely what you are asking for: evidence. So I already have given you what you say you want. Did you even read my previous posts?

There is no evidence for life on Europa but that doesn't make the statement " there is no life on Eurpoa" a justifiable one.

Not if you intend the claim as a statement of certainty. But science does not make statements of certainty. It only makes statements about what our best current evidence says. Having no evidence for life on Europa does make the statement "we have no evidence for life on Europa, therefore our best current model does not include it" justifiable. And that's precisely the kind of statement I made about our best current models of the universe. So again, did you even read my previous posts?

 

[windy miller]

I didn't claim "there isn't". I said there is no evidence for it. Which is precisely what you are asking for: evidence. So I already have given you what you say you want. Did you even read my previous posts?
Not if you intend the claim as a statement of certainty. But science does not make statements of certainty. It only makes statements about what our best current evidence says. Having no evidence for life on Europa does make the statement "we have no evidence for life on Europa, therefore our best current model does not include it" justifiable. And that's precisely the kind of statement I made about our best current models of the universe. So again, did you even read my previous posts?

Yes Peter i read your post. Did you read mine? Science may not make statements of certainty , but individual scientists and forum members do and they are not always justified in making them. I was responding to this comment ""there is nothing outside which it expands in to.". Where is there any hint of doubt in this statement? It could have been phrased as we have nor reason to think there is anything its expanding input, or our current modes don't require it to expand into , or we have no evidence of something its expanding into, something like that I would have no objections to these comments . But that isn't what was said. Please read other peoples posts before you accuse them of not reading yours,

 

[PeterDonis]

I was responding to this comment ""there is nothing outside which it expands in to.".

Which I clarified: that statement, as I understand it, refers to our best current model and is not a claim of certainty. If you're going to be fixated on something @rootone said and ignore my clarification, you should quote him in all of your posts. When you quote me, I assume you're trying to respond to what I said, not to what @rootone said, and your statements are not responsive to what I said.

Please read other peoples posts before you accuse them of not reading yours,

Please quote who you're actually responding to if you're going to insist on your interpretation of their words instead of responding to my words.

 

[windy miller]

As far as I can see lots of cosmologists says things like "the universe isn't expanding into anything" without the slightest caveat that they don't really know this to be the case. This isn't the only example of this. Lots of cosmologists say the universe began with a big bang but we don't really know the big bang was the beginning of the universe and some other cosmologists are careful in saying that. So we can't even say the universe is 13.8 bio years old. Maybe one should say its at least 13.8 bio years old but it might be much older. Its one thing to mean something that isn't a statement of absolute confidence but its a totally different thing to convey that to others. I did quote rootone in my reply. I thought that was clear. You gave an answer about what he meant, but I don't see how you get to speak about what other people mean. But the question isn't just about what some people have in their head when they say something but its about what their language conveys. and statements like the universe isn't expanding into anything or the universe is 13.8 bio years old convey a confidence that may not be justified.

Peter , I think this will be my last post (although I reserve the right to change my mind) on the subject because the conversation is clearly deteriorating. So feel free to have the final say.

 

[PeterDonis]

As far as I can see lots of cosmologists says things like "the universe isn't expanding into anything" without the slightest caveat that they don't really know this to be the case.

I won't try to justify what cosmologists say in pop science articles, videos, etc., because I think scientists in general are too careless in such venues. But in textbooks or peer-reviewed papers, cosmologists are (and should be) more careful. Statements in such sources are always made in the context of some model or set of models.

Lots of cosmologists say the universe began with a big bang

In textbooks and peer-reviewed papers, the term "big bang" has a particular technical meaning, and statements about it are made accordingly. We do know, based on many different lines of evidence, that the earliest state of the universe of which we have good evidence, which is what the term "big bang" properly refers to in textbooks and peer-reviewed papers, was a very hot, very dense, rapidly expanding state. That is what a statement using the term "big bang" refers to, properly interpreted, and statements about it are not speculation or hypothesis, they are based on evidence.

If you can find textbooks or peer-reviewed papers that use the term "big bang" differently, and that go beyond what is justified by our best current models and evidence, feel free to give specific references. But I strongly suspect that your statements are based on pop science sources, not textbooks or peer-reviewed papers. The fix for that problem is simple: don't try to learn actual science from pop science sources. That's why PF has rules about acceptable sources.

