Does Space Expand? What Do You Think?

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In summary: In this theory, distance is not absolute, but rather depends on the curvature of spacetime and the observer's frame of reference. The proper distance, as mentioned before, is the length between two events in a frame of reference where they occur simultaneously. However, in GR, this distance can change over time as the curvature of spacetime changes. This allows for the possibility of objects to move away from each other at a rate faster than the speed of light, as long as they are not in the same inertial frame of reference. This is why, in an expanding universe, distant galaxies can appear to be moving away from each other at speeds greater than the speed of light
  • #36
MeJennifer said:
It may be a convention fo some but that does not make it any way true.

The issue is simple and directly related to the principle of equivalence in general relativity.

The assumption that a photon changes due to the curvature of space-time is a direct violation of the principle of equivalence.

So "stretching photons" is, as Pauli could have said: "not even wrong". :smile:
The question is "How do we measure mass, length and time across cosmological distances?"

To be able to make cosmological measurements we need something that stays constant when transported across the universe as a standard, therefore a conservation principle is absolutely essential, not just a vague "assumption", but the question is which principle?

As we cannot prove that anything remains constant over cosmological space-time all we can do is define a conservation principle and test to see whether it is concordant and internally consistent.

At the heart of GR is the conservation of energy-momentum, which leads to the atom, atomic 'regular' clocks and 'rigid' steel rulers, being the standard by which to measure the universe. GR has been tested, and is still being tested and so far has not be falsified. Perhaps Saturday will tell! :wink:

Garth
 
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  • #37
Garth said:
The question is "How do we measure mass, length and time across cosmological distances?"
There is no such thing as a spatial distance in a dynamic space-time. The FRW model is a dynamic space-time.
 
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  • #38
MeJennifer said:
There is no such thing as a spatial distance in a dynamic space-time. The FRW model is a dynamic space-time.
Of course there is, the concept of a standard ruler demands such.

It is measured along the space-like foliation in the frame of the co-moving cosmological fluid identified by the isotropic CMB.

Garth
 
  • #39
Garth said:
...the concept of a standard ruler demands such.

It is measured along the space-like foliation in the frame of the co-moving cosmological fluid identified by the isotropic CMB.
The concept of a standard ruler demands what? :confused:

It seems that my and your understanding of general relativity are quite different.

I think discussing the concept of spatial distance in non-stationary spacetimes is a bit beyond the scope of this topic.

So let's just assume that I am wrong and don't know what I am talking about and that you are right about it, surely that will make everybody happy here. :smile:
 
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  • #40
MeJennifer said:
The concept of a standard ruler demands what? :confused:

It seems that my and your understanding of general relativity are quite different.

I think discussing the concept of spatial distance in non-stationary spacetimes is a bit beyond the scope of this topic.

So let's just assume that I am wrong and don't know what I am talking about and that you are right about it, surely that will make everybody happy here. :smile:

Hi MeJennifer,
Don’t agree.
I have read this thread twice and certainly will do it once again.
Where I had (and still) have problems with the notions of space and space-time and was just about posting some questions, I got much sympathy for your statements e.g. in post#29 “Absolutely nothing happens to the photon”. I can indeed see the advantage of the conservation of energy-momentum which leads to the atom, atomic ‘regular’ clocks and the ‘rigid’ steel rulers being the standard by which to measure the universe (your post #36). I had (or still have) my questions about rulers like Planck-length or light-years in following a non stationary universe as an observer (in “my position”) in our universe in the past and in the future and especially how they behaved and will behave. Do I interpret you well if I now conclude, “nothing happens to those rulers”. That answer could be helpful for me to make some nice cartoons.
I am still happy with Marcus in his post #10 where he, not obliged, explains GR. It seems to me that this forum is not only for those who already know but also and especially for those who are curious, anxious and able to understand more and more. So I would suggest you and others like Marcus, Garth, Chronos and others to continue explaining, or discussing, the concept of spatial distance in a non-stationary space-time even if your understanding of GR seems quite different.
Finally, am I right if I think that space-time needs to be discussed (and only) in relation to energy density, because together and inseparably they are the base of existence?
By the way I am reading provocative books like L.Smolins’ “The problems with Physics” and even J. Magueijo’s controversial “faster than the speed of light”. I must say I love them, but I need to be very critical to finally develop my own ideas. Your contribution is very welcome.
Kind regards
Hurk4.
 
  • #41
Brand new memeber here. This is my second day here and i wanted to tell you both/all how fascinating and interesting your discussions are. Please, please keep up the ongoing discussions. How else will you or I ever really learn or formulate from the ideas that are expressed here. There are some very brilliant people here with sound and logical ideas. This is what makes it one of the best Forum sites I've encountered on the net. Unfortunately i have no other input into this discussion except to keep checking in and see where this goes. Very interesting stuff people!

