Are galaxies moving faster than the speed of light?

[Gregory.gags]

Its been noted through the Doppler Effect (or red shift maybe? not 100% sure atm) that some of the farthest galaxies seem to be moving away from us faster than the speed of light
(c+). Since there is no axis point to measure the speed of these galaxies from, other than the milky way, could it not be said that our galaxy is/is also moving through space at ≥c? (once i get some feedback on this i'll get to my main point :D)

 

[alexg]

From the point of view of the farthest galaxies, yes.

But remember, it's not motion through space.

 

[Gregory.gags]

its NOT motion through space? Would that mean that were not physically traveling THROUGH spacetime but more like spacetime is being stretched out and were just being pulled along with it? like a marker dot on an inflating balloon

 

[alexg]

Exactly.

 

[Gregory.gags]

OK, on to my second point...
because of space contraction and time dilation it is widely accepted that the faster you go through space the slower you move through time...
so moving at c, time would = 0 and >c time would = <0
therefore going 'backwards in time"

 

[Chalnoth]

OK, on to my second point...
because of space contraction and time dilation it is widely accepted that the faster you go through space the slower you move through time...
so moving at c, time would = 0 and >c time would = <0
therefore going 'backwards in time"

On these scales, you have to use general relativity, not special relativity. No, they don't ever appear to go backward in time from our perspective. In fact, you can get the apparent time dilation directly from their redshift. An object at z=3, for example, has a recession velocity greater than the speed of light (by the usual definition of recession velocity), but the time dilation factor is 1/(z+1) = 1/4, so that to us, time on that galaxy appears to be moving at 1/4th the speed it moves here on Earth.

 

[Mark M]

Its been noted through the Doppler Effect (or red shift maybe? not 100% sure atm) that some of the farthest galaxies seem to be moving away from us faster than the speed of light
(c+). Since there is no axis point to measure the speed of these galaxies from, other than the milky way, could it not be said that our galaxy is/is also moving through space at ≥c? (once i get some feedback on this i'll get to my main point :D)

Remember, relativity forbids objects from moving through spacetime faster than light.
Galaxies barely move, for example, the Milky Way moves about 600 KPS. The galaxies appear to be red shifted and moving away from us because the space in between the galaxies is expanding.

You can imagine putting two dots on the surface of a balloon. As you blow up the balloon, they move away from each other, but only because the balloon is expanding.

GR places no limit on how fast space can expand.

EDIT: As Marcus pointed out, the 600 KPS is in respect to the Cosmic Microwave Background, mistake on my part.

 

[marcus]

Mark, this is clear accurate concise. I tend to be more talky and I think this brief style communicates better to most people. I offer one small change:

Remember, relativity forbids objects from moving through spacetime faster than light.
Galaxies barely move, for example, the Milky Way moves about 600 [KPS]. The galaxies appear to be red shifted and moving away from us because the space in between the galaxies is expanding.

You can imagine putting two dots on the surface of a balloon. As you blow up the balloon, they move away from each other, but only because the balloon is expanding.

GR places no limit on how fast space can expand.

It's moving about 600 kilometers per second relative to the ancient light, the microwave background, in the general direction of the southern constellations Hydra and Centaurus. All our local group of galaxies is headed that way pretty much. A little fleet of a dozen or so, the main other one being Andromeda. I checked one time and the exact southern constellation is a small little-known one called Crater which is in the general region of Hydra and Centaurus.

It's easy to forget and write KPH instead of KPS. If you see the error and are still able to edit your post I will delete this. I hope I'm not misunderstanding. The most common way to express the small local random motions of galaxies is relative to the CMB. Those KPS speeds (relative microwave background) certainly are small compared with either the speed of light or with the rate at which distances to most galaxies are expanding! Distances to most of the galaxies we can see are indeed expanding faster than the speed of light, but that is just geometry change, not motion thru space (as you say.)