 

[windy miller]

I won't try to justify what cosmologists say in pop science articles, videos, etc., because I think scientists in general are too careless in such venues. But in textbooks or peer-reviewed papers, cosmologists are (and should be) more careful. Statements in such sources are always made in the context of some model or set of models.
In textbooks and peer-reviewed papers, the term "big bang" has a particular technical meaning, and statements about it are made accordingly. We do know, based on many different lines of evidence, that the earliest state of the universe of which we have good evidence, which is what the term "big bang" properly refers to in textbooks and peer-reviewed papers, was a very hot, very dense, rapidly expanding state. That is what a statement using the term "big bang" refers to, properly interpreted, and statements about it are not speculation or hypothesis, they are based on evidence.

If you can find textbooks or peer-reviewed papers that use the term "big bang" differently, and that go beyond what is justified by our best current models and evidence, feel free to give specific references. But I strongly suspect that your statements are based on pop science sources, not textbooks or peer-reviewed papers. The fix for that problem is simple: don't try to learn actual science from pop science sources. That's why PF has rules about acceptable sources.

Peter, I totally had the intention of walking away form this conversation but since you've asked for specific references I feel the need to answer so apologies for keeping this going beyond what I intended.
I do think this problem of cosmologists giving statements like these is not restricted to pop science tv shows and the like. But is a wider problem within the cosmological community.
Lets take two essays from two extremely reputable institutions. One gets its totally right in my opinion and the other gives the misleading confidence that I am talking about. I will come on to peer review in a moment. But these two institutions web sites I don't think should be classified as "pop science". Perhaps you disagree,
The first is NASA WMAP web site.
https://map.gsfc.nasa.gov/universe/uni_age.html
They clearly state that WMAP can measure the age of the universe. But I assert that the age of the universe can't be measured because we don't know if time really began at the big bang or not. Therefore they are misleading the public. If we contrast this with an essay that our old friend Marcus used to post a lot ( and others have to) then we see this from the MAX Planck Institute for Gravitational Physics:
http://www.einstein-online.info/spotlights/big_bangs.1.html
They explain something something I hope you will agree with. The phrase "big bang" can have multiple meanings. One meaning is expansion from a singularity and the other is expansion from a super hot dense state. We know that latter is true but we don't know the former to be true. In fact I suspect very few cosmologists today believe the former to be true. It seems from your previous post you agree with this. The problem is to assert the universe has a finite age of about 13.8 bio years is to assert there was no time before the big bang. Thats a valid statement if the singularity theorems represent some physical reality. But if they just represent the mis application of GR to the Planck regime then it is not a valid statement. Then we can't say the universe is 13.8 bio years old. What I think should be said is the universe is at least 13.8 bio years old and it could much older but beyond that we don't know. 13.8 bio years is NOT NECESSARILY THE AGE OF THE UNIVERSE its the lower limit for the age of the universe. But that isn't what cosmologists say; they say simply that's its 13.8 bio years old.
Now on to the peer reviewed literature. The very first article i found when i put in a search for peer reviewed articles on the age of the universe was this one:
https://arxiv.org/pdf/1607.00002.pdf
Yes I know arxiv isn't peer reviewed but this article did get peer reviewed and published in a reputable journal.
So let's suppose someone has watched a pop science tv show and hears that the universe has a finite age of 13.8 bio years. Then they run into me and I tell that's not right. The age of the universe is unknown I say. All we can do is trace the evolution back to the inflationary epoch ( although even there there is some controversy but it won't change our conclusion). What happened before inflation ( or if you want replace inflation with something else) is unknown. We certainly don't know that time and all things began just a tiny fraction of a second before then. So they decide to go the peer reviewed literature and the above is likely to be the first article they found ( it was the first one I found).
they read this in the introduction:
"The discovery of cosmic ac- celeration solved this riddle: with a cosmological constant of the amount required by the supernova observations, ΩΛ ∼ 0.7 (Riess et al. 1998; Perl- mutter et al. 1999), the age of the Universe im- plied by a Hubble constant near 70 km s−1Mpc−1 was about 14 Gyr, "
The peer reviewed literature seems to back what the pop science show said and not what I, Marcus, the Max Planck Institute say or other more careful cosmologists say.
I should add I am no cosmologist. but I did take an undergraduate course in cosmology at university ( a reputable one) and this is also what I was taught ( the universe is 13.8 bio years old and there was no before the big bang).
As I said some cosmologists are more careful . Sean Carroll is a good example and makes the two big bang distinction that the Max Planck article does. But this carefulness is in my opinion not common, or certainly not common enough. What is for more common ( and yes its not just in pop science as I believe I have shown) is a careless use of language by cosmologists that hides real uncertainties about the full nature of reality and what we can actually know.
This carelessness can seriously undermine science as a whole. It is a serious issue and not a technical quibble. I hear all the time from friends we shouldn't trust scientists because they are always changing their minds.
Let us suppose that some day that a model with a pre big bang phase ( e.g. like loop quantum cosmology or the Ekpyortic model ) is eventually shown to be correct. A lot of the public are going to add this to their perceived list of things science got wrong. Of course they would be incorrect because science read more carefully, isn't really telling us the universe has a finite age. But that brings us back to the key point. Science is communicated by scientists and i think it not an uncommon feature for cosmologists in general to overstep the mark in what we can be really confident of when we are talking about such grand statement as the entirely of all physical existence.

