Far away galaxies moving faster than light?

[CarlosLara]

Good evening. I just watched a documentary called The Universe-Light Speed. In it, astrophysicist Alexei Filippenko said that galaxies which are very far are moving away from us faster than the speed of light. Also, cosmologist Alan Guth said that during inflation the universe expanded faster than the speed of light. What did they mean exactly? I know that neither matter or energy can break the limit imposed by the speed of light, so I am a little confused.

Thank you in advance.

 

[Mark M]

Carlos, nothing can move through spacetime faster than c. But, the expansion of the universe is the growth of space itself, which is not limited. So, essentially, relativity places a limit on local frames of reference, but there is no limit on global frames on an expanding universe. (e.g. receding galaxies in an expanding universe)

Also, remember that it isn't the galaxies themselves that are receding. They appear to be receding because new space is being created in between galaxies.

 

[CarlosLara]

Oh ok, then it is not accurate to say that the galaxies are moving away from us faster than light, but rather it's spacetime in between us and those galaxies that is moving (expanding) faster than light. But, if we measure the time dilation of such a galaxy from the reference frame of Earth, how will we perceive it? How do we perceive the characteristics of an object which appears to move faster than the speed of light?

 

[Chalnoth]

Oh ok, then it is not accurate to say that the galaxies are moving away from us faster than light, but rather it's spacetime in between us and those galaxies that is moving (expanding) faster than light. But, if we measure the time dilation of such a galaxy from the reference frame of Earth, how will we perceive it? How do we perceive the characteristics of an object which appears to move faster than the speed of light?

Well, if we try to measure the speed directly, such as through the redshift, we don't get a speed faster than light. The "speed" that is talked about is the inferred speed due to the change in distance over time. This speed is faster than light.

But the time dilation can be read directly off of the redshift of the object.

 

[Calimero]

Also, remember that it isn't the galaxies themselves that are receding. They appear to be receding because new space is being created in between galaxies.

I think that you should be much more careful with that kind of interpretation. Galaxies are separating due to the inertia, that is due to the fact that they were doing so in the past. As a result space between them is expanding.

 

[Chalnoth]

I think that you should be much more careful with that kind of interpretation. Galaxies are separating due to the inertia, that is due to the fact that they were doing so in the past. As a result space between them is expanding.

Both ways of looking at this situation are equivalent.

 

[sweet springs]

Hi.

As a result space between them is expanding.

Not only between the galaxies, at anywhere, for an example between my head and foot, do space expand?

Regards.

 

[Chalnoth]

Not only between the galaxies, at anywhere, for an example between my head and foot, do space expand?

Regards.

No. The expansion of space is an average effect on large distances. The local matter density is more than enough to overcome the expansion.

 

[Calimero]

Both ways of looking at this situation are equivalent.

Well, in most aspects I agree, but then again you can't say that galaxies themselves are not receding as a physical interpretation.

 

[Chalnoth]

Well, in most aspects I agree, but then again you can't say that galaxies themselves are not receding as a physical interpretation.

Sure you can. Why couldn't you? Perhaps the simplest way of looking at the expansion is that galaxies are mostly stationary with respect to a space-time that is expanding.

 

[sweet springs]

Hi.

No. The expansion of space is an average effect on large distances. The local matter density is more than enough to overcome the expansion.

If the expansion of space is an average effect on large distances, would not it contradict expansion of wavelength of each photon from the stars which is very short of 10^-6 m or so. It suggests that expansion occurs homogeneously in such micro scale.

Regards.

 

[Chalnoth]

If the expansion of space is an average effect on large distances, would not it contradict expansion of wavelength of each photon from the stars which is very short of 10^-6 m or so. It suggests that expansion occurs homogeneously in such micro scale.

Regards.

No, that's an incorrect understanding of the situation. Perhaps a better way to understand it is that different parts of the universe expand at different rates, so that when a photon of any wavelength passes through an expanding region, it expands. It is the parts of the universe with very little matter, such as the vast space between galaxies, that expand.