 

[Mark M]

Mark, this is clear accurate concise. I tend to be more talky and I think this brief style communicates better to most people. I offer one small change:

It's moving about 600 kilometers per second relative to the ancient light, the microwave background, in the general direction of the southern constellations Hydra and Centaurus. All our local group of galaxies is headed that way pretty much. A little fleet of a dozen or so, the main other one being Andromeda. I checked one time and the exact southern constellation is a small little-known one called Crater which is in the general region of Hydra and Centaurus.

It's easy to forget and write KPH instead of KPS. If you see the error and are still able to edit your post I will delete this. I hope I'm not misunderstanding. The most common way to express the small local random motions of galaxies is relative to the CMB. Those KPS speeds (relative microwave background) certainly are small compared with either the speed of light or with the rate at which distances to most galaxies are expanding! Distances to most of the galaxies we can see are indeed expanding faster than the speed of light, but that is just geometry change, not motion thru space (as you say.)

Thanks for the correction! I just did a quick Google search for the velocity of the Milky Way galaxy, I should have been more specific.

 

[Drakkith]

Marcus or Chalnoth, is there a local redshift due to actual motion away from us equivalent to z=3 redshift? IE would an object moving away from us here in local space have the same redshift at a certain velocity?

 

[marcus]

Marcus or Chalnoth, is there a local redshift due to actual motion away from us equivalent to z=3 redshift? IE would an object moving away from us here in local space have the same redshift at a certain velocity?

To translate local velocities into doppler one uses the relativistic doppler shift formula
If β is the speed (as a fraction v/c of speed of light) then
1+z = sqrt((1+β)/(1-β))
so if you want the speed that would give a doppler shift (not a cosmological redshift but an actual doppler shift) of 3, then you have to set that sqrt = 1+3 = 4
so what's inside the sqrt must = 16.
And you can solve for β.

Let's see what that would be, in the example of shift=3 that you proposed.

16(1-β) = (1+β)
15 = 17β
β = 15/17 of the speed of light.

That is a purely special rel. thing, the calculation applies in local nonexpanding geometry and the speed you get has essentially nothing to do with expansion that occurs while light is traveling long distances.

As I'm sure you know but other readers might not, if you go here
http://www.einsteins-theory-of-relativity-4engineers.com/cosmocalc.htm
and plop 3 into the redshift box and press calculate then it tells you that the recession rate was 1.6 c when
the light was emitted and 1.5 c when the light was received here on earth.
The cosmological redshift is the result of all the expansion that was happening all while the light was in transit from there to here. It does not depend just on one instantaneous relative velocity, like doppler does.

That's obviously different from the doppler 15/17 c.

 

[Drakkith]

Awesome, thanks Marcus!

 

[Drakkith]

Hmmm. Just thought of another one.
How can we tell the difference between doppler and cosmological redshift?

 

[turbo]

Hmmm. Just thought of another one.
How can we tell the difference between doppler and cosmological redshift?

In our local neighborhood, peculiar motions can be dominant, but as we look farther and farther out, peculiar motions become negligible in the total redshift of an object.

 

[Drakkith]

In our local neighborhood, peculiar motions can be dominant, but as we look farther and farther out, peculiar motions become negligible in the total redshift of an object.

I'm sorry turbo, I have no idea what you are saying here. What are "Peculiar Motions"?

 

[turbo]

I'm sorry turbo, I have no idea what you are saying here. What are "Peculiar Motions"?

Peculiar motions are motions of objects relative to us. For instance, M31 is quite close to us, so any redshift/blueshift that we observe can be mostly attributed to the motions of our galaxy and M31 relative to each other. If we want to observe a galaxy at the limits of our observational instruments, it can safely be assumed that those galaxies are not flying away from ours (peculiar motion) and their redshifts can be attributed to cosmological effects.

This is OK, though not comprehensive, IMO.
http://en.wikipedia.org/wiki/Peculiar_velocity

 

[marcus]

Hmmm. Just thought of another one.
How can we tell the difference between doppler and cosmological redshift?