All the best,
Maj.MattMason
 
  • #42
It is cheering to hear your approval. Welcome Major Matt!
 
  • #43
Matt, since you urge continued discussion, I'll say what the main issue in this thread is for me.

Mainstream professional cosmologists (Wallace is one, SpaceTiger also) use a particular model (associated with names Friedman Lemaitre Roberson Walker and abbreviated FRW sometimes) into which you can plug various parameters ---and it gives you nice simple solutions to the main (Einstein) equation that you can try to fit to observational data.

And this FRW model has an idea of universal time: At each given moment of time there is a spatial metric---a slice of spacetime which is space at that particular moment and a metric (distance function) describing the geometry.
This is only APPROXIMATELY right because the FRW construction is based on everything looking uniform at large scale as if all the lumps were smoothed out by averaging. that is approximately realistic but not perfectly right---reality is lumpy.

So professional cosmologists have an absolute time idea and they also have an idea of being absolutely AT REST, called being at rest with respect to the Hubble flow or at rest with respect to the CMB.

The sun and planets are moving about 370 km/second wrt CMB because there is a Doppler hotspot ahead of us and a Doppler cold spot behind. The difference in temperature can be accurately measured. The hotspot is in the contellation Leo---we know our motion relative to the UNIVERSE REST and we can allow for it.

And the professionals do allow for that 370 km/sec speed and correct their observations for it (and other known motions).

---------------------
Matt, the PURE THEORY of Gen Rel does not have an idea of absolute rest or absolute motion, or an idea of universal time. those are things which you get from being in a PARTICULAR SOLUTION of the general equations. It is something that comes in with working cosmologists studying our particular universe.
---------------------
Now I will tell you what the issue is, for me, in this thread.

Cosmologists keep very detailed catalogs of all the energy in the universe, at different times. (counting matter as a form of energy)
and one form of energy is the CMB PHOTONS

these started out being a mix of wavelengths rather like sunlight or the glow of something at 3000 kelvin (a bit redder than the sun, like a reddish star)

So cosmologists can tell you at any moment in the past 13 billion years, in a cubic lightyear of space, how much energy is represented by the CMB photons in that cubic lightyear
They have to have this accurate ENERGY INVENTORY because the density of energy of various kinds actually affects how the universe evolves.

The dynamics of the universe will not work right unless you assume that at any given moment or era in time the CMB photons have wavelengths and those wavelengths are GRADUALLY GETTING LONGER. Because the wavelength determines how much energy the photon represents (longer means less energetic)

They start out short (3000 kelvin light) and at present they are long (2.7 kelvin infrared/microwave) and in the intervening span of time they are constantly getting longer----as the energy density of the CMB gradually diminishes.

this is what they teach you in an advance undergrad or graduate course in cosmology.

However, it seems to me that some people in this thread DISAGREE
with that. So that is what the issue is. I don't know whether folks will want to discuss it any more, but if they do discuss that is what I will be listening for.

for a mainstream professional, the spatial metric changes over time and distances increase by a tinytiny percentage each second or day or year.
this does not affect the size of OBJECTS, because objects like steel rods are held together by atomic and molecular forces which determine crystal lattice bond lengths etc. and planet orbit radii etc.
but a LIGHTWAVE propagating according to MAXWELL equation is not bound by lattice forces like a steel rod:smile: so it does experience the tiny percentagewise increases in distance and it must be affected by them.

so little by little a lightwave is extended by this, and in fact this is what we have observed to have happened to the old light that we see!

But other people seem to have a different take on this:smile:
so stay posted and maybe there'll be more said.
 
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  • #44
marcus said:
... a LIGHTWAVE propagating according to MAXWELL equation is not bound by lattice forces like a steel rod:smile: so it does experience the tiny percentagewise increases in distance and it must be affected by them.

so little by little a lightwave is extended by this, and in fact this is what we have observed to have happened to the old light that we see!

But other people seem to have a different take on this:smile:
so stay posted and maybe there'll be more said.
Ok, I take a small step into the lion's cosmological den :wink:

I think that if we assume that the proper frequency of an inertially traveling photon is modified by spacetime we invalidate general relativity. We would have to explain how spacetime interacts with the photon even when it travels inertially. In general relativity an inertially moving object is supposed to be "left alone" even when the spacetime is not flat.

Redshift does not mean that the photon changes its frequency instead it means that the emitter and absorber measure a different frequency due to the curvature or kinematics of spacetime.