 

[laymanB]

@windy miller I think @PeterDonis is always very careful to stick to what the best model shows and qualify his statements as such. I never read him making broad, sweeping statements about metaphysical realities and he is always quick to point out that science can not prove things in a logically deductive manner. @kimbyd is answering your question directly by making a stronger claim in post #11 that General Relativity describes spacetime in it's entirety and that by embedding 4-D spacetime in some higher "meta-spacetime", the math no longer makes sense.

I agree with your general tone and the premises of your argument that dogmatic statements should not be made without giving the caveats and nuances about it's domain of applicability, but I think you're arguing with someone who does not represent what you find distasteful.

 

[PeterDonis]

these two institutions web sites I don't think should be classified as "pop science". Perhaps you disagree

Yes. I already said what are acceptable sources: textbooks and peer-reviewed papers. Neither of the web sites are either of those.

The very first article i found when i put in a search for peer reviewed articles on the age of the universe was this one

This paper clearly says that the "age of the universe" as they are defining it is derived from a specific model, which is extrapolated back to a time in the model when the scale factor is zero. (It is true that the paper uses the term "Big Bang" to refer to this time, instead of to the earliest state of which we have actual evidence; that's somewhat disappointing, but at least they explicitly define what they mean by the term, which pop science sources typically do not do.)

The paper is not claiming that this is the "real" age of the universe; what it is saying (or at least implying) is that this "age" is an upper bound on the time that things like stars and galaxies in our present universe could have existed. That is the point of the statement in the introduction that "the objects in the Universe should not be older than the time from the Big Bang". Calling the time since the scale factor was zero in the model the "time from the Big Bang" is not precisely accurate, but for that purpose it's quite good enough; the earliest state of which we have good evidence occurred at a time in the model about $10^{-32}$ seconds, IIRC, after the time in the model when the scale factor was zero, so the difference between the two is entirely negligible when we are talking about how old stars and galaxies can be.

For the intended audience of the paper, all this doesn't need to be spelled out explicitly; working cosmologists are already well aware of it. It would be nice if there were pop science sources that explained it in more detail, though (see further comment below).

Science is communicated by scientists and i think it not an uncommon feature for cosmologists in general to overstep the mark in what we can be really confident of when we are talking about such grand statement as the entirely of all physical existence.

I agree that scientists (and not just cosmologists) do a very bad job, when communicating with non-scientists, of conveying the actual level of confidence we have in various scientific statements; which means that, when scientists complain about the public not understanding science or trusting what scientists tell them, they have only themselves to blame. I have complained about this myself in threads here on PF (including a previous post in this thread), and also in other venues.

But as I said before, here at PF we have rules about acceptable sources, and we try to discuss the actual scientific models and what they actually say, not what some scientists have said in pop science venues. Perhaps we should have a separate forum for discussing how science is communicated to non-scientists.

