A thought experiment regarding the nature of space expansion

In summary, the conversation revolves around the concept of expanding space and its effects on various objects and phenomena. The participants discuss a nifty device that can cause spatial distortion and its potential implications. They also touch on the idea of relativity and how it applies to expanding space. The conversation ends with one participant expressing their confusion about the accepted physics of spatial expansion.
  • #1
salvestrom
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I devised this using a setup that I found easiest to imagine, while trying to be clear on how modern physics considers space to expand.

We have a nifty device that can cause spatial distortion (no, you cannot has one!). Before we use it, we lay down two meter rulers, end to end. They both look the same, so we get down to business. Turning on our device, we expand a cubic meter of space to twice its original size. I have three basic questions. I'll provide my current answers, based on how I understand space to work. Number 2 is my most hazy. I treat the cut off between normal and expanded space as being sharp rather than smooth, for convenience.

1) What has become of the one meter ruler where our space has been expanded?
2) What happens if we pick up the unaffected ruler and place it halfway into the expanded space?
3) What happens when we shoot a beam of light across the expanded space?

I say "we" to make you partly culpable if the experiment breaks the universe.

My answers are:

1) To the external observer the ruler appears twice as large.
2) The part we put in the expanded space will become larger, and then shrink to a perceived normal once removed.
3) This seems to be a nice example of relativity. Since the external observe would measure the external dimensions of the space as 2m, light apparently moves twice as fast. So we must assume that light, at least, behaves as if the expanded space were two meters across. However, an 'internal' observer inside the space now would see light moving at half speed. And so we must conclude apparent time dilation.

I was interested to note that the external observe can use the internal observers clock and ruler to get the correct speed of light, instead of his own, but he can't use his ruler and the internal observers clock, or vice versa.

So there's my take. One question about it is the use of relativity here. Part of me sees that the light can clearly be seen by both observers to travel at the correct speed, since the external observer can see the ruler and understand that the light isn't actually moving faster, just that space is bigger. Is the issue then that light will naturally take twice as long to travel across the expanded space, therefore requiring relativity to keep c constant? In a sense, light is not affect by expanded space in the same way the ruler is (assuming I even answered question 2 correctly?

Comments?
 
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  • #2
Assuming that your machine works the same way as normal expansion:

1. Your ruler is still the same size. Gravity, the EM force, and the Strong Force are all much much stronger than the expansion of space.

2. This cannot happen, so I don't know what to tell you.

3. Nothing. The space is no longer expanding and acts exactly like normal space. Normal expansion is continual and will stretch the wavelength of the light as it travels.
 
  • #3
Drakkith said:
Assuming that your machine works the same way as normal expansion:

1. Your ruler is still the same size. Gravity, the EM force, and the Strong Force are all much much stronger than the expansion of space.

2. This cannot happen, so I don't know what to tell you.

3. Nothing. The space is no longer expanding and acts exactly like normal space. Normal expansion is continual and will stretch the wavelength of the light as it travels.

So, when expanding space is discussed, the idea is that there is more space coming into existence? I want to be really clear on this.
 
  • #4
salvestrom said:
So, when expanding space is discussed, the idea is that there is more space coming into existence? I want to be really clear on this.

Either that or space is simply expanding. Either way works fine as far as I know.
 
  • #5
We have to be careful about saying what an outside observer can see. There is no way to be an observer outside the universe and no way to make a machine in the universe that can expand space. We can visualize scenarios; but what you're doing in this case is breaking the laws of physics so we can observe the laws of physics in action. We can't create a separate chunk of universe one cubic meter in size and expand it. We don't know exactly why the real universe began to expand, though we have theories. These are fun games; but don't expect real answers to questions about impossible physical experiments.
 
  • #6
Impetus said:
We have to be careful about saying what an outside observer can see. There is no way to be an observer outside the universe and no way to make a machine in the universe that can expand space. We can visualize scenarios; but what you're doing in this case is breaking the laws of physics so we can observe the laws of physics in action. We can't create a separate chunk of universe one cubic meter in size and expand it. We don't know exactly why the real universe began to expand, though we have theories. These are fun games; but don't expect real answers to questions about impossible physical experiments.