 

[Calimero]

Sure you can. Why couldn't you? Perhaps the simplest way of looking at the expansion is that galaxies are mostly stationary with respect to a space-time that is expanding.

I have to go, will respond later. Hope we can continue this discussion.

 

[sweet springs]

Hi. OK. As for the vast space between galaxies that expand,

different parts of the universe expand at different rates, so that when a photon of any wavelength passes through an expanding region, it expands.

the expansion is very homogeneous in micro scale as homogeneous expansion of visible light wavelength show it. Is it OK?

Regards.

 

[Chalnoth]

the expansion is very homogeneous in micro scale as homogeneous expansion of visible light wavelength show it. Is it OK?

No, the expansion is only homogeneous at very large scales. At smaller scales, different parts of the universe expand at different rates (with no expansion at all in gravitationally-bound areas, such as within galaxy clusters or galaxies). But the expansion impacts all objects within the expanding region identically. The wavelength of a photon passing through an expanding region just does not matter.

 

[sweet springs]

Hi.

I still feel wonder that during traveling in expanding space the top and the tail of the wave train of one photon, say 1 meter length or so is expanded as pulled rubber. Expansion is very homogeneous so wave length is exactly proportional everywhere in this 1 meter. All the colleague photons are expanded perfectly in the same manner, not like that some are red light but others remain violet.

Regards.

 

[Chalnoth]

Hi.

I still feel wonder that during traveling in expanding space the top and the tail of the wave train of one photon, say 1 meter length or so is expanded as pulled rubber. Expansion is very homogeneous so wave length is exactly proportional everywhere in this 1 meter. All the colleague photons are expanded perfectly in the same manner, not like that some are red light but others remain violet.

Regards.

No, expansion isn't about stretching anything. Photons aren't "pulled" by the expansion.

A better way to think about it is that because of the curved space-time they traverse, when they arrive at their destination it is as if they arrive with a relative speed difference compared to where they arrived from.

 

[and9]

NO spacetime is neither created nor destroyed. Whoever said that spacetime is being created between two galaxes to accommodate for the expansion is wrong. It is called expansion for a reason -- we are literally expanding, or stretching if you will, the fabric of spacetime as the universe grows.

The spacetime can be stretched out to all infinities as well as compacted to the singularity of Planck length in all directions.

 

[Chalnoth]

NO spacetime is neither created nor destroyed. Whoever said that spacetime is being created between two galaxes to accommodate for the expansion is wrong. It is called expansion for a reason -- we are literally expanding, or stretching if you will, the fabric of spacetime as the universe grows.

The spacetime can be stretched out to all infinities as well as compacted to the singularity of Planck length in all directions.

I don't understand your objection here. There is no conservation of space-time.

 

[Radrook]

Before we begin viewing such galaxies as somehow unusually exotc due to their near-faster light redshift, please keep in mind that any denizen of those far-flung galaxies looking our way would see us receding from them at near-faster speed of light as well. In fact, to them we would seem on the brink of going over the universal horizon of visibility ready to disappear forever from their detection just as we see them.

 

[Chalnoth]

Before we begin viewing such galaxies as somehow unusually exotc due to their near-faster light redshift, please keep in mind that any denizen of those far-flung galaxies looking our way would see us receding from them at near-faster speed of light as well. In fact, to them we would seem on the brink of going over the universal horizon of visibility ready to disappear forever from their detection just as we see them.

We don't observe their speed directly. If we interpreted their entire redshift as due to recession velocity, for example, their speeds would always be measured as slower than light.

The faster-than-light recession velocity comes from estimating the change in distance over time of those galaxies. And for many of them, for the most obvious measures of distance, that distance is and has always been increasing with time faster than light.

 

[Radrook]

We don't observe their speed directly. If we interpreted their entire redshift as due to recession velocity, for example, their speeds would always be measured as slower than light.