Turbo is right in everything he says here but I'll try explaining from a different angle:

both have the same effect of shifting the wavelength, so we CAN'T tell the difference just by looking at the bright and dark lines of the spectrum but we can use commonsense or plausible reasoning. If the galaxy is in what looks like a cluster then probably it is roughly the same distance as the other ones in the cluster. If the distance is the same then the cosmo redshift should be the same. So if the shift is very different from the others then probably that is the add-on contribution of a doppler. It is probably moving towards or away with its own individual motion.

And if it is NOT different from the others in the cluster then the natural tendency is to attribute it to cosmo redshift. Unless you have some other handle, like an independent distance determination say by means of some supernova or variable star that can serve as a standard candle. If you can tell the distance you may be able to infer that the whole cluster is moving towards or away and has a doppler contribution adding on to the usual cosmo redshift that happened while the light was in transit.

Things have their own individual motions (called their "peculiar" motions) and it takes some detective work to sort it out.

Stars on the righthand edge of a galaxy might be less red and those on the left edge more red---so you can figure that the thing is rotating! That effect is doppler and is superimposed on the generally more dominant cosmo redshift effect.

 

[Drakkith]

Thanks Turbo and Marcus. All that makes sense. However, what I am asking is how to tell, fundamentally, whether redshift is caused by the doppler effect or cosmological effects. Since we can't tell just by looking at the spectrum, what causes us to say that redshift is due to the expansion of space and not simple motion away from us? AKA we can look at a galaxy cluster and say that redshift due to expansion is z=.3, but why do we say its because space is expanding and not doppler shift?

I've taken it for granted that we can tell, but I've never known what our reasons are.

 

[turbo]

Thanks Turbo and Marcus. All that makes sense. However, what I am asking is how to tell, fundamentally, whether redshift is caused by the doppler effect or cosmological effects. Since we can't tell just by looking at the spectrum, what causes us to say that redshift is due to the expansion of space and not simple motion away from us? AKA we can look at a galaxy cluster and say that redshift due to expansion is z=.3, but why do we say its because space is expanding and not doppler shift?

I've taken it for granted that we can tell, but I've never known what our reasons are.

Actually, we can't tell by measuring redshifts, but we can make educated guesses about which mechanisms are responsible for the redshift of individual galaxies, as Marcus explained. If a galaxy appears to be physically associated with others in a string or a cluster, it's probably safe to assume that its redshift is due to a common (cosmological) mechanism. If its redshift is anomalous WRT to its apparent neighbors, we have to consider that it might be a foreground or background galaxy projected by chance on its apparent neighbors. Absent other distance-indicators, we have to rely on best-estimates.

 

[Drakkith]

Actually, we can't tell by measuring redshifts, but we can make educated guesses about which mechanisms are responsible for the redshift of individual galaxies, as Marcus explained. If a galaxy appears to be physically associated with others in a string or a cluster, it's probably safe to assume that its redshift is due to a common (cosmological) mechanism. If its redshift is anomalous WRT to its apparent neighbors, we have to consider that it might be a foreground or background galaxy projected by chance on its apparent neighbors. Absent other distance-indicators, we have to rely on best-estimates.

Perhaps I am wording my question incorrectly.

Since we can't tell by looking at redshift, why do we say "space is expanding" instead of "galaxies are moving away from us THROUGH space". I'm guessing that this is a much bigger can of worms than I thought it was.

 

[turbo]

Perhaps I am wording my question incorrectly.

Since we can't tell by looking at redshift, why do we say "space is expanding" instead of "galaxies are moving away from us THROUGH space". I'm guessing that this is a much bigger can of worms than I thought it was.

Much bigger can of worms! Can we assume that we are the center of the observable universe, and that individual galaxies and entire clusters are racing away from us? That's not a tenable position. You could invoke another mechanism (like the discredited "tired light" idea) to explain cosmological redshift, but it is not reasonable to assume that all objects at cosmological distances are rushing away from us. I'm tired and may not be explaining this well, but I'll try again later if you still have questions.