Yet he did provide real answers. Well, as real as they can be, since we lack the ability to fly into the voids between clusters and see what's going on. In order to begin answering the question, we need to have an idea of what we think that answer might be. He provided a framework in which to consider a variety of issues that might lead to a practical hypothesis that might also be testable. He particularly highlighted a flaw in my thinking, reminding me that the inherent gravitational field of the ruler would counter a lot of the affect.

But I still feel like I'm not on top of the accepted physics of spatial expansion. Growing, expanding and stretching are three very different words that I hear used interchangeably.

Clearly from my own answers to the thought experiment I considered space to stretch, such that each point of spacetime was pulled further apart from every other point. An overly literal quantisation of spacetime? It naturally begs the question of what I think is occupying the spaces between the spaces. Obviously not crystal aliens. :P

If energy only exists in discrete amounts, why not spacetime? /shrug. It's only half a thought at the moment. For now, I'm interested in understanding what is considered the mainstream description of expanding space.
 
  • #7
Your idea of spacetime existing in the equivalent of quanta in matter and energy is pretty much what I am thnking, as well as others. See the latest Scientific American, Feb. 2011. The article on pg. 30 is entitled Is Space Digital? by Michael Moyer. Re: growing, expanding and stretching, growing is more or less colloquial. Expanding (of space) takes matter with it and simply separates matter without expanding it. The reason it appears that energy is expanding is that the matter further from us that sends light is traveling faster and faster with distance from us, as you well know. This is simply doppler shift. Talking about the expansion of the universe stretching energy is just confusing the issue. Think doppler. Rulers don't expand, though, if we could see rulers lightyears away, they would appear shorter in the direction of their motion away from us. Stretching, in my experience, is simply an easy word to express the doppler shifting of light frequencies. In fact, nothing actually stretches. It is just that, as the electromagnetic radiation was being generated, the generating object was moving away at a speed that made the "pitch" lower. I know most of this is simplified and I apologize if any of it makes you feel I am talking down to you. I have simply found that simpler is usually best for visualization as long as it's not so simple as to be wrong.
I'm not sure the "expansion" of space (or spacetime) is not more than a geometrical construct. If we get to the point that we know what a gravitational field is, we will probably find out. I have postulated that gravity is the time gradient generated by a massive object; but I don't know exactly how this gradient (or field) is generated. Physicists talk about gravity traveling at the speed of light; but gravity is always just there. It doesn't have to travel like a light beam to get to the object. It affects any object instantly at any point. So, spacetime really is gravity, or the intermix of time gradients throughout the universe. And, I think it's digital. You can imagine time "stretching" or dilating. Things speed up in the absence of gravity (no time dragging). In space far from any massive body, time clicks at its fastest rate. The expansion of space dilutes gravity (time gradients) by the simple inverse square law. Michael Greene has stated that objects tend to migrate toward regions where they age slower; but even he has not actually equated the time gradient with gravity. He has said that people think of warped space; but they can't visualize warped time, which is what is really taking place around massive bodies. That's close to my theory of gravitational impetus; but not there yet. His book, The Fabric of Space, is nevertheless the way to get the best "mainstream description of expanding space." Putting mathematics to words is fraught with danger, as was indicated by your questioning of the words growing, expanding and stretching. There's no such confusion in the mathematics, though every solution seems to have two explanations for the same end result. Does the airplane get lift by the differential between pressures above the wing and below, or is it because the mass of air diverted downward by the wing equal the mass of the airplane? Both. It's like that.
 
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  • #8
Impetus, I don't believe that the increasing redshift we see when looking at far away galaxies can be explained by doppler shift due to velocity through space.
 
  • #9
If space is getting... 'more' ... then I have questions about conservation of energy.

When I talk about stretched, I mean pulled apart in all directions, so that each point of spacetime is separated from every other point. It naturally creates a void between these points - rather, straightforward quantisation does. I'm not happy with that - a series of bubbles sperated by utter nothingness. Seems redundant. I am currently thinking more in terms of a quilt, where, in 2D, squares of spacetime are side by side and distort as space is literally stretched out.

I was thinking in terms of stretching like this as it seemed a logical opposition to the compression of spacetime as it is curved by mass. I've never heard it ever suggested there is less space when curvature occurs, so it seems odd to then hear it said there is more space in the absence of matter. I have been thinking a lot lately of space being elastic and highly malleable. I put less emphasis on time, but doing so tends to lead me... further astray ;)
 
  • #10
Conservation of energy does not apply at the cosmological scale, only locally. A perfect example is the redshift of light due to expansion. Where has the energy gone??