For instance take the simple case of a purely gravitational redshift. It is not because the emitter's clock runs slower compared to the absorber's clock, ideal clocks run at the same speed everywhere in the universe, it is also not because the photon looses energy, but instead, spacetime curvature, and in particular, geodesic convergence and divergence, causes the absorber to measure a different frequency than the emitter.
 
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  • #45
I tend to agree, photons are not reshifted by traveling through the universe, they are redshifted only because they are observed in a different frame from what they are emitted in.
 
  • #46
Any theory is sound to the point it does not forbid what is observed. A genius sees all possibilities, a fool sees even more.
 
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  • #47
The trouble with all the stuff and nonsense that is written about "expanding space" (not in this thread, I hasten to add, but in cosmology and in books like Brian Greene's "The Fabric of the Cosmos") is that there is no accepted definition of "space" itself ( Unless I can persuade folk to adopt my definition: “Space is what you can swing a cat in”.

In cosmology it is obvious that talking of "expanding space" , as if space was an elastic continuum is just plain silly. For instance, Matter --- crystals, atoms, nuclei and suchlike --- are mostly empty "space", as is the dispersion of galaxies that constitutes the universe. If you describe change in the universe as the expansion of "space" you are then faced with the difficulty of
distinguishing between two sorts of "space", non-expanding "space" in microscopic interstices and inter-galactic- cluster “ expanding space”.

Better be more circumspect, like Wallace and Me Jennifer in this thread.
 
  • #48
Wallace said:
I tend to agree, photons are not redshifted by traveling through the universe, they are redshifted only because they are observed in a different frame from what they are emitted in.

Well you are the cosmologist here! I think you do it for a living. My habit, or general policy, is to go along with what I see as mainstream pro definitions.

So basically I have to shut up and not argue:biggrin:

But I am not comfortable with that because among other things I see cosmologists doing inventories of the energy density which are implicitly estimated IN A CMB FRAME.

I could probably dig around and find an inventory of all the energy, of all different kinds, in some standard volume. And it would be estimated relative to a frame at rest wrt Hubble flow. And all that energy matters because it affects the evolution of space as described by Friedmann equation. (which also involves an idea of rest).

And almost every time I look at a cosmology paper there will routinely be some equation that radiation energy density evolves with time according to the fourth power of scale.

So it looks to me as if in a practical or operational way, cosmologists live with an objective idea of the energy in the radiation background regardless of whether it is being observed or not---that the energy is well defined regardless of any specified observer, or (if you prefer) constantly defined by a standard observer at rest wrt Hubble flow.

I think their models depend on that.

To repeat, in different terms, it looks to me that in their actual work cosmologists have a definite idea of the wavelength mix of the CMB at any given era in the history of the universe----regardless of whether it is being observed (it is always interacting gravitationally with the rest anyway).

And moreover that wavelength mix is subject to the usual scalefactor ratio

wavelength (t)/wavelength(then) = z+1 = scale(t)/scale (then)

where "then" is the time the wave was emitted.

=======================

It may come down to Occam. Perhaps there are two explanations one involving trillions of observations all thru time, by fiduciary observers all at rest wrt Hubble flow, to provide for the smooth change of wavelength appropriate to the dynamical model.

and the other explanation (which seems simpler to me) that Maxwell's equations, applied to a context where distances gradually increase, result in gradually extending wavelengths.

Then if you, as pro, say you prefer the former I shall take that to mean that the pros in their wisdom think that it is conceptually simpler to have trillions (a continuum) of fiduciary observers distributed thru time.

Or there may be another alternative which hasn't dawned on me:smile:
 
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  • #49
Or there may be another alternative which hasn't dawned on me
There is ... it was shot down a long time ago. The steady state.
A new and improved version that takes into consideration the quantization of space would be ... just add more units of space then what you take away.
jal
 
  • #50
jal said:
There is ... it was shot down a long time ago. The steady state.
A new and improved version that takes into consideration the quantization of space would be ... just add more units of space then what you take away.
jal

Hi jal,
Personally I want to steer clear of thinking of "units of space" or anything suggesting that space is a medium or a substance. (though in a quantum geometry/gravity context the idea of adding vertices might come up)

what I am tending towards as a formulation (this is CLASSICAL relativity based cosmology now) is this

from the standpoint of an observer at rest wrt CMB
an electromagnetic wave has its wavelength enlarged in accordance with the scalefactor a(t)
and this process goes on constantly as the wave travels thru the universe.

[tex]\frac{\lambda_t}{\lambda_{emitted}} = \frac{a(t)}{a(t_{emitted})}[/tex]

if you know the wavelength when the wave was emitted and you know the ratio by which the universe has expanded since the time when it was emitted---that is the ratio on the righthand side---then you know the wave's wavelength at any given time.

