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Author Topic: How can galaxies receding at faster-than-light-speed be observed?  (Read 8576 times)

Offline Benni The Bull

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I was reading a column today from an ongoing article that ended a couple of years ago. It was explaining the basics of the expansion of the universe. In the article, it adresses the question "are there galaxies that are moving away from us faster than the speed of light?" they answer, that yes, the distance between us and those galaxies is growing at a faster rate than the speed of light. If that statement is true, how is it that we would be able to observe these galxies if the distance between is us growing more quickly than the light can cover it?
« Last Edit: 21/10/2013 23:43:21 by chris »


 

Offline Phractality

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Re: A question concerning expansion
« Reply #1 on: 19/10/2013 06:16:15 »

The break-even point, where the distance between us and light source is growing exactly at the speed of light, is call the Hubble limit. If the rate of expansion of space, i.e. the Hubble constant H0 were actually constant in cosmological time, you could easily calculate the Hubble limit. It is simply c/H0, which comes to about 14 billion light years.

In one big bang scenario, I think, the Hubble limit divided by the speed of light (i.e. 1/H0.) is the age of the universe. If that is the case, then nothing in our universe exists beyond the Hubble limit. Other big bang scenarios, with a period of very rapid expansion, do allow for things to exist beyond the Hubble limit.

We are now fairly certain that the expansion has been accelerating for at least the last few billion years. So the Hubble limit might actually be farther than 14 billion light years (I think?  :-\ ).

Anyway, you are correct to point out that light from beyond the Hubble limit can never reach us, unless the expansion slows down.

You could debate semantically whether or not things beyond the Hubble limit have left our universe, or whether they even exist.
« Last Edit: 19/10/2013 06:21:51 by Phractality »
 

Offline Pmb

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Re: A question concerning expansion
« Reply #2 on: 21/10/2013 14:38:28 »
I was reading a column today from an ongoing article that ended a couple of years ago. It was explaining the basics of the expansion of the universe. In the article, it adresses the question "are there galaxies that are moving away from us faster than the speed of light?" they answer, that yes, the distance between us and those galaxies is growing at a faster rate than the speed of light. If that statement is true, how is it that we would be able to observe these galxies if the distance between is us growing more quickly than the light can cover it?
Since this is due to the expansion of space its possible for light to cover those distances as described. The only way I know how to explain more than this is in mathematical terms.
 

Offline chris

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The space itself has increased in scale. The galaxy is not moving faster than light. Light can still pass through the space, but the expansion causes red shifting of the light as it moves through.
 

Offline CPT ArkAngel

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This explanation is one of the basic principles on which the standard model of cosmology has been founded.

But there is no experimental proof of an expansion of space. This is a major problem in my opinion.

The problem of observing the universe is we can only do it from our frame of reference so the data cannot really validate a cosmological theory unless it is validated entirely in a laboratory where you are in proximity of the interactions. If there was a perfect standard "yardstick" to measure the universe it would solve the problem. But we need the solution to get the perfect "yardstick".
« Last Edit: 22/10/2013 00:36:11 by CPT ArkAngel »
 

Offline Pmb

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The space itself has increased in scale. The galaxy is not moving faster than light. Light can still pass through the space, but the expansion causes red shifting of the light as it moves through.
I disagree. I think it's quite fair to say that galaxies with high z really are moving away from us FTL if its understood that its in the sense that the change in distance between us and the galaxy during unit time has a value greater than c. We just have to understand that this is from a calculation of the form v = L/T which is a global measurement of speed where was v = dL/dT cannot exceded speed since that's a local measurement of speed and dL/dT < c.
« Last Edit: 22/10/2013 01:12:21 by Pete »
 

Offline Pmb

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Quote from: CPT ArkAngel
But there is no experimental proof of an expansion of space. This is a major problem in my opinion.
This is a very misleading statement since in physics one never assumes that any experiment can prove something to be true. Experiment cannot prove space expands just like no experiment or series of experiments can prove relativity to be correct. The sole purpose of experiments is to test theories by verifying predictions. Experiments can only show consistency with a theory. They can't prove them to be right.