 

[Drakkith]

Much bigger can of worms! Can we assume that we are the center of the observable universe, and that individual galaxies and entire clusters are racing away from us? That's not a tenable position. You could invoke another mechanism (like the discredited "tired light" idea) to explain cosmological redshift, but it is not reasonable to assume that all objects at cosmological distances are rushing away from us. I'm tired and may not be explaining this well, but I'll try again later if you still have questions.

I'm not trying to invoke anything here. I've just never seen an explanation for why we say cosmological redshift isn't due to doppler shift. Like why isn't doppler shift compatible with observations of redshift. If both are indistinguishable from each other by comparing redshift alone, then I'm assuming that the reason lies elsewhere?

 

[marcus]

It can be thought of either way but becomes very complicated if thought of as doppler.

The basic underlying thing is the assumption that we live in a solution to the Einstein GR equation. GR has been tested a lot and is considered pretty reliable. Plus the model derived from GR fits the data well.

So there is an expansion history a(t) and the redshift satisfies 1+z = a(now)/a(then).

Now this CAN be treated as the result of a concatenation of an infinite number of infinitesimal doppler effects, occurring in little almost flat Minkowski patches along the way. You can hop from one coordinate patch to the next all along the way the light traveled, and perform a cumulative doppler shift.

Because the cumulative redshift depends NOT ON A SINGLE SPEED or rate of recession, say at the time of emission, but on a whole history of recession rates that prevailed at different times during the light's travel.

But you can analyze the cosmo redshift as the cumulative result of many many small dopplers. And therefore you can say that it IS A DOPPLER. what you say depends on how you treat it mathematically. Poincaré said that mathematics is not right it is *convenient*. It is a sophisticated attitude, he was a smart man. Don't ask what nature IS, ask what is the most convenient way to describe it. Call it a succession of many tiny doppler shifts if you like. Or call it stretching out the lightwave, if you like. They give the same answer.

 

[Drakkith]

You're saying that because of the way GR works simply thinking of the redshift as doppler only works if you make little short hops and stay "local" the whole time? Alright, I can understand that. Thanks Marcus.

 

[Khashishi]

It seems to me there are two different ways to create redshift. The first is if the light source is moving away from us in a static spacetime. The second is if spacetime is expanding and the source is being carried away with it. In general there would be some combination. How can we observe the difference? Is the difference real or just in the coordinate system used?

 

[cng99]

Well, I know it might be too late to point out, but there's this really awesome link for anyone who hasn't understood.
http://www.exploratorium.edu/hubble/tools/center.html

The tiny widget they have created, makes things really clean.

 

[Chalnoth]

Hmmm. Just thought of another one.
How can we tell the difference between doppler and cosmological redshift?

Cosmological redshift is correlated with distance measures. Doppler redshift isn't.

 

[Chalnoth]

It seems to me there are two different ways to create redshift. The first is if the light source is moving away from us in a static spacetime. The second is if spacetime is expanding and the source is being carried away with it. In general there would be some combination. How can we observe the difference? Is the difference real or just in the coordinate system used?

If you're using these to describe the expansion of the universe, there is no difference whatsoever between these two descriptions. It's just a coordinate system choice.

 

[Drakkith]

If you're using these to describe the expansion of the universe, there is no difference whatsoever between these two descriptions. It's just a coordinate system choice.

Wouldn't there be a difference in recession velocity, since doppler shift doesn't really work well with galaxies receding faster than light? Or would either work fine as long as you are using GR appropriately?

 

[Chalnoth]

Wouldn't there be a difference in recession velocity, since doppler shift doesn't really work well with galaxies receding faster than light? Or would either work fine as long as you are using GR appropriately?

Recession velocity isn't well-defined anyway. There is no absolute definition of the relative velocities between far-away objects. So it shouldn't be a surprise that you'll get different velocities in different coordinate systems.

 

[Drakkith]

Recession velocity isn't well-defined anyway. There is no absolute definition of the relative velocities between far-away objects.

I was not aware of this. Got any links that explain?