Also, just ignore quantization of space. Unless you are on the cutting edge of theoretical physics and understand the math used, you don't have a chance of really finding out anything. For now just realize that if two points in space are separated by expansion, there is still space in between those original points.
 
  • #11
Drakkith said:
Conservation of energy does not apply at the cosmological scale, only locally. A perfect example is the redshift of light due to expansion. Where has the energy gone??

Also, just ignore quantization of space. Unless you are on the cutting edge of theoretical physics and understand the math used, you don't have a chance of really finding out anything. For now just realize that if two points in space are separated by expansion, there is still space in between those original points.

I think it would be fairer to say I have little chance of proving anything. Quantisation of space is a fairly natural conclusion to reach given the direction physics has generally been heading. But since it isn't mainstream, let's set it aside, and discuss your opening statements.

Conservation of energy. I know the virtual particlees also don't obey the conservation, but do not exist long enough to matter, but it all sounds an awful lot like playing games. The same goes for notions of new space. If new space is created then it must contain new vacuum energy. On what basis has the law of conservation of energy been put aside on the cosmological scale? Is it no longer consider so that the universe contains no more energy than when it began?

I can't actually respond to your question in context of the accepted expansion - but the wording implies that energy is lost by the increased wavelength and goes nowhere. I could answer it vaguely using the subject I agreed to set aside, but that's not going to be helpful to you, who won't be interested, or me, who actually wants to understand the current model.
 
  • #12
Just a quick reply about where the energy goes when the expansion of spacetime "stretches" the frequency of light: The light had a higher frequency over a shorter distance before expansion. After expansion it has a lower frequency over a longer distance. Higher frequency, greater energy; lower frequency, lesser energy. But, the same number of light cycles represents the same amount of total energy per cone of light volume. In general terms, a lot of red light has the same energy as a little bit of violet light.
 
  • #13
Impetus, the key is that single photons are being redshifted. They are not being turned into more lower energy photons. I've also never heard of this "cone of light volume".

Salvestrom, I wish I could elaborate on the conservation of energy relation to Cosmology, but I am not educated enough on the subject. All I know is that, to my knowledge, energy is not conserved on a universal scale. Of course the fact that I've repeatedly seen the statement "energy is not well defined in GR" could have something to do with this.
 
  • #14
Drakkith, The definition of doppler shift is the changing of frequency due to the approach or recession of the object producing the vibration, whether sound or light, to or from the observer. In the case of sound, the air between doesn't expand. In space, we say that spacetime expands, but that doesn't preclude it still being a doppler shift. In the case of spacetime, it seems that there is nothing required to "carry" electromagnetic waves. They are self-carrying; but doppler effects still occur.
 
  • #15
Drakkith, You are right. More, lower energy photons are not being produced by the expansion of space; but every photon is a wave that travels through space creating, in effect, the "appearance" of a long wave passage, a long string of individual photons. The emitting object can be considered as moving away from us, or we can be considered as moving away from the emitting object. Either way, that chain of waves appears to have a lowered frequency and less energy. Here's a thought experiment: If you get in your spaceship and travel toward the light source, it will appear to blue shift, even though it was red shifted due to the expansion of the universe. If you go fast enough, you will observe blue, violet, ultraviolet and on up to x-rays just because you are heading into the oncoming frequency and increasing its effect on you. The light didn't change; but your observation sees it as x-rays; and they will go through your body, even though an observer on the Earth you left would eventually see the same chain of waves as, say, yellow.
 
  • #16
Re: conservation of energy universally -- Gravity is generally considered negative energy. Matter and energy are equivalent; so we will call them both energy for the purposes of this comment. Energy in the form of matter and the energy associated with it are considered positive energy. The total of Gravity and energy is zero and this is conserved. If you think about it, where could energy go. It can be exchanged with gravity; but neither gravity nor energy has anyplace to go since, for us, there is no "outside" to the universe.
 
