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.