I wish scientists would rid their vocabulary of the term proof. All too often far too many of them use it in inappropriate ways. If I had it my way you'd have to have a license to use that term. :)
 

Offline CPT ArkAngel

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Yes, you are right about the word 'proof' and experiment.

Expansion of space is a parameter that is being adjusted according to the observation of redshift. So it is a free parameter. You can't use redshift vs distance (or time) to say it is in agreement with the theory of expansion of space...
 

Offline Pmb

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Quote from: CPT ArkAngel
Expansion of space is a parameter that is being adjusted according to the observation of redshift. So it is a free parameter.
Expansion of space is a phenomenon, not a parameter. The parameter you're thinking of is the Hubble parameter which quantifies speed vs. distance via the relationship v = Hr known as Hubble's law.

Quote from: CPT ArkAngel
You can't use redshift vs distance (or time) to say it is in agreement with the theory of expansion of space...
That is incorrect. Would you care to provide a proof of that assertion?
 

Offline CPT ArkAngel

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The expansion of space as a parameter is defined by the redshifts vs distances of clusters of galaxies. There is no theory fixing it. It is the other way around, the measurements defines the theory. It simply means we don't know!
 

Offline Pmb

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Quote from: CPT ArkAngel
The expansion of space as a parameter is defined by the redshifts vs distances of clusters of galaxies.
I'm sorry to inform you of this, again, but you have once again erroneously asserted that expansion is a parameter. That is truly a meaningless assertion. Therefore since you haven't corrected it when I first pointed it out I see no reason to continue this line of discussion. All you keep doing is repeating an erroneous assertion anway and it's a waste of time to keep correcting this.
« Last Edit: 22/10/2013 04:24:40 by Pete »
 

Offline CPT ArkAngel

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A parameter may be a variable in space and time. The expansion of space in the standard model is supposed to depend on time alone (from the big bang).
« Last Edit: 23/10/2013 21:34:12 by CliffordK »
 

Offline Pmb

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Quote from: CPT ArkAngel
A parameter may be a variable in space and time.
The reason I terminated my participation in your line of reasoning is because you're not using the term parameter in a way consistent with how the term is defined in mathematics. No mathematician would ever attempt to use it the way you did. And that's what makes your assertion vauge and meaningless.

You would have been better off just admitting you made a mistake rather than being rude like this again. Next time please do everone a favor and simply look the term up in a dictionary.
« Last Edit: 22/10/2013 05:43:52 by Pete »
 

Offline flr

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Why would space grow/expand? What is made of to expand? Could it generate itself?

Is it possible that light get redshifted by yet to be discovered phenomenon? Like for example: the photon looses small amount of energy as it passes through empty space (or quantum vacuum)?
 

Offline Pmb

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Quote from: flr
Why would space grow/expand?
Why? questions are notoriously difficult to answer in physics. By its very nature physics only describes How questions.

Quote from: flr
What is made of to expand?
Space isn't made of anything. It's expansion means that there is simply more of it being created as time progresses. This is where the balloon analogy comes in handy. Think of ants crawling around on the surface of a balloon. The ants have the ability to determine how much space exists within that surface. If the balloon is enlarging with time then that ants can determine that as well. Now think of the balloon having a bunch of magic marker dots on its surface. Those marks can't move along the surface like ants can. But as time changes and the balloon is getting larger (whereby I mean that the surface area is increasing with time) the distance between those spots increases with time, not because their moving, but because more space is being created. The same thing can happen for a closed universe. I.e. we can measure more space being created in the universe. That means we can measure greater and greater amounts of volume existing as time increases.



Quote from: flr
Could it generate itself?
No. What leads you to think so? I.e. what would that mean to you?

Quote from: flr
Is it possible that light get redshifted by yet to be discovered phenomenon?
I have a friend that used to love to say that Anything is possible. I cured him of that when I asked him Is it possible that youíre wrong? Lol!

To be precise the answer to all questions which start off with Is it possible Ö is Yes. That is because science cannot prove that any of its theories are correct and as such what the laws actually state could be wrong and anything it previously predicted as impossible once again becomes possible. We could ask ďIs it possible ÖĒ questions all day long. That and a dollar would get you a cup of coffee.