(everything is always understood to be seen from the standpoint of an observer at rest wrt Hubble flow)

Now I have to see if that is compatible or not with what the experts say.
 
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  • #51
I'm also interested in the expert opinions.
My understanding may need some revising.
Since the speed of light is constant, (unit of distance/unit of time) then the ratio always has to 1/1, 2/2, etc. otherwise there would be a change in the speed of light.
In order to keep the speed of light constant both the distant units and the time units have to change at the same moment.
Has this been observed? What changes would we observe? Only Red shift?
I'll listen.
jal
 
  • #52
In this discussion AFAIK nobody is talking about the UNITS of time or distance changing, Jal.

distances between utterly separate disconnected things change, and they change as measured by units which are assumed to be constant and reliable

the cosmological redshift epitomizes a process which for better or for worse has always been described as the expansion of space

==============

If anyone is familiar with what Galileo is supposed to have said or muttered:
Eppur' si muove.

it could be adapted to modern context:
Eppur' si 'spanda.

In spoken Italian a lot of syllables get dropped, if you want to use babelfish to translate you have to write the words out
Eppure si espanda.
 
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  • #53
marcus said:
But I am not comfortable with that because among other things I see cosmologists doing inventories of the energy density which are implicitly estimated IN A CMB FRAME... (etc)

This and the rest of that post isn't wrong, we can and do define things wrt to the CMB 'rest' frame and the energy of the CMB photons does decline with the fourth power of the scale factor. The interpretation that they do this by traveling through some stretching stuff we call space is this issue. How many time have you seen a demo where someone draws a wave on a balloon then blows it up? The problem with thinking of expanding space like this is that implies the expansion acts like a viscous force, dragging photons and galaxies apart.

My view on galaxies and expanding space has, I think, been explained already in this thread. For photons I do think it is better to think of them as being redshifted by being observed in a different frame. However, to be clear what a mean by this, consider the FRW metric for a flat universe:

[tex] d\tau^2 = -dt^2 + a(t)[dr^2 + d\Omega^2] [/tex]

Now at t ticks along, the scale factor a(t) increases. Therefore two observers who are both at rest wrt to the CMB, but who have different times t will therefore be in different frames (have different metrics). This is what leads to photons being redshifted when observed and emitted at different times.
 
  • #54
marcus said:
Hi jal,

from the standpoint of an observer at rest wrt CMB
an electromagnetic wave has its wavelength enlarged in accordance with the scalefactor a(t)
and this process goes on constantly as the wave travels thru the universe.

[tex]\frac{\lambda_t}{\lambda_{emitted}} = \frac{a(t)}{a(t_{emitted})}[/tex]

if you know the wavelength when the wave was emitted and you know the ratio by which the universe has expanded since the time when it was emitted---that is the ratio on the righthand side---then you know the wave's wavelength at any given time.

(everything is always understood to be seen from the standpoint of an observer at rest wrt Hubble flow)

Now I have to see if that is compatible or not with what the experts say.

This relationship can be derived without any notion of "stretchy space", given that one assumes that the universe has a FRW metric. The FRW metric can be derived assuming the universe is isotropic, and that GR is valid.

If you have MTW's "Gravitation", see for instance 29.10 on pg 778.

The basic approach is simply to solve for the geodesics of light. Assuming all the motion is in the radial direction

-dt^2 + a(t)^2 dr^2 = 0 implies that dt = a(t) dr, or

[tex]r(t) = \int \frac{dt}{a(t)}[/tex]

The rest is just algebra.
 
  • #55
I am totally on board with Wallace on this issue. My hangup is my inability to mathematically describe our observable universe without at least one more degree of freedom. I vaguely recall a similar problem with too few degrees of freedom that predicts the energy potential of empty space is absurdly large. Is this a modern day 'ultraviolet catastrophe'?
 
  • #56
Wallace said:
How many time have you seen a demo where someone draws a wave on a balloon then blows it up? The problem with thinking of expanding space ...

Let me point out that the reason for much confusion is that the familiar word "expansion" has connotations which have nothing to do with changing metrics --- which are of the essence in your thread, Wallace.

In practical ordinary life, e.g. in introductory physics, "expansion", say of a heated rod, is measured relative to a practical invariant standard, say an invar measuring tape. Our familiarity with such a typical use of the word "expansion" has unfortunately come to colour our thinking about cosmology.