The faster-than-light recession velocity comes from estimating the change in distance over time of those galaxies. And for many of them, for the most obvious measures of distance, that distance is and has always been increasing with time faster than light.

Please show me where I said that we can observe their speed directly. The fact remains that any DETECTOR, of our galaxy from that vantage point would DETECT us in the same manner as we DETECT them. Neither did I attribute the velocity of their receding from us to the proper motion of the galaxies themselves since nothing material can move THROUGH space faster than light. Neither can we see the light they have emitted since "they" "went" supraluminal since that light will never reach us because space is being added faster than light can traverse it at that point.

BTW
By that I mean that although it traverses space it doesn't do so fast enough to cover the distance between us.

 

[Chalnoth]

Please show me where I said that we can observe their speed directly. The fact remains that any DETECTOR, of our galaxy from that vantage point would DETECT us in the same manner as we DETECT them. Neither did I attribute the velocity of their receding from us to the proper motion of the galaxies themselves since nothing material can move THROUGH space faster than light. Neither can we see the light they have emitted since "they" "went" supraluminal since that light will never reach us because space is being added faster than light can traverse it at that point.

BTW
By that I mean that although it traverses space it doesn't do so fast enough to cover the distance between us.

What you did say was this:

Before we begin viewing such galaxies as somehow unusually exotc due to their near-faster light redshift

This statement is only coherent if you think redshift is a measure of velocity.

However, there is a sort of asymmetry in the way we detect other galaxies. For a galaxy currently receding at faster than the speed of light, they will never observe us at all. They can observe our distant past, but they can never observe our present. Similarly, we observe their distant past. So it turns out to be a bit of an over-simplification to just say that they see what we see. It's true in a way, but it neglects the fact that we have moved out of our respective horizons.

 

[Radrook]

What you did say was this:

This statement is only coherent if you think redshift is a measure of velocity.

However, there is a sort of asymmetry in the way we detect other galaxies. For a galaxy currently receding at faster than the speed of light, they will never observe us at all. They can observe our distant past, but they can never observe our present. Similarly, we observe their distant past. So it turns out to be a bit of an over-simplification to just say that they see what we see. It's true in a way, but it neglects the fact that we have moved out of our respective horizons.

Sigh!
Please show me where I said that they are observing our present. Obviously such far- flung objects cannot have their own relative presents observed. In fact, even our nearby stars and our sun can't because of the time it takes for light to reach us. LOL!

BTW

I clearly mentioned that we have moved beyond visible detectable horizons by mentioning how the increasing velocity of added space makes visual detection of present locations impossible. Are you really reading what I write carefully? It seems as if ignorance of basics is being assumed at first glance.

 

[marcus]

...
However, there is a sort of asymmetry in the way we detect other galaxies. For a galaxy currently receding at faster than the speed of light, they will never observe us at all. They can observe our distant past, but they can never observe our present. Similarly, we observe their distant past. So it turns out to be a bit of an over-simplification to just say that they see what we see. It's true in a way, but it neglects the fact that we have moved out of our respective horizons.

Chalnoth, I'm going to pick a nit. On the whole, nearly 100% you are doing a great job patiently and accurately answering Radrook's questions.

The nit is that you KNOW that there are galaxies just slightly beyond the Hubble radius, receding just slightly faster than c, that we could send a signal to, today, and it would get there. We've talked about it before, I just don't recall in what thread. I suspect you are one of a number of us who've taught that to newcomers and explained how it can happen. So in this case you are oversimplifying when you suggest it can't happen. It "almost can't" but just barely can.

Anyway thanks for the patient hard work. You and Brian Powell and others are doing a great job.

====================
For anybody else, not familiar with this effect, let's use Jorrie's model and try z = 1.45.
You should get the recession speed is >c. Yes something like 1.02c
Google "cosmocalc 2010" to get the calculator and put 1.45 in the z box.