 

[Chalnoth]

I was not aware of this. Got any links that explain?

I'm not aware of any popular sources for this, sorry. You might be able to find something by searching for "relative velocity general relativity".

But in the mean time, in General Relativity the only time vector subtraction is well-defined is at a single point. When you try to subtract one vector at one point from a vector at another point in space-time, ambiguities arise as to how to do that.

To take a simple example, one method in General Relativity that allows you to subtract two vectors at different locations is through parallel transport. Parallel transport moves one vector across some path towards the other vector, keeping this vector parallel to itself along the entire path.

The problem is that in curved space-times, parallel transport can lead to different answers depending upon which path you choose. And there is no a priori way of saying that one path is better than any other path.

 

[Drakkith]

I see. Alright, I'll see if I can find some more information on it. Thanks guys.

 

[emailanmol]

I would like to point out that the quality of discussion on this thread has been phenomenal.
They have given a huge boost to my conceptsThanks :-)

 

[phinds]

Drakkith, another way of saying it is that is is a difference between "moving in space" and "space expanding". Far distant galaxies ARE "moving in space" relative to us (towards, away, sideways, whatever), but by utterly trivial amounts compared to the effect of the expansion of space. Nearby objects, however, are NOT moving relative to us due to the expansion of space, they really are moving IN space relative to us.

Hope I haven't just made it less clear instead of more clear.

EDIT: Hm ... I think I skipped a full page of replies and didn't say anything that wasn't already said. Guess I just like the sound of my own keystrokes.

 

[Drakkith]

So I was thinking...if an object is moving THROUGH space, as opposed to WITH space, then wouldn't the CMB be noticeably anisotropic?

 

[alexg]

So I was thinking...if an object is moving THROUGH space, as opposed to WITH space, then wouldn't the CMB be noticeably anisotropic?

It is. The temperature variations are small, but they're there.

http://www.astro.ucla.edu/~wright/CMB-DT.html

Yea, 100 posts.

 

[phinds]

So I was thinking...if an object is moving THROUGH space, as opposed to WITH space, then wouldn't the CMB be noticeably anisotropic?

The anisotropy of the CMB is tiny because the motion THROUGH space is tiny. A MUCH larger effect would be due to the fact that areas that we see as CMB could only have been in casual distance of each other (and thus be at very nearly the same temperature) if it is true that there was this incredible inflation that occurred from (I forget the exact figures) something like 1E-47 to 1E-33 after the singularity when the universe expanded by a factor of something like 10E80. All very incomprehensible stuff to me but if you posit it as real then it DOES explain a number of things including what would otherwise be an unexplainably tiny anisotropy of the CMB.

 

[bapowell]

There is a relatively large anisotropy that results from our motion through space relative to the CMB -- it's called the CMB dipole. Higher-order anisotropies, however, are independent of our motion and result from primordial inhomogeneities. These are the ones that we study to understand the composition and evolution of the universe; the dipole is ignored.

Is this what you had in mind Drakkith?

 

[Chalnoth]

So I was thinking...if an object is moving THROUGH space, as opposed to WITH space, then wouldn't the CMB be noticeably anisotropic?

Others have provided good answers here, but I thought I'd add in a little caveat.

There is no absolute distinction between moving "through" or "with" space. Space itself has no absolute motion. The CMB rest frame is only a convenient choice of rest frame when doing cosmology. It is not an absolute frame.

Interestingly, the movement of galaxy clusters with respect to the CMB changes how the cloud of hot gas within those clusters interacts with the CMB. So we can, in principle, measure the movement of these galaxies with respect to the CMB simply by examining their brightness. In practice this is fantastically difficult because it turns out that the spectrum of this deviation in brightness is exactly the spectrum of the CMB itself, so I'm not sure that we've ever had a confirmed example of the kinetic SZ effect (that's the name of this effect).

 

[Drakkith]

Awesome, thanks all.

 

[RUTA]

Others have provided good answers here, but I thought I'd add in a little caveat.

There is no