In cosmology, "expansion", refers to something altogether more strange and unfamiliar to practical ordinary life: a change in a metric coefficient a(t) in the expression:

the FRW metric for a flat universe:

[tex] d\tau^2 = -dt^2 + a(t)[dr^2 + d\Omega^2] [/tex]

"Expansion", used to describe this change, is shorthand for "the effect produced by that change in the ratio of the metric coefficients of the FRW metric that leaves physics invariant". We have no everyday word for such a change. But the historically natural choice of "expansion" as shorthand for this phrase has turned out to be unfortunate. It produces much confusion about "expanding space".

Also, it seems, there are confusions about what happens to photons as they travel between emission and absorption (as expressed in this thread by Marcus). The correct view of redshift is, as you have explained, Wallace:

I do think it is better to think of (photons) as being redshifted by being observed in a different frame ...Now at t ticks along, the scale factor a(t) increases. Therefore two observers who are both at rest wrt to the CMB, but who have different times t will therefore be in different frames (have different metrics). This is what leads to photons being redshifted when observed and emitted at different times.

I guess it's far too late now to invent a new word to describe the changes in the metric of spacetime that cosmologists have uncovered. They will just have live with the semantic confusions about expansion that they have inherited.
 
  • #57
P.s.

Wallace said:
two observers who are both at rest wrt to the CMB, but who have different times t will therefore be in different frames (have different metrics). This is what leads to photons being redshifted when observed and emitted at different times.

I agree fully with what you say here, Wallace, as I said in my previous post. This post is to unpack some implications of what you said.

It is that the observers and the photons that are emitted or absorbed (all of which are very localised, in our case deep inside a gravitationally bound galaxy) will have as their local metric the FRW metric of the universe that prevails at the time of emission or absorption.

This means that at a particular instant of universal time the FRW metric of the universe prevails on all scales and everywhere, even inside gravitationally bound structures or within atoms. And that using it with the appropriate scale factor will always yield the same physics. The only caveat is that this metric, which changes dramatically over times comparable with the age of the universe, may be perturbed by local mass concentrations. In a laboratory or observatory these perturbations are hopefully not large enough to perceptibly affect spectral light.

Using the word "expansion" to describe what is going on in the universe doesn't sit well with this situation. Something like "Universal rescaling" might fit better.
 
  • #58
oldman said:
In cosmology it is obvious that talking of "expanding space" , as if space was an elastic continuum is just plain silly. For instance, Matter --- crystals, atoms, nuclei and suchlike --- are mostly empty "space", as is the dispersion of galaxies that constitutes the universe. If you describe change in the universe as the expansion of "space" you are then faced with the difficulty of
distinguishing between two sorts of "space", non-expanding "space" in microscopic interstices and inter-galactic- cluster “ expanding space”.
You alluded to other difficulties but did not specify them so they may be cogent, I can't say. Furthermore, I am not an expert in GR so the following comments may be subject to correction. However, I don't see this "two sorts of space" as a difficulty at all. IF you accept the "expanding space" view, THEN: Within an atom, the particles are bound by forces as strong as the chain mentioned in post #3 in this thread. As space expands, it oozes right out of the atom like bread rising around raisins that are held fixed by chains.
 
  • #59
jimmysnyder said:
...I don't see this "two sorts of space" as a difficulty at all. IF you accept the "expanding space" view, THEN: Within an atom, the particles are bound by forces as strong as the chain mentioned in post #3 in this thread. As space expands, it oozes right out of the atom like bread rising around raisins that are held fixed by chains.

I agree with your IF...THEN; there is always a way of resolving problems by exercizing one's imagination. The as-yet unexplored rheology of "space" may yet come to be of interest to cosmologists! Thanks for the comment.
 
  • #60
oldman said:
This means that at a particular instant of universal time the FRW metric of the universe prevails on all scales and everywhere, even inside gravitationally bound structures or within atoms. And that using it with the appropriate scale factor will always yield the same physics. The only caveat is that this metric, which changes dramatically over times comparable with the age of the universe, may be perturbed by local mass concentrations. In a laboratory or observatory these perturbations are hopefully not large enough to perceptibly affect spectral light.

Ouch! I have to step in a disagree with you here oldman (I permit you to ignore me as a young whipper snapper if you wish :biggrin: ). The FRW metric, and the 'expansion' (I agree that's an awful word to describe the effect) does not have an effect on small scales in our universe. Local mass concentrations are far more than merely a perturbation to FRW on familiar scales. If anything there is a vanishingly small FRW element to the metric of bound structures.

If the FRW metric 'prevail(ed) on all scales and everywhere, even inside gravitationally bound structures or within atoms' then why do galaxies maintain a constant size as the distance between them expands? Commonly we are told that the local mass concentration 'overcomes' the expansion preventing this from occurring. This is one of the worst and most fallacious explanations you could possibly give someone! What really happens then?