The point is that the Hubble radius, which is currently around 14 billion LY, is expected to extend out to around 1/sqrt(0.73) of that as matter density thins out. Please correct me if I'm wrong, anybody!
So you calculate 14/sqrt(.73) and get 16.4.

So if, today, a guy in a z=1.45 galaxy sent us a message, say a photon, that photon would only be drifting away at rate 0.02c. So it would basically be hanging around at distance about 14 billion LY from us for ages and ages. Its speed towards us barely almost matching the expansion of distance.

Eventually the Hubble radius has to grow from 14 to 16, so it would reach out and take the photon into our Hubble sphere, and then it would be smooth sailing and the photon would make steady progress and eventually get to us. This isn't a detailed quantitative argument, it's just intuitive and conceptual, but I hope anyone who is interested can see how would work.

 

[Radrook]

I have posted absolutely no questions that need answering.

 

[marcus]

... Neither can we see the light they have emitted since "they" "went" supraluminal since that light will never reach us because space is being added faster than light can traverse it at that point.

BTW
By that I mean that although it traverses space it doesn't do so fast enough to cover the distance between us.

I have posted absolutely no questions that need answering.

Quite true, Radrook! Chalnoth has been answering your assertions, some of which he thought needed answering, not your questions. I looked back in thread and didn't see you asking questions at least about cosmology/physics. You may have but I didn't notice.

Keep in mind that the vast majority of the galaxies which we can see today emitted the light which we are now receiving when they were already receding FTL.

That would be true for any galaxy with redshift z > 1.7. Which is the vast majority.

To check that, google "cosmocalc 2010" and put 1.7 in the redshift box.

Your earlier post gives me the impression that you think we cannot receive the light from something that is going superluminal. That it is over beyond some visibility horizon. But most of the galaxies we see were receding superluminal at the time they emitted the light.

There is something called the cosmological event horizon but the definition is a bit more complicated than the Hubble distance (the distance which is today increasing at rate exactly c.)

 

[Radrook]

Fact: galaxies cannot travel through space faster than light.

Fact: If galaxies appear to go superluminal it is an illusion caused by universal expansion.
Even if they were motionless the superluminal appearances would remain due to that expansion.

Fact: If the space between galaxies is expanding faster than light the light emitted by such galaxies at that juncture will never fully traverse it.

Fact: Emitted light of galaxies once not superluminal now gone superluminal is presently reaching us. The light not reaching us is the light being emitted once the space between us and them went superluminal.

The only way to get around this is to have light traverse an area of space that is being added faster than it can outrun it.

It is compared to a runner on a treadmill that is going faster than he can run. All literature I have read negates any contrary phenomenon that nullifies this fact and until proven otherwise and accepted by reputable physicists as indisputable and not mere conjecture, that will remain my view as well.

As for having this matter addressed by Chalth prior to it being introduced by Marcus, after reading all his previous writing again I see absolutely no evidence of it. The matter was never brought up as far as I can see.

Kind regards

Radrook

Is the universe expanding faster than the speed of light?You might be wondering how we could possibly see a galaxy that is moving away from us faster than the speed of light! The answer is that the motion of the galaxy now has no effect whatsoever on the light that it emitted billions of years ago. The light doesn't care what the galaxy is doing; it just cares about the stretching of space between its current location and us. So we can easily imagine a situation where the galaxy was not moving faster than the speed of light at the moment the light was emitted; therefore, the light was able to "outrun" the expansion of space and move towards us, while the galaxy moved away from us as the universe expanded.

Dave Rothstein
Dave is a former graduate student and postdoctoral researcher at Cornell who used infrared and X-ray observations and theoretical computer models to study accreting black holes in our Galaxy. He also did most of our website development.http://curious.astro.cornell.edu/question.php?number=575

 

[Chalnoth]

Fact: galaxies cannot travel through space faster than light.

This is not a correct understanding of the situation. Space has no frame of motion, so the statement you made above makes no sense, because it is impossible to talk about how fast anything is moving through space.

The correct statement is that nothing can outrun a light beam.