That said, when I see people ask questions that start off with ďIs it possible ÖĒ I give them examples of people using that to back their theories. Have you ever watched the history channels series Ancient Aliens? Two of the show hosts are well-known crackpots. They make a great deal of money playing on this point. E.g. Is it possible that aliens taught the Egyptians how to build the pyramids? The answer to that kind of question might be entertaining but it doesnít have a place in science for the most part. Those people abuse it.



Quote from: flr
Like for example: the photon looses small amount of energy as it passes through empty space (or quantum vacuum)?
Can you clarify this for me please? You wrote this as if it were a statement of fact but add a question mark that seems to indicate that itís a question. Did you mean it as a statement or a question? In any case photons donít loose energy as they move through space other than the fact that there is a cosmological redshift due to their recession velocity. For small enough cosmological distances you can think of that redshift as being due to the Doppler effect.
« Last Edit: 22/10/2013 16:47:05 by Pete »
 

Offline alancalverd

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I wish scientists would rid their vocabulary of the term proof. All too often far too many of them use it in inappropriate ways. If I had it my way you'd have to have a license to use that term. :)

It's actually mathematicians and lawyers who misuse the word. Etymologically it derived from the latin probare, to test (e.g. with a probe!) . Experiment is indeed proof (as with proof spirit, and proof-tested gun barrels), and exceptio probat regulum makes sense.
 

Offline Pmb

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Quote from: alancalverd
It's actually mathematicians and lawyers who misuse the word.
That is incorrect. As any mathematician can tell you they are the only ones using it correctly to refer to the truth of a statement.
Quote from: alancalverd
Etymologically it derived from the latin probare, to test (e.g. with a probe!) . Experiment is indeed proof (as with proof spirit, and proof-tested gun barrels), and exceptio probat regulum makes sense.
Why are you attempting to use the etymology of the term to establish the current usage?

Etymologically refers to the history of the term, not its current use. There are several meanings to that word and most of them do not mean what you appear to think  it means. E.g. see http://www.merriam-webster.com/dictionary/proof
The one people mean when they use it is as follows
Quote
proof - a :  the cogency of evidence that compels acceptance by the mind of a truth or a fact b :  the process or an instance of establishing the validity of a statement especially by derivation from other statements in accordance with principles of reasoning
etc.
The meaning you're referring to is the archaic meaning. The meaning has changed since then and no longer means simply to test. A good example of misuse can be found at http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.htm
Quote
NASA Finds Direct Proof of Dark Matter
(...)
"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona at Tucson, and leader of the study. "These results are direct proof that dark matter exists." ect proof that dark matter exists."
The definition that is above how every one else uses the term
Quote
proof - a :  the cogency of evidence that compels acceptance by the mind of a truth or a fact b :  the process or an instance of establishing the validity of a statement especially by derivation from other statements in accordance with principles of reasoning
etc.
As stated above, the meaning has changed since then and no longer means simply to test.
« Last Edit: 23/10/2013 05:15:37 by Pete »
 

Offline flr

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It's expansion means that there is simply more of it being created as time progresses.
You mean to say that space is something that it is being created?
What do you mean by "more of it"? You mean that in the past it was less of "it"?

 

Offline Pmb

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Quote from: flr
You mean to say that space is something that it is being created?
No. I do not mean to say that.

Quote from: flr
What do you mean by "more of it"? You mean that in the past it was less of "it"?
Neither yes or no would be the correct response here.

If you find that confusing then that's the nature of nature in this case. You seem to be thinking of space, which can expand and contract, in the same way as you would a rubber band. I.e. that if something can expand and contract then there must be an "it" which is expanding and contracting. The answer is No.