 

[KurtLudwig]

I am learning from your discussions. I now have read "metric expansion" on Wikipedia and will read it again.
1. The distance between galaxies is increasing, yet no space is added or created. Where are they all headed to? Space must be somehow expanding! Yet our Earth and solar system are not getting any bigger or further apart, at least not to my knowledge. Please explain.
2. Is space a field? If space is a field then there must be "particles" associated with it. (My technical level: I have been attempting to read textbooks on quantum mechanics and the standard model. I do follow the general ideas, but I have problems following some of the mathematics. I am at the level of second order differential equations. I have been reading Scientific American for over 50 years and understood most of the articles. In my professional life, I have been designing production machinery.)
3. After having read too many "pop science" books I tend to believe the following, without any proof at this time: In physics, at exceedingly small dimensions everything turns digital, including space. Is anyone conducting an experiment to find out if space is "granular"?
4. Time is just a convenience to be used in calculus. Time is the count of the number of cycles, such as in a mechanical pendulum clock or a vibrating crystal in a digital watch.
Please comment or explain.
Kurt Ludwig

 

[PeterDonis]

The distance between galaxies is increasing

Yes.

no space is added or created

This isn't really a meaningful statement either way; you can't say space is being created and you can't say it isn't. "Space being created" isn't a well-defined concept.

Where are they all headed to? Space must be somehow expanding!

No; spacetime is curved. Your intuitions are used to flat spacetime; they don't work in curved spacetime.

Is space a field?

No. Depending on what theory of quantum gravity finally pans out, it might turn out that spacetime is a field, or at least that it can be usefully modeled as one. But that goes beyond GR. As far as GR is concerned, spacetime isn't a field either, it's just geometry.

I have been attempting to read textbooks on quantum mechanics and the standard model

Which ones?

After having read too many "pop science" books I tend to believe the following, without any proof at this time: In physics, at exceedingly small dimensions everything turns digital, including space.

This is a good example of why it's a bad idea to try to learn science from pop science books.

Is anyone conducting an experiment to find out if space is "granular"?

According to our best current understanding of quantum gravity, if spacetime (not space) is in fact quantized (which is a better word than "granular"), we would not expect to see effects due to that until we got down to length scales on the order of the Planck length. That's about 20 orders of magnitude smaller than the smallest length scales we can currently probe. So we don't have any expectation of being able to test for quantum effects of spacetime or gravity any time soon.

Another possible test would be trying to spot violations of Lorentz invariance; but so far no such violations have been observed.

Time is just a convenience to be used in calculus. Time is the count of the number of cycles, such as in a mechanical pendulum clock or a vibrating crystal in a digital watch.

These two statements are mutually inconsistent. Which one do you want to discuss?

 

[rootone]

find out if space is "granular"?

It can appear that way if quantum mechanics is true. which seems seems very likely.
Here is something to look at which is sort of pop-sci, but not too bad,

 

[phinds]

1. The distance between galaxies is increasing

Yes, as long as the galaxies are not in local clusters, which are bound systems and do not expand

, yet no space is added or created

Corrrect

. Where are they all headed to?

They are not "headed" anywhere, except apart

Space must be somehow expanding!

No, space is just geometry, not a thing that can be streched. Re-read "metric expansion"

Yet our Earth and solar system are not getting any bigger or further apart, at least not to my knowledge. Please explain.

Bounds systems such as galactic clusters, galaxies, solar systems, planets, you, atoms, etc do not expand.

2. Is space a field?

No, space is just geometry

If space is a field then there must be "particles" associated with it.

There are particles IN it, along w/ planets, stars, etc.

3. After having read too many "pop science" books I tend to believe the following, without any proof at this time: In physics, at exceedingly small dimensions everything turns digital, including space

There is no reason to believe this. Please avoid personal speculation. It MAY in fact be true but there is not currently any evidence for it.

. Is anyone conducting an experiment to find out if space is "granular"?

It is very much an open area of discussion in physics. I don't know about experiments; I don't think we can measure things small enough. If things ARE granular, it is believe that the granularity will be on the order of Planck measurements or smaller and right now, the Planck Length is TWENTY ORDERS OF MAGNITUDE below our ability to measure things.

4. Time is just a convenience to be used in calculus. Time is the count of the number of cycles, such as in a mechanical pendulum clock or a vibrating crystal in a digital watch.

That's a reasonable way of looking at it.

EDIT: I see Peter beat me to it. Again. Damn speed typist

EDIT AGAIN: Actually, I'M a speed typist. He's a speed thinker.

 

[phinds]

By the way, @KurtLudwig, I suggest the link in my signature

 

[phinds]

It can appear that way if quantum mechanics is true.

Do you have any refereed citations that connect quantum mechanics to the necessity that space and time are granular? I do not believe any such exists.