  • #17
Impetus said:
Drakkith, The definition of doppler shift is the changing of frequency due to the approach or recession of the object producing the vibration, whether sound or light, to or from the observer. In the case of sound, the air between doesn't expand. In space, we say that spacetime expands, but that doesn't preclude it still being a doppler shift. In the case of spacetime, it seems that there is nothing required to "carry" electromagnetic waves. They are self-carrying; but doppler effects still occur.

I'm not sure whether you have changed your mind about the cosmological redshift. Are you still treating it as a doppler effect? I had a response to that but if you now agree with Drakkith about it I won't bother.
 
  • #18
Marcus, The speed of the expansion of the universe at the time of the CMB emissions was extreme. It was, as I recall, just after the time of inflation. (Correct me if I am wrong; I am not taking the time to look it up.) You don't have to sum every photon in a string of photons to get the red shift. All you have to do is know the recession speed of a galaxy. It doesn't take big math to calculate it. I won't belabor the point; but, whether or not you accept that doppler shifting takes place, you can use it to determine the frequency of light. If you want to also consider the expansion of spacetime, it can be treated as a separate problem. Physicists that I know haven't yet even agreed that the expansion of spacetime (or its warping around a gravitating body) is more than a geometric construct. We do know that the universe is expanding; but there's not universal agreement that spacetime is. For example, the space between atoms is huge compared to the nucleus and the electrons of the atom. That space is not expanding. And the space between the molecules of our body is not expanding. At what point can we say space IS expanding?
At some point out at the edge of the universe we can (theoretically) observe, galaxies are receding at the speed of light. At that point, the red shift of all light reaches a frequency of zero. The visible edge of the universe is as if it were a black hole. No light can come to us from beyond it. Forgive me. I'm introducing much to much.
 
  • #19
Actually, the speed of expansion when CMB photons were being emitted was rather pedestrian, inflation was a distant memory. I take it you don't know many physicists.
 
  • #20
I'm actually having some trouble determining how Doppler Redshift and Relativistic Redshift are actually told apart. I understand the Redshift concept: a light spectrum contains black lines that indicate the elements the light has passed through, shifted toward the red end of the spectrum. The amount of shift indicates the speed of recession. But what is it that makes relativistic redshift 'stand out'? Is it an extreme shifting, indicating FTL speeds, which requires explanation? Is there anything else coming into play?

Also, is a photon able to radiate heat? 13 billion years in absolute zero? They aren't strictly massless either, right, it's just considered negligable, kinda leading me toward the idea that 13 billion years of even a minute gravitational affect must surely add up to something meaningful? Again, though, I'd be interested to hear more on just what defines a redshift as relativistic over doppler.

Oh, also, just to test my own knowledge, growing at pedestrian speed: the CMB photons were emitted prior to the inflationary period, which is at least part of the reason we see them entirely around us and it has taken them so long to get here.
 
  • #21
Photons do not radiate heat, as thermal radiation IS EM Radiation. And yes, they are considered to be massless. The most sensative experiments have determined that a photon cannot be more massive than 1×10−18 eV/c2 or else we would have detected it. So while we cannot know for 100% sure that it is massless, if it does have mass then it's so small that it has almost zero effect on anything. Future experiments will probably reduce that maximum even smaller.

The CMB was emitted about 377,000 years after inflation ended. The reason we see them everywhere is because they emitted from everywhere. Also, we have been bombarded by CMB photons since they were created, it's just that over time the photons that reach us have been traveling further. The close ones have already interacted with us or passed us by to be seen by other possible observers billions of light-years away.
 
  • #22
Impetus said:
Drakkith, The definition of doppler shift is the changing of frequency due to the approach or recession of the object producing the vibration, whether sound or light, to or from the observer. In the case of sound, the air between doesn't expand. In space, we say that spacetime expands, but that doesn't preclude it still being a doppler shift. In the case of spacetime, it seems that there is nothing required to "carry" electromagnetic waves. They are self-carrying; but doppler effects still occur.

I am not sure what point you are making. Cosmologists do not normally treat the redshift z as a doppler effect because there is no obvious relative speed (when we are talking about cosmological distances, not nearby stuff).
Emitter and receiver are not in the same reference frame. It gets very messy to try to analyze it doppler-fashion because one has to use a concatenation of many small dopper effects to get the right answer. An analysis like that was done by Ted Bunn to demonstrate the principle. But it is cumbersome and messy. As a practical matter, nobody treats redshift (over large distances) as doppler.