General relativity is not a theory whose purpose it is to explain things to us. Theories don't explain anything whatsoever. If that seems too strange and unexpected to you then I recommend reading the following chapter of a book I have

http://www.worldscientific.com/doi/suppl/10.1142/6220/suppl_file/6220_chap01.pdf

I canít stress reading this chapter strongly enough. After you read it youíll understand why the phrase scientific explanation is circular. :o
« Last Edit: 23/10/2013 11:31:36 by Pete »
 

Offline SimpleEngineer

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I was reading a column today from an ongoing article that ended a couple of years ago. It was explaining the basics of the expansion of the universe. In the article, it adresses the question "are there galaxies that are moving away from us faster than the speed of light?" they answer, that yes, the distance between us and those galaxies is growing at a faster rate than the speed of light. If that statement is true, how is it that we would be able to observe these galxies if the distance between is us growing more quickly than the light can cover it?

1. Expansion of the universe.
Is expansion of the universe the outward moving of mass and energy from the centre of the universe (if there are such things) or does this mean that every little 'bit' of space is growing including the space between elementary particles (if considered as 'static')

2. If we can put boundaries on the universe, having a centre point and an edge, we could determine the 'expansion' of the universe as a rate, dependant on definitions, light would be transmitted 'out' of the universe as fast as inwards, so the universe could be defined as expanding at the speed of light in all directions, so an observer at one side of the universe (travelling at the speed of light to ensure he stays on the edge) would transmit his signal which would never arrive at the other edge as that would be travelling at the speed of light also. if the expansion was anything less than c the signal would eventually get there.

3. Regardless of expansion, if the relative velocities are less than double the speed of light we would get some form of observation.. and theres no point saying things cant go faster than the speed of light.. if I travel 2/3c in one direction and something traveled 2/3c the opposite then our relative velocity would be greater than the speed of light (4/3c) but then neither of us ARE travelling faster than light, and a signal fired from him would eventually reach me

The question for me is that we haven't really defined Expansion of the universe, but any premise of not being able to see things means they must be traveling away from us at or faster than TWICE the speed of light (relative velocity) which is pretty much as fast as anything can go.. so unless the theories are all wrong, there cannot be anything that we cannot or will never see from any point in the universe.

However if ALL space is expanding (using the balloon theory) then we could surely measure this looking at subatomic particles.         
 

lean bean

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In the article, it adresses the question "are there galaxies that are moving away from us faster than the speed of light?" they answer, that yes, the distance between us and those galaxies is growing at a faster rate than the speed of light. If that statement is true, how is it that we would be able to observe these galxies if the distance between is us growing more quickly than the light can cover it?
Ben, the light from a galaxy has taken time to reach us, what we are receiving is old light which left the galaxy before the expansion of space between us and galaxy was FTL. Once the expansion of space between us is FTL, then you have...to put it in your words... ''the distance between us is growing more quickly than the light can cover it?'' That's when we stop 'seeing' the galaxy.

« Last Edit: 23/10/2013 14:19:50 by beany »
 

Offline Pmb

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Quote from: SimpleEngineer
1. Expansion of the universe.
Is expansion of the universe the outward moving of mass and energy from the centre of the universe (if there are such things) or does this mean that every little 'bit' of space is growing including the space between elementary particles (if considered as 'static')
It means that space itself is expanding and not merely the distance between objects. Although the distance between objects does increase with time it only does so because space is being created between objects. See http://en.wikipedia.org/wiki/Metric_expansion_of_space#Is_the_expansion_of_the_universe_felt_on_small_scales.3F


Quote from: SimpleEngineer
3. Regardless of expansion, if the relative velocities are less than double the speed of light we would get some form of observation.. and theres no point saying things cant go faster than the speed of light.. if I travel 2/3c in one direction and something traveled 2/3c the opposite then our relative velocity would be greater than the speed of light (4/3c) but then neither of us ARE travelling faster than light, and a signal fired from him would eventually reach me
There certainly is a point saying that things canít go faster than the speed of light. I fail to see how your example clarifies your assertion.