 

[DanMP]

... Bounds systems such as galactic clusters, galaxies, solar systems, planets, you, atoms, etc do not expand...

Why exactly? And how is established the upper "limit" (galactic clusters?)?

 

[PeroK]

Why exactly? And how is established the upper "limit" (galactic clusters?)?

If you take the Earth's orbit round the Sun as an example. First, without considering the expansion of space, using only gravity you get an equation for a stable elliptical, almost spherical orbit.

Now, the expansion of space is so small on this scale that you can effectively ignore it. But, if you do include it in your calculations - perhaps imagining it was much greater so that it has some influence - then it acts like a small outward "force". This would move the Earth into a slightly larger stable orbit. In other words, the Earth orbits slightly further away from the Sun than it would if space were not expanding.

What does not happen is that the expansion of space gradually pushes the Earth further away without gravity being able to do anything about it!

So, on the scale of the solar system, the expansion of space has a tiny, negligible effect.

On an intermediate scale, you would have to take both factors into account. An orbit at a very large distance would be different from the one calculated using gravity alone.

And, on a greater scale, it is gravity that becomes negligible and the recesssion of a distant galaxy is entirely calculated from the expansion of space.

 

[DanMP]

... on the scale of the solar system, the expansion of space has a tiny, negligible effect...

So phinds was wrong/inaccurate, as I thought. Thank you.

This expansion of the Universe is derived mainly from the redshift of all distant galaxies, right? What reasons do we have (other than the absence of a good, overall acceptable, model) to reject the ideea that this redshift (or at least a part of it) is caused by / related to the distance that light has to "travel"?

 

[phinds]

So phinds was wrong/inaccurate, as I thought. Thank you.

I was, I suppose, unclear when I said there was no expansion. What I should have said was that there is no ONGOING expansion. Whatever effect DE has on bound systems is already built into their orbits/conditions. Also, DE on the scale of atoms, for example, is REALLY negligible. Possibly a rounding error somewhere around the 20th decimal place.

 

[PeroK]

This expansion of the Universe is derived mainly from the redshift of all distant galaxies, right? What reasons do we have (other than the absence of a good, overall acceptable, model) to reject the ideea that this redshift (or at least a part of it) is caused by / related to the distance that light has to "travel"?

Why would the travel time of light induce a redshift?

When Einstein first developed General Relativity the conclusion was an expanding universe. As it was assumed (in 1915) that the universe was not expanding he fudged his theory to remove this characteristic. Later, when Hubble discovered the redshift, Einstein realized his mistake. You can read about that here, for example:

https://en.wikipedia.org/wiki/Cosmological_constant

 

[DanMP]

Why would the travel time of light induce a redshift?

I can't answer this question now/here, that's why I specified "other than the absence of a good, overall acceptable, model" ... so please focus on other reasons to reject the idea.

 

[PeroK]

I can't answer this question now/here, that's why I specified "other than the absence of a good, overall acceptable, model" ... so please focus on other reasons to reject the idea.

That's as good a reason as I can think of!

Actually, once you understand the concept of an expanding universe, it's not so problematic as popular science would have you believe. There's no sense in which producing a theory that explained the universe without metric expansion would lead to a general sigh of relief that a conceptual problem was solved.

In fact, the whole question "where does space expand", although worth asking and worth trying to answer, is actually based on a misconception of the theory that a proper study would eliminate!

 

[DanMP]

That's as good a reason as I can think of!

Ok, but there are more?

Actually, once you understand the concept of an expanding universe, it's not so problematic as popular science would have you believe. ...

Maybe, but to explain dark energy is problematic, as far as I understand ...

 

[PeroK]

Maybe, but to explain dark energy is problematic, as far as I understand ...

That's a good point. But, if we assume that the universe is spatially static, then we not only have the redshift to explain, but everything else the Big Bang theory does explain. And, it explains a lot.

So, we go from one problem (dark energy) back to a whole range of inexplicable data and phenomena.

 

[phinds]

And, DanMP, another way of looking at it is that we don't know what Dark Energy IS, but we know what it DOES and what it does is incontrovertible so "Dark Energy" is a placeholder name for whatever it is that does what it does. I'm basically saying the same thing as PeroK, just in a different way.

 

[timmdeeg]

What does not happen is that the expansion of space gradually pushes the Earth further away without gravity being able to do anything about it!

So, on the scale of the solar system, the expansion of space has a tiny, negligible effect.