Fact: If galaxies appear to go superluminal it is an illusion caused by universal expansion.
Even if they were motionless the superluminal appearances would remain due to that expansion.

Galaxies don't appear superluminal in any sort of direct sense. That is, there is no direct measurement of the speed of an observed galaxy that would ever produce v > c.

Furthermore, relative velocity isn't a well-defined notion for far-away objects. It is only possible to subtract velocities and get a well-defined answer at a single point. But measuring the velocity of a far-away galaxy relative to us is arbitrary: there are many equally-correct ways of doing it. Because there is no absolute way to talk about the speed of a far-away galaxy, there cannot possibly be any speed limit either.

Fact: If the space between galaxies is expanding faster than light the light emitted by such galaxies at that juncture will never fully traverse it.

Not true. Marcus pointed out an example above. Most of the galaxies we observe today are now and always have been receding at greater than the speed of light according to the normal definition of recession velocity.

Fact: Emitted light of galaxies once not superluminal now gone superluminal is presently reaching us. The light not reaching us is the light being emitted once the space between us and them went superluminal.

That doesn't make any difference. Once the light has left the galaxy, the galaxy's motion is irrelevant to whether or not the light beam reaches us. It is the motion of the light beam that is important, not the motion of the galaxy.

 

[Flustered]

It is not possible for galaxies to be seemingly motionless and that space is the only thing moving. If that was the case than galaxies would not be able to be held together if there were no motion to them.

 

[Mark M]

It is not possible for galaxies to be seemingly motionless and that space is the only thing moving. If that was the case than galaxies would not be able to be held together if there were no motion to them.

That is what cosmology holds, that the universe is expanding. This is because of Hubble's Law - there is a linear relationship between the proper distance to a galaxy, and its redshift. Photons emitted from distant galaxies traverse more expanding space than do closer galaxies, and are redshifted more - the expanding space stretches their wavelength. The gravitational pull of the galaxies makes the expansion of space absolutely negligible inside of them.

 

[Flustered]

Galaxies would not be elliptical or irregular if they were not moving.

 

[Mark M]

Galaxies would not be elliptical or irregular if they were not moving.

Why do you conclude that? Galaxies rotate, and are filled by large amounts of matter and dark matter that have gravitational pulls far stronger than the expansion of the universe.

 

[Chalnoth]

Galaxies would not be elliptical or irregular if they were not moving.

Perhaps you mean that galaxies would not collide if they were not moving? This is strictly true, but the statement that "galaxies are not moving" must be understood in context. A few points:

1. It is not an absolute statement. It is a relative statement. Galaxies are not moving relative to the CMB.
2. It is not an exact statement, but an approximate one. Individual galaxies can have quite a bit of motion with respect to the CMB, sometimes as high as 1000km/s. But when you take a large collection of galaxies, say, over a few hundred million light years, the average motions with respect to the CMB nearly cancel.

 

[Lino]

Chalnoth, I understand most of what is being discussed here, but in relation to your post

1. It is not an absolute statement. It is a relative statement. Galaxies are not moving relative to the CMB.

could I ask you to explain this a little further. I appreciate the second part of the post (that this isn't a hard and fast rule, but a general statement based on large scales), but do you mean that the distance to the galaxy is growing at the same relative rate as the distance to the CMB is growing, as the universe is expanding? Alternatively, if this statement is less about distance and more about left / right /up / down relative to the CMB, could you point me at something to read about how this has been measured.

Regards,

Noel.

 

[Chalnoth]

Chalnoth, I understand most of what is being discussed here, but in relation to your post


could I ask you to explain this a little further. I appreciate the second part of the post (that this isn't a hard and fast rule, but a general statement based on large scales), but do you mean that the distance to the galaxy is growing at the same relative rate as the distance to the CMB is growing, as the universe is expanding? Alternatively, if this statement is less about distance and more about left / right /up / down relative to the CMB, could you point me at something to read about how this has been measured.