It will help you understand why if you consider a familiar example. I'm sure you know, if you follow cosmo at all, that the CMB redshift is about 1100. It came from hot gas at temp of 3000 kelvin and the temp is now a bit less than 3 kelvin.
The presentday distance to the material which formed the hot gas (whose glow we see) is estimated at 45 billion ly. To confirm, google "wright calculator" and type 1100 in for z.

That is what you would measure if you could stop the expansion process at this moment and use some conventional method like radar ranging.

At the time of emission the distance to that matter from our matter is estimated to be about 41 million ly. That is, 45 billion divided by 1100.

So what is the SPEED that you want to have caused the doppler effect?

What speed corresponds to the z = 1100 redshift we are talking about? (if you insist on treating it as a doppler effect of some speed).

I think you will appreciate the awkwardness, if you think about it, of trying to treat redshift as doppler. It is very inconvenient. Cosmologists normally use a different formula which is not doppler but is simply based on the expansion history a(t). 1+z = a(now)/a(then)
The ratio of distances now to distances back then when the light was emitted.
IOW the wavelengths are enlarged by exactly the same factor by which distances increased during the light's transit time.
 
  • #23
Marcus, using the calculator here (http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redshf.html) I input 1100 for z and got 0.9999983501099652 c. This is the velocity required to give the same redshift correct? That would mean that at the time of emission, the matter in that area was receding from us at that same speed?
 
  • #24
That [energy lost to the gravitational field] is one of the popular explanations for cosmological redshift. I think it is merely diluted [yes, I know - by what]. Would a redshifted photon think it 'lost' energy during its journey. I doubt it. But, photons have no concept of time.
 
  • #25
Personally I don't see how it would be lost to gravity, as given the homogenous and isotropic state of the Universe on the largest scales there should be equal gravity pulling it in all directions over the course of a long long journey of billions of years, right?
 
  • #26
Drakkith said:
Marcus, using the calculator here (http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redshf.html) I input 1100 for z and got 0.9999983501099652 c. This is the velocity required to give the same redshift correct? That would mean that at the time of emission, the matter in that area was receding from us at that same speed?

Thanks for the link! But that calculator is pretty much useless in an actual cosmology context. It does not include the expansion history of the universe in the form of the scale-factor. It is just a simple Doppler shift calculator---not a cosmological redshift calculator.

To answer your question: the recession rate THEN is typically different from the recession rate NOW. For example put in z=9 which is around the redshift of the first proto galaxies ---the earliest star formation.

If you are using a cosmological redshift calculator you will get something like recession rate NOW is 2.2c
and recession rate THEN was 3.7c.
Obviously there is a substantial difference!

My favorite* redshift calculator continues to be the simplest one mainly just because it is the simplest. It is not as precise as Ned Wright's (google "cosmo calculator") or Jorrie's (google "cosmocalc 2010") but for a lot of stuff I don't feel I need 5 decimal place accuracy :biggrin:

(Especially when the 3 basic model parameters you have to plug in are only known to at best 2 or 3 significant digits! Some like to use Hubble parameter 70, some like 71, some like 72.)

*My favorite for many purposes is Siobhan Morgan's. (google "cosmos calculator") She makes you put in values for the three basic parameters, so you realize that this is involved. I type in the same ones that Ned Wright uses:

.27 for matter fraction
.73 for cosmo constant
71 for presentday Hubble rate
 
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  • #27
As a rough consistency check, let's get the then and now recession rates for z=9 using two online calculators.
The link to Morgan (google "cosmos calculator") is in my sig at the end of the post, but googling is nearly as quick.

Morgan (after plugging in .27, .73, 71) gives now=2.2c and then=3.7c (or 3.67 if I don't round).

Jorrie (google "cosmocalc 2010") with Jorrie's DEFAULT parameters gives the SAME! 2.2c and 3.7c (or 3.68 if I don't round).

That is nice. They agree to two significant figures even though I just used Jorrie's default case (which is .272, .728, 70.4).

You can see Jorrie's default parameters are very close to .27, .73, 71 (which Ned Wright uses and which I type into Morgan's) so it is not surprising they give essentially the same answers---they agree to two significant figures.
It is not surprising they agree, but it is reassuring. It would suck if they were noticeably different.