The FRW metric is the inevitable result of the cosmological principle, which is that the universe is homogeneous and isotropic. The metric is only valid if these principles hold. Consider now a galaxy, solar system or planet. Does the CP hold? No. Is it a remotely useful approximation? Not at all! Unsurprisingly then the dynamics of bodies in these systems and on these scales bears no resemblance to the dynamics of galaxies. So for instance, there is no redshift of light due to a(t) when we observe light from the other side of our galaxy, or from say Andromeda. The FRW metric simply is not valid on these scales.

Remember that there is no force due to gravity. However when we say that a galaxies mass 'overcomes' the expansion we on the one hand mistreat the expansion of the universe as acting as some kind of viscous force then on the other hand treat the galaxy self gravity as a force overcoming this global effect. This is a horrible hodjpog situation. The better way to look at it is that the presence of the mass in the galaxy gives the metric of space-time around this mass a form that would look much more like a Schwarzschild metric than FRW (though we cannot fully solve GR for a galaxy..). The point is though that there is not expansion to 'overcome' since the 'expansion' is merely the result of the metric formed by a homogeneous and isotropic mass distribution. If the mass dosn't obey these principles we shouldn't be surprised that we don't see any 'expansion'.

If you don't believe me hold an object in each hand with outstretched arms. When you let them go what happens? I think you will find that they both plummet towards the local centre of mass (the centre of the Earth) rather than drift off into the Hubble flow! The local mass concentration can hardly be described as a mere perturbation to the FRW metric!

Sorry for all the !'s I get rather excited when I talk about this stuff :redface:
 
  • #61
Wallace said:
Commonly we are told that the local mass concentration 'overcomes' the expansion preventing this from occurring. This is one of the worst and most fallacious explanations you could possibly give someone!
I could not agree more on this.

Wallace said:
Sorry for all the !'s I get rather excited when I talk about this stuff :redface:
Well, keep talking about it! :smile:

By the way an excellent explanation Wallace!
 
  • #62
Wallace said:
The FRW metric, and the 'expansion' does not have an effect on small scales in our universe. Local mass concentrations are far more than merely a perturbation to FRW on familiar scales. If anything there is a vanishingly small FRW element to the metric of bound structures.

I guess our disagreements are more a case of crossed wires (due to my careless writing about gravitationally bound structures) than a disagreement. At least I hope so!

I see it like this:

First, consider a classic dust-filled fluid-like universe described by the FRW metric, but filled with CMB radiation (no structures). Let all dust particles be at rest in their local CMB frames (no peculiar motions). The Cosmological Principle rules strictly here. Yet the proper separations of dust particles vary as a(t) changes with time! Strange but true! This is GR, remember ... not simple expansion.


In bound structures with finite volumes cohesion (due to any kind of nature's interactions ... gravitational, electroweak or strong) endows individual components of the structure with kinematic or coasting velocities relative to local CMB frames. These velocities can ensure that proper distances between components remain constant while H remains constant. (Please don't ask me what happens in an inflating, accelerating or decelerating universe!). Ask Pervect!

Thus, in a gravitationally bound galaxy, stars on opposite sides have coasting velocities relative to their local CMB frames. And, atoms in a molecule in deep space have (very, very, very tiny) velocities relative to their local CMB frames.

This is how, as you say:

galaxies maintain a constant size as the distance between them expands?

while the FRW metric nevertheless "prevails" everywhere (a poor word choice of mine, I'm afraid). I should have said "applies but imperceptibly in cases of practical interest". Perturbation was also the wrong word to use.

But then I could accuse you of my own sin, when you say:
The FRW metric simply is not valid on these scales.

valid?
 
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  • #63
I think we pretty much agree on clarification and reflection. I just think it's important to be clear that the 'expansion' (which we both definitely agree is a bad term for it!) is a result of the FRW metric, in particular a(t). The metric in the region of bound structure looks nothing like the FRW metric, in particular it has no global time dependence (though will of course evolve). For this reason I stand by the statement that the FRW metric is not valid on scales which are significantly inhomogeneous, since the metric has no component that reflects the global a(t), and hence the FRW picture does not relate to the dynamics of the system.
 
  • #64
I can't refrain from pointing out that during inflation, where H increases exponentially (if I remember correctly), even the tiniest structures experience a huge disruptive force as a result of the rule of the FRW metric. Pervect convinced me that these forces are vast enough to tear apart just about any structure we can imagine. Yet nobody seems to take notice of this extreme effect! Strange.
 