I think what one can say for sure is that if we pick a region in the ideal fluid model which is comparable in size to the solar system then there would be tiny effect on this scale. But how would you justify a tiny effect on the scale of the solar system even though this is gravitationally bound?

 

[PeroK]

I think what one can say for sure is that if we pick a region in the ideal fluid model which is comparable in size to the solar system then there would be tiny effect on this scale. But how would you justify a tiny effect on the scale of the solar system even though this is gravitationally bound?

I'm not sure I understand your question. If space is expanding, then it's expanding at all scales. There's no justification needed. What is needed is a calculation to confirm that it has a negligible effect on a given scale.

 

[timmdeeg]

I'm not sure I understand your question. If space is expanding, then it's expanding at all scales. There's no justification needed. What is needed is a calculation to confirm that it has a negligible effect on a given scale.

Well I was reasoning that the accelerated expansion creates tidal forces. And that these are way too week to stretch gravitationally bound systems like the solar system at all, but am not sure if this is correct. Whereas the ideal fluid model lacks gravitationally bound systems by definition.

The solar system is part of the milky way. So the average density exceeds the critical density. From this I wonder whether we expect any expansion locally at all.

 

[PeroK]

Well I was reasoning that the accelerated expansion creates tidal forces. And that these are way too week to stretch gravitationally bound systems like the solar system at all, but am not sure if this is correct. Whereas the ideal fluid model lacks gravitationally bound systems by definition.

The solar system is part of the milky way. So the average density exceeds the critical density. From this I wonder whether we expect any expansion locally at all.

I understand your question and I don't know the answer. In our solar system you would have to combine the local Schwarzschild geometry with the expansion geometry in some way. But, both are idealised solutions to spacetime on vastly different scales. And, of course, the local expansion is/would be infinitesimal in any case.

 

[PeterDonis]

I was reasoning that the accelerated expansion creates tidal forces

It creates (more precisely, it is a form of) tidal gravity. But tidal gravity, in itself, is not a force. What are often called "tidal forces" are really internal non-gravitational forces inside objects that prevent the individual atoms of those objects from moving on the trajectories that they would move on if they were moving solely in response to tidal gravity.

these are way too week to stretch gravitationally bound systems like the solar system at all

If we ignore the fact that gravity is not a force in GR, and think of a gravitationally bound system like the solar system as being held together by internal forces, which prevent its parts from moving on the exact trajectories that they would move on in response to the tidal gravity created by accelerated expansion, then it is true that the effect of that tidal gravity on the bound system is way, way too small for us to measure. But in principle the effect is there.

However, gravity isn't a force in GR, and a full GR analysis of something like the solar system embedded in a universe with accelerated expansion requires finding an appropriate metric. As it happens, there is one, at least heuristically; see below.

In our solar system you would have to combine the local Schwarzschild geometry with the expansion geometry in some way.

The Schwarzschild-de Sitter geometry (basically Schwarzschild spacetime with a cosmological constant added) is a known exact solution. I haven't seen a detailed analysis done of the case where the cosmological constant is very small (so the solution can be approximated by Schwarzschild plus a small perturbation), but intuitively I would expect it to show basically what I said above: you would still have stable orbits, they would just have slightly different parameters for the same Schwarzschild mass.

 

[timmdeeg]

I haven't seen a detailed analysis done of the case where the cosmological constant is very small (so the solution can be approximated by Schwarzschild plus a small perturbation), but intuitively I would expect it to show basically what I said above: you would still have stable orbits, they would just have slightly different parameters for the same Schwarzschild mass.

As I understand it the FRW model predicts the dynamical evolution of the universe globally thereby assuming an average matter density. In our universe the data are consistent with the assumption of a small cosmological constant. In the milky way however the matter density is higher than the critical density. I'm not sure if the Friedmann equations can be applied locally. If yes then it seems that from this point of view the milky doesn't participate in the global expansion. Hm, perhaps overdensity and gravitationally bound means the same.

 

[PeterDonis]

As I understand it the FRW model predicts the dynamical evolution of the universe globally thereby assuming an average matter density.

I'm not sure if the Friedmann equations can be applied locally.

I wasn't talking about using the FRW model to model something like the solar system. I was talking about using the Schwarzschild-de Sitter model, which is different: it is basically a model of an isolated gravitationally bound system in the presence of a cosmological constant, which, with a suitably chosen (very small) value of the cosmological constant, is a reasonable approximation to a model of a bound system like the solar system in our universe.