Regards,

Noel.

Not really. There isn't a "distance to the CMB", as the CMB is everywhere. What it means instead is that when the CMB is observed from a reference frame at rest with respect to the CMB, the observer doesn't see any particular direction where there is a redshift or blueshift: it looks pretty much the same in every direction.

 

[Lino]

Not really. There isn't a "distance to the CMB", as the CMB is everywhere. What it means instead is that when the CMB is observed from a reference frame at rest with respect to the CMB, the observer doesn't see any particular direction where there is a redshift or blueshift: it looks pretty much the same in every direction.

Thanks Chalnoth, but how does that relate to the precieved movement of the galaxy? I understand your comment on the CMB / redshift / blueshift, but am having difficult relating this galaxy redshift - I understand that something (the galaxy) won't move relative to something (the CMB) that exists everywhere / all of the time, but I'm having difficulty appreciating the context ("... the statement that "galaxies are not moving" must be understood in context").

(I think that) I do understand universal expansion and the precieved impact on distances to galaxies. Are you saying that (at large scales - thus ignoring gravitational interactions) the galaxies have no motion through space other than that associated with universal expansion?

Regards,

Noel.

 

[Calimero]

Sure you can. Why couldn't you? Perhaps the simplest way of looking at the expansion is that galaxies are mostly stationary with respect to a space-time that is expanding.

Sure it is. But those are just coordinates. Since we are fundamentally incapable to detect motion through space (people tryied that, but failed miserably), it doesn't make sense at all to make physical distinguish between motion through space as opposed to motion with space. Difference is only in coordinates you are using. Distances are increasing, but space is doing exactly nada, let alone dragging galaxies along faster then light.

 

[Whovian]

Sure it is. But those are just coordinates. Since we are fundamentally incapable to detect motion through space (people tryied that, but failed miserably), it doesn't make sense at all to make physical distinguishing between motion through space as opposed to motion with space. Difference is only in coordinates you are using. Distances are increasing, but space is doing exactly nada, let alone dragging galaxies along faster then light.

And there's the catch. Velocity doesn't matter. We can detect the acceleration of space, though, which is what matters here.

 

[Calimero]

Nice you brought it up. What do you think would happen with galaxy left at proper cosmological distance at rest with respect to us? Universe (space) is still expanding, but expansion is decelerating due to the gravity. Should that galaxy just go on with expanding space, or maybe do something else?

 

[Whovian]

...
but expansion is decelerating due to the gravity.
...

Huh? I'm pretty sure our current models predict that expansion will go on forever. And I didn't mean the acceleration of expansion space, I mean the acceleration of space.

 

[Mark M]

Nice you brought it up. What do you think would happen with galaxy left at proper cosmological distance at rest with respect to us? Universe (space) is still expanding, but expansion is decelerating due to the gravity. Should that galaxy just go on with expanding space, or maybe do something else?

As Whovian said, the universe is believed to have a non-zero cosmological constant (referred to as 'dark energy'). Analysis in the 1990's of distant supernovae show that after redshift z = 0.5, (which corresponds to ~5 billion years ago) Hubble's Law, the linear relationship between redshift and distance, is slightly violated. The 2011 Nobel Prize in Physics was awarded for the discovery of the accelerating universe.

 

[Whovian]

As Whovian said, the universe is believed to have a non-zero cosmological constant (referred to as 'dark energy'). Analysis in the 1990's of distant supernovae show that after redshift z = 0.5, (which corresponds to ~5 billion years ago) Hubble's Law, the linear relationship between redshift and distance, is slightly violated. The 2011 Nobel Prize in Physics was awarded for the discovery of the accelerating universe.

Wait. I thought the Cosmological Constant was different from Dark Energy? The CC was from a different version of GR that didn't require a Big Bang, while Dark Energy is what we think is causing the Universe to expand?