  • #65
Isn't it possible that close galactic clumps currently receding could in a matter of say 10 billion years, be seen as a part of a larger system, by which anyone of them could have a blue shift from our vantage point? This is something tantamount to the universe evolving, wherein our current universal state of evolution is that of clumps of galaxies held together by gravity, and a later state of evolution we will see clusters of clumps held together by gravity, and further down the line ... clumps of clusters held together by gravity?

It would seem realistic to assume that earlier on in the history of the universe, that all galaxies were seen to be receding from our vantage point, and much earlier than that, matter the size of a standard average every day sun would see all suns receding from the vantage point of every sun. Taken much earlier - all atoms are seen from the vantage point of any atom to be receding.

If this is so, then the idea of space expanding seems frivolous. An accelerated expansion even more so.
 
  • #66
Castlegate said:
Isn't it possible that close galactic clumps currently receding could in a matter of say 10 billion years, be seen as a part of a larger system, by which anyone of them could have a blue shift from our vantage point? This is something tantamount to the universe evolving, wherein our current universal state of evolution is that of clumps of galaxies held together by gravity, and a later state of evolution we will see clusters of clumps held together by gravity, and further down the line ... clumps of clusters held together by gravity?

It would seem realistic to assume that earlier on in the history of the universe, that all galaxies were seen to be receding from our vantage point, and much earlier than that, matter the size of a standard average every day sun would see all suns receding from the vantage point of every sun. Taken much earlier - all atoms are seen from the vantage point of any atom to be receding.

If this is so, then the idea of space expanding seems frivolous. An accelerated expansion even more so.

I'm not quite sure what your point is. A hypothetical universe where everything was inititally redshifting, continuously evolving towards less and less redshift, then started to blueshift is imaginable. Furthermore, it is imaginablie in the context of GR - such a universe would have a matter density over the critical, and would be a universe ending in a "big crunch".

However, experimental data does not support this. As is widely known, the universal expansion appears to be accelerating.

I don't understand why you think the experimental data that says that the universe is expanding and that the expansion is accelerating is "frivolous".

The redshift data is pretty clear evidence that the universe is currently expanding. Even more convincing is that fact that distant supernovae explosions appear to happen "in slow motion" due to time dilation - this basically rules out "tired light" type theories. So for starters, I hope we can agree that the universe is currently expanding.

It does require GR to go beyond saying that the universe is currently expanding to interpret the data as saying that the expansion is accelerating and will continue to accelerate. It is possible using some non-GR theories ,to have a non-accelerating expansion that fits the data. AFAIK there isn't any alternative theory "in play" that suggests that the expansion could reverse, however.

Basically, GR isn't "frivolous" - it's being tested even as we speak. One of the simplest forms of the alternative gravitational theories that would allow non-accelerating expansion has recently been ruled out by the Gravity probe B experiments, for instance. Other more complex forms of this theory may still be viable, but GR has a long history of making good, correct predictions.
 
  • #67
oldman said:
I can't refrain from pointing out that during inflation, where H increases exponentially (if I remember correctly), even the tiniest structures experience a huge disruptive force as a result of the rule of the FRW metric. Pervect convinced me that these forces are vast enough to tear apart just about any structure we can imagine. Yet nobody seems to take notice of this extreme effect! Strange.

I think that the people concerned with large scale structure have to take this effect into account - but they don't tend to popularize their work in such simple terms.

There is a rather subtle point here - when one looks at the actual tidal forces generated by the Riemann, it is not the expansion of the universe that causes these forces, but the acceleration of the expansion of the universe. This is a point that's been discussed before, but I'm too lazy to look up the threads unless there is some renewed interest. Basically, if one considers a universe with no cosmological constant and no "dark energy", there could never be "tearing apart" tidal forces, there could only be compressive tidal forces.

Of course, inflation has effectively an extremely large cosmological constant - and this is what causes the "tearing apart" forces in the usual inflationary schenario.
 
  • #68
Wallace said:
I think we pretty much agree on clarification and reflection. I just think it's important to be clear that the 'expansion' (which we both definitely agree is a bad term for it!) is a result of the FRW metric, in particular a(t). The metric in the region of bound structure looks nothing like the FRW metric, in particular it has no global time dependence (though will of course evolve). For this reason I stand by the statement that the FRW metric is not valid on scales which are significantly inhomogeneous, since the metric has no component that reflects the global a(t), and hence the FRW picture does not relate to the dynamics of the system.


While it may not make sense to use a pure FRW metric to describe a bound system, there are some papers that take the approach of using for instance a Schwarzschild De-sitter metric. These approaches can be justified more rigorously if one imagines that the dark matter and dark energy are isotropically distributed, and that the lumpiness is only in the normal matter.