Folks should be warned from using the hyperphysics online calculator. It's OK for small redshifts and small distances where there has not been significant expansion, but it does not even have the answer depend on the Hubble parameter. Translating from redshift to presentday and past distances and recession rates etc etc is model dependent in the sense that it depends on the basic model parameters which it is the cosmologists job to measure. If a calculator doesn't have boxes for the model parameters this tells me that probably whoever posted it on the web is cosmically naive :biggrin:
 
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  • #28
So, basically, the photon doesn't react with the proposed Higgs field, assuming it really does have absolutely NO mass, not even a 0.0...1 to some absurdly small magnitude. But that sounds immeasurable. So any proof of photon mass would actually need to come from experimental or observational evidence of gravitational interaction, beyond following curved space?

On the other hand its energy should still interact? Is this popular explanation you mention intended to remove the need for expansion to explain the redshift?

Also, thanks for clearing up the CMB thing. Pretty sure I understood that, but blanked out while typing, especially since the post above mine said the exact opposite of what I was putting.

Time. You guys ever heard of anyone explaining the expansion of space as a product of the passage of time? Can you point me at some literature?

Does the light experience any blue shifting as it enters any gravitationally curved space? In other words, can the light wave start to compress again in shrinking space?
 
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  • #29
To all, I wrote stuff at 1 a.m. this morning that I reviewed in the light of day.Chronos, I looked at my comment about physicists not agreeing on the expansion of spacetime. The fact of spacetime expansion was not what I meant was at issue. You surely have the right to question how many physicists I know based on that misstatement. What I meant to say was that physicists don't agree on what the expansion of spacetime is, or what warped space really is. It's easy to visualize a two dimensional surface warping or expanding into three dimensions. The Earth is a two dimensional surface that is warped (if I can use the term in this context) into a spherical, three dimensional shape. There is no way for the human mind to visualize three dimensions warping or stretching into a fourth dimensional shape, or four dimensional spacetime warping or stretching. It can be done mathematically in any number of dimensions is seems, just not visualized.
The other thing is that the doppler shift is more than the basic doppler shift when the speeds of recession significantly approach the speed of light. The equation for figuring that include, one way or another, two factors: One is basic doppler shift. The second accounts for the dilation of time which affects the frequency of light emitted with respect to the observer (us). And you could say that there is a third factor. The theoretically)visible edge of the universe where galaxies were moving away from us at the speed of light at the time of emission are now racing beyond the cosmic horizon at greater than light speed with respect to us. Of course, they might as well not exist. They are forever beyond any communication with us.
I will drop out of the conversation at this point as you all seem to have everything in hand.
One last comment. The physicist I most admire was Richard Feynman who I met when he was in Florida in '86 for the Challenger explosion investigation. I met and talked to him about my gravitational impetus theory. He showed interest in the idea. I had been working in aerospace some time prior to that with Ball Research Corp. in Boulder, Colo. I worked on the Apollo Telescope Mount on Skylab back in '68-'70. I know other physicists but none who I more admired. I have, since talking to Feynman, fleshed out more details of my impetus theory. I'm retired now. Bye all.
 
  • #30
Impetus said:
...The other thing is that the doppler shift is more than the basic doppler shift when the speeds of recession significantly approach the speed of light. The equation for figuring that include, one way or another, two factors: One is basic doppler shift. The second accounts for the dilation of time which affects the frequency of light emitted with respect to the observer (us). And you could say that there is a third factor. The theoretically)visible edge of the universe where galaxies were moving away from us at the speed of light at the time of emission are now racing beyond the cosmic horizon at greater than light speed with respect to us. Of course, they might as well not exist. They are forever beyond any communication with us.
I will drop out of the conversation at this point as you all seem to have everything in hand.
One last comment. The physicist I most admire was Richard Feynman who I met when he was in Florida in '86 for the Challenger explosion investigation. I met and talked to him about my gravitational impetus theory. He showed interest in the idea. I had been working in aerospace some time prior to that with Ball Research Corp. in Boulder, Colo. I worked on the Apollo Telescope Mount on Skylab back in '68-'70. I know other physicists but none who I more admired. I have, since talking to Feynman, fleshed out more details of my impetus theory. I'm retired now. Bye all.