 

[Calimero]

As Whovian said, the universe is believed to have a non-zero cosmological constant (referred to as 'dark energy'). Analysis in the 1990's of distant supernovae show that after redshift z = 0.5, (which corresponds to ~5 billion years ago) Hubble's Law, the linear relationship between redshift and distance, is slightly violated. The 2011 Nobel Prize in Physics was awarded for the discovery of the accelerating universe.

Well, thank you Mark for that novel info you are providing me with. First, when subject like this is discussed, cosmological constant is left aside. We may be discussing universe 10 billion years ago when total amount of CC was negligible. Second, if you are aiming for that 2012 Nobel Prize, you should explain what exactly dark energy is. Is it some pre-curvature of spacetime, or what? Furthermore, in phinds thread the balloon analogy (please critique), Naty1 initiniaded some discussion about it (expanding space). There are quotes from highly regarded PF members, and also links to some very good papers and old threads. I can provide more links if you want to delve deeper in the subject.

 

[Mark M]

Wait. I thought the Cosmological Constant was different from Dark Energy? The CC was from a different version of GR that didn't require a Big Bang, while Dark Energy is what we think is causing the Universe to expand?

The cosmological constant is a generic term in the Einstein Field Equations, given by \Lambda. You can see it in the Field EquationsR_{\mu \nu }-\frac{1}{2}g_{\mu \nu }R+g_{\mu \nu }\Lambda =\frac{8 \pi G}{c^{4}}T_{\mu \nu }It represents an energy of 'space itself' that can drive expansion. Einstein's cosmological constant was negative - it drove the contraction of the universe. This would balance out the expansion effects of the density of the matter and energy in the universe.

The cosmological constant (dark energy) today is positive - it increases the rate of expansion. An estimate of the cosmological constant is \Omega = 0.7. In Planck units, this is equivalent to 10^-122.

The cosmological constant is a much better explanation for dark energy than quintessence (quantum vacuum energy), because the estimated value of quintessence totally overshoots the estimated value of dark energy.

Also, keep in mind dark energy is not the reason the universe is expanding - it's the reason the universe is accelerating. Normal expansion can be driven by the presence of matter or energy. You can imagine an empty spacetime, filled with a uniform gas - in general relativity, this can drive expansion. But, when regular matter and energy drive expansion, they lose density (every time the universe doubles in size, it falls by 8 fold). Dark energy, on the other hand, doesn't lose density, ever. That's why it is associated with the cosmological constant.

 

[bapowell]

I should point out that dark energy is a generic term given to any fluid with an equation of state parameter w = p/\rho < -1/3. The CC corresponds to the case w=-1.

 

[Chalnoth]

Are you saying that (at large scales - thus ignoring gravitational interactions) the galaxies have no motion through space other than that associated with universal expansion?

Right, that's exactly it. The way the expansion works, the expansion of the universe itself forces this to happen, in fact. Basically, if something is moving in some direction with respect to the overall expansion, then it will, in time, catch up to other matter that is moving in the same direction. Over time, then, this object's motion will always slow down relative to the local matter that is going along with the expansion.

 

[Chalnoth]

Sure it is. But those are just coordinates. Since we are fundamentally incapable to detect motion through space (people tryied that, but failed miserably), it doesn't make sense at all to make physical distinguish between motion through space as opposed to motion with space. Difference is only in coordinates you are using. Distances are increasing, but space is doing exactly nada, let alone dragging galaxies along faster then light.

There is no way of distinguishing "moving with space" vs. "moving through space". Both are meaningless statements.

 

[Calimero]

There is no way of distinguishing "moving with space" vs. "moving through space". Both are meaningless statements.

If they are meaningless, which I agree, why people here often use them as explanation for some phenomena, or even worse, try to interpret them as physical phenomena itselves?

 

[Mark M]

If they are meaningless, which I agree, why people here often use them as explanation for some phenomena, or even worse, try to interpret them as physical phenomena itselves?

It's an easy explanation to people as to why galactic recessional velocities greater than the speed of light do not violate special relativity.