This approach won't model effects due to "lumpiness" in the dark matter, for instance, but one can probably gain some insight into the effects of lumpiness in the dark matter by the effects of lumpiness in the non-dark matter.

The whole dark matter issue is rather annoying if one isinterested in gravity - basically, the source of most of the gravity in the universe appears to be invisible :-(.
 
  • #69
I'm not quite sure what your point is. A hypothetical universe where everything was inititally redshifting, continuously evolving towards less and less redshift, then started to blueshift is imaginable. Furthermore, it is imaginablie in the context of GR - such a universe would have a matter density over the critical, and would be a universe ending in a "big crunch".
Your assumption of a 'big crunch' would be true if you assumed a 'big bang'

I don't understand why you think the experimental data that says that the universe is expanding and that the expansion is accelerating is "frivolous".
I'm not saying that an accelerating expansion is frivolous. Only that if the assumption posted earlier by me were true, that the expansion of space is a frivolous thought. Do you consider the idea of greater masses evolving to even greater masses as frivolous? It is at least a logical progression one might expect irrespective of other known measurements.
My way of creating new angles by which to look at the same thing. In other words - If galaxy clusters are to eventually cluster themselves, what model of a universe would bring this about? Big bang does not fit this bill. This is just thinking out of the box that most of us put ourselves in, and I leave the box because all six sides represent dead ends.

A model that produces an evolving scenerio as described is possible by the way. Doesn't make it right ... but possible?
 
  • #70
Castlegate said:
Do you consider the idea of greater masses evolving to even greater masses as frivolous? It is at least a logical progression one might expect irrespective of other known measurements.
My way of creating new angles by which to look at the same thing. In other words - If galaxy clusters are to eventually cluster themselves, what model of a universe would bring this about? Big bang does not fit this bill. This is just thinking out of the box that most of us put ourselves in, and I leave the box because all six sides represent dead ends.

A model that produces an evolving scenerio as described is possible by the way. Doesn't make it right ... but possible?

Hmm the current model of the universe predicts that mass gets increasingly lumpy as time goes on. Lumps of mass do indeed get more massive as time goes on, clusters form etc etc. I don't follow your previous logic as to how this is a problem but rest assure that far from being incompatible with the Big Bang it is a prediction (and observation!) of current cosmology.
 
<h2>1. Does space really expand?</h2><p>Yes, according to current scientific theories and observations, space does expand. This expansion is known as the expansion of the universe.</p><h2>2. How does space expand?</h2><p>Space expands through a process known as cosmic inflation, which is believed to have occurred shortly after the Big Bang. This inflationary period caused the rapid expansion of the universe and continues to this day, although at a much slower rate.</p><h2>3. What evidence supports the idea of space expanding?</h2><p>There are several pieces of evidence that support the idea of space expanding. One of the most notable is the observation of galaxies moving away from each other, which is known as the Hubble's law. Additionally, the cosmic microwave background radiation, leftover radiation from the Big Bang, also supports the idea of cosmic inflation and the expansion of space.</p><h2>4. Is there a limit to how much space can expand?</h2><p>The current understanding is that space does not have a limit to how much it can expand. However, the expansion rate may change over time, and there are theories that suggest that the expansion may eventually slow down or even reverse.</p><h2>5. How does the expansion of space affect objects within it?</h2><p>The expansion of space does not directly affect objects within it. This is because the expansion happens at a much larger scale than the objects in our everyday lives. However, the expansion does have an impact on the distance between objects, causing them to move further apart over time.</p>

1. Does space really expand?

Yes, according to current scientific theories and observations, space does expand. This expansion is known as the expansion of the universe.

2. How does space expand?

Space expands through a process known as cosmic inflation, which is believed to have occurred shortly after the Big Bang. This inflationary period caused the rapid expansion of the universe and continues to this day, although at a much slower rate.

3. What evidence supports the idea of space expanding?

There are several pieces of evidence that support the idea of space expanding. One of the most notable is the observation of galaxies moving away from each other, which is known as the Hubble's law. Additionally, the cosmic microwave background radiation, leftover radiation from the Big Bang, also supports the idea of cosmic inflation and the expansion of space.

4. Is there a limit to how much space can expand?

The current understanding is that space does not have a limit to how much it can expand. However, the expansion rate may change over time, and there are theories that suggest that the expansion may eventually slow down or even reverse.

5. How does the expansion of space affect objects within it?

The expansion of space does not directly affect objects within it. This is because the expansion happens at a much larger scale than the objects in our everyday lives. However, the expansion does have an impact on the distance between objects, causing them to move further apart over time.

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