I wish you a happy retirement. And good luck with your "Gravitational Impetus Theory".

It should be pointed out that the great majority of the galaxies which one can see, for instance with the Hubble Space Telescope, were receding faster than the speed of light at the time they emitted the light which we are now receiving from them. There is an elementary explanation of how their light is able to reach us despite superluminal recession, which is given in the "charley" link in my sig. We've gone over it many times here at PF.

The overall expansion of largescale distances is unlike ordinary motion because nobody gets anywhere by it. No destination is approached. Distances simply increase according to an overrall approximately uniform pattern.

To illustrate, the vast majority of currently observed galaxies have redshift > 1.7. The distance to a z=1.7 galaxy WAS increasing superluminal back when it emitted the light and STILL IS increasing at a superluminal rate. This has not made it impossible for the light to reach us. The reason is simple and explained in that Scientific American article by Charley Lineweaver that I mentioned.

To confirm this google "cosmocalc 2010" go to the top hit and put 1.7 in the z box. You will see that the "then" and "now" recession rates are both greater than the speed of light.

Or skip the google search by going directly to:
http://www.einsteins-theory-of-relativity-4engineers.com/cosmocalc.htm

So galaxies which were receding at rate c at the time of emission are certainly NOT at the "edge" of the visible universe! They are just z = 1.7 galaxies. The vast majority of visible galaxies are out beyond them. Way out beyond. It is in no sense an edge.

It does not make sense to say that galaxes with z > 1.7 "might as well not exist". In fact we study them with great interest and are learning more about the universe by observing them.
 
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  • #31
Since we get this kind of discussion fairly often as newcomers join the forum maybe I'll try to boil it down to something easier to remember. As a rough rule of thumb to get the recession rate (in units of c) from redshifts like z = 3 and 4, divide by 2. Or if you like decimal numbers multiply z by 0.5.

This gives the approximate recession rate when light was emitted. For instance for z = 3 the recession was a bit over 1.5 c, so that's right.
For z = 4 the recession rate was a bit over 2.0 c, so right again.
For z =5 this rule gives 2.5c and the right answer is 2.4 c, so still close.

As a crude approx therefore, you get the recession (when light was emitted) by multiplying redshift by a factor of 0.5. This works roughly in the range z = 3 to 5.

For larger redshifts up to z = 10 the factor to use is more like 0.4, which is usually a bit on the conservative side.

For z = 6 this gives 2.4c (the right answer is 2.75)
For z = 7 this gives 2.8c (the right answer is 3.0)
For z = 8 it gives 3.2c (the right answer is 3.4)
For z = 9 it gives 3.6c (the right answer is 3.7)
For z = 10 it gives 4.0c (the right answer!)

And the earliest protogalaxies are around z = 10. In a sense we don't have to go any farther with our approximation. At z=10 they were just beginning to form. Beyond that we can SEE stuff (e.g. CMB at z = 1100) but we don't normally see galaxies because they mostly haven't formed yet. So that factor of 0.4 works over a useful range.

check this by googling "cosmocalc 2010"
 
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1. What is a thought experiment regarding the nature of space expansion?

A thought experiment regarding the nature of space expansion is a hypothetical scenario created in the mind to explore and understand the concept of the universe expanding. It involves using logic and imagination to consider the potential outcomes and implications of such expansion.

2. Why is a thought experiment used to explore space expansion?

A thought experiment is used because it allows scientists to consider and test ideas that may not be possible to observe or replicate in a physical experiment. It also helps to challenge and expand our understanding of the universe.

3. How does a thought experiment about space expansion relate to real-world observations?

A thought experiment about space expansion can help us understand and interpret real-world observations, such as the redshift of distant galaxies and the cosmic microwave background radiation. It can also lead to new theories and predictions that can be tested through observation.

4. Are there any limitations to using thought experiments to study space expansion?

Yes, there are limitations to using thought experiments. They are based on assumptions and simplifications, so they may not accurately reflect the complexity of the real world. Additionally, they cannot provide concrete evidence and must be supported by observational data.

5. How can a thought experiment about space expansion contribute to scientific progress?

A thought experiment can contribute to scientific progress by providing new insights and perspectives on the nature of space expansion. It can also inspire further research and experimentation, leading to the development of new theories and advancements in our understanding of the universe.

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