 

[timmdeeg]

I wasn't talking about using the FRW model to model something like the solar system. I was talking about using the Schwarzschild-de Sitter model, which is different: it is basically a model of an isolated gravitationally bound system in the presence of a cosmological constant, which, with a suitably chosen (very small) value of the cosmological constant, is a reasonable approximation to a model of a bound system like the solar system in our universe.

Whereby regarding the cosmological constant there seems to be a bridge to the FRW model in this approximation.

I understand that one can't model a bound system using the FRW model. Would you say that any considerations by interpreting the Friedmann equations locally like I attempted in #41 don't make any sense? Strictly these equations are based on the assumption of ideal homogeneity. I'm interested if they nevertheless provide conclusions regarding local inhomogeneities such, as regions with "overdensity" don't participate in the accelerated expansion or as super voids would expand faster than the universe in average.

 

[PeterDonis]

regarding the cosmological constant there seems to be a bridge to the FRW model in this approximation.

You would want to use the value for the cosmological constant that corresponds to our best current FRW-based model, yes.

Would you say that any considerations by interpreting the Friedmann equations locally like I attempted in #41 don't make any sense?

You can interpret the Friedmann equations locally, but you have to meet the conditions, one of which is that the density is the same everywhere. If that condition is not met, the equations aren't valid. But if, for example, you have a spherically symmetric (another condition for the FRW model to be valid) "bubble" of uniform density inside a spherically symmetric universe of some other density, you can apply the Friedmann equations inside the "bubble" just fine. The problem with applying the Friedmann equations locally in our actual universe is that the density is very, very far from being uniform on local scales.

There is ongoing work to develop more sophisticated models, not using the precise FRW metric (but the models could be, I think, described as FRW plus perturbations), that allow for variations in density. But AFAIK nobody has tried to apply them on scales as small as a single galaxy; we are still talking scales of tens to hundreds of millions of light years.

 

[timmdeeg]

You can interpret the Friedmann equations locally, but you have to meet the conditions, one of which is that the density is the same everywhere. If that condition is not met, the equations aren't valid. But if, for example, you have a spherically symmetric (another condition for the FRW model to be valid) "bubble" of uniform density inside a spherically symmetric universe of some other density, you can apply the Friedmann equations inside the "bubble" just fine.

Ok and thanks, this clarifies my question.

 

[KurtLudwig]

Yes.
This isn't really a meaningful statement either way; you can't say space is being created and you can't say it isn't. "Space being created" isn't a well-defined concept.
No; spacetime is curved. Your intuitions are used to flat spacetime; they don't work in curved spacetime.
No. Depending on what theory of quantum gravity finally pans out, it might turn out that spacetime is a field, or at least that it can be usefully modeled as one. But that goes beyond GR. As far as GR is concerned, spacetime isn't a field either, it's just geometry.
Which ones?
This is a good example of why it's a bad idea to try to learn science from pop science books.
According to our best current understanding of quantum gravity, if spacetime (not space) is in fact quantized (which is a better word than "granular"), we would not expect to see effects due to that until we got down to length scales on the order of the Planck length. That's about 20 orders of magnitude smaller than the smallest length scales we can currently probe. So we don't have any expectation of being able to test for quantum effects of spacetime or gravity any time soon.

Another possible test would be trying to spot violations of Lorentz invariance; but so far no such violations have been observed.
These two statements are mutually inconsistent. Which one do you want to discuss?

What is time? How does time arise? Is time the count of cycles?

 

[Mordred]

Simply a measure of rate of change or duration. It isn't some mythical substance but another property. Even under spacetime as per GR this definition does not change.

 

[PeterDonis]

What is time?

You're the one that made statements about what it is. I just asked you to make up your mind which of the two inconsistent things you said you want to pick.

As far as GR is concerned, time is just like length, except it's measured along timelike curves instead of spacelike curves.

 

[KurtLudwig]

Thanks for your reply. I have read the article on Special Relativity in Wikipedia. I do not yet fully understand all of the mathematics involved.

 

[Mordred]

Wiki in this case isn't particularly helpful particularly with its latest modifications.

While I can't buy you a textbook, I can offer a link to an open source written by another forum member

http://www.lightandmatter.com/sr/