Quote from: SimpleEngineer
The question for me is that we haven't really defined Expansion of the universe,Ö
The expansion of the universe is already well defined through the metric tensor. See http://en.wikipedia.org/wiki/Metric_expansion_of_space

See also http://en.wikipedia.org/wiki/Metric_expansion_of_space#How_is_the_expansion_of_the_universe_measured_and_how_does_the_rate_of_expansion_change.3F

Quote from: SimpleEngineer
but any premise of not being able to see things means they must be traveling away from
However if ALL space is expanding (using the balloon theory) then we could surely measure this looking at subatomic particles.         
That is incorrect. The expansion of space cannot be determined by observing subatomic particles. Why you would claim otherwise is not clear since all you did is to make the assertion and not argue as to why thatíd be the case.
 

Offline Phractality

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 3. Regardless of expansion, if the relative velocities are less than double the speed of light we would get some form of observation.. and theres no point saying things cant go faster than the speed of light.. if I travel 2/3c in one direction and something traveled 2/3c the opposite then our relative velocity would be greater than the speed of light (4/3c) but then neither of us ARE travelling faster than light, and a signal fired from him would eventually reach me     
 
That's not how velocities add in SR. The formula is   V'_x=\frac{ V_x - v }{ 1 - \frac{V_x v}{c^2} }.
 
Let Vx =2/3 c and v = -2/3 c.
Vx - v = 4/3 c, and 1 - Vxv/c2 = 1 + 4/9 = 13/9
So V'x = (4/3 c) / 13/9 = 36/39 c = .92 c
 
If two space ships are moving at 2/3 c in opposite directions in Earth's reference frame, then each ship is moving away from the other ship at .92 c in its own reference frame.
 
In special relativity, relative velocities cannot exceed c. In general relativity, two objects cannot pass one another faster than c, but the distance between two objects can change faster than c, due to expansion or contraction of space in the direction of the acceleration.
 

Offline Pmb

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Quote from: Phractality
That's not how velocities add in SR.
Itís unclear from what he wrote who was doing the measuring/observing of the motion. Thatís why I ignored it. He very well could have meant that as measured in frame S there is an object moving in the Ėx direction with speed v = -2c/3 as measured in S and one moving in the +x direction with speed v = 2c/3 as measured in S and the relative speed of one body with respect to the other is 4c/3 as measured in S. Since no body is moving FTL then thereís no conflict with relativity.
 

Offline Phractality

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However if ALL space is expanding (using the balloon theory) then we could surely measure this looking at subatomic particles.       
Without going too deeply into my new theory, let me just describe how it quantifies the problem at hand. I believe space has substance. There is an aether, and the aether is stationary relative to a preferred reference frame, probably the same as the reference frame of the CMB. I believe space expands one Planck volume at a time. We are moving thru the aether at about 500 km/s. So 16 seconds after a new Planck volume of space appears on the forward side of Earth, it exits the trailing side of Earth. That makes it rather difficult to isolate and observe that new piece of space.
  • Distances expand at the rate of the Hubble constant, about 2.3 x 10^-18/s.
  • Volume of space expands at three times that rate, or 6.9 x 10^-18/s.
  • A Planck volume is about 4.22◊10^-105 m3; that's 2.37◊10^104 Planck volumes per cubic meter.
  • 6.9 x 10^-18/s * 2.37◊10^104 = 1.635 x 10^87 Planck volumes per cubic meter per second. That how often Planck volumes of new space are created.
  • A proton's charge radius is 4.475◊10^-14 meters. So it occupies about 4 x 10^-51 m3; that's about 10^54 Planck volumes.
  • 6.9 x 10^-18/s * 10^54 = 6.9 x 10^36. So in one second, about 6.9 x 10^36 Planck volumes of new space will appear inside a proton.
  • This does not change the size of the proton, instead that new space is squeezed out of the proton because of the forces that give the proton its characteristics, including its fixed size.
  • If that proton is stationary relative Earth, moving thru the aether at about 500 km/s, each of those new bits of space will traverse the diameter of the proton in about 5◊10^-14 meters / 500 km/s = 10^-19 s. About 10^18 new Planck volumes of new space form inside the proton in the time it takes for them to traverse the proton's diameter.
  • Since an electron is much smaller and usually moves much faster, one bit of new space forming inside an electron might be gone before the next bit of new space appears.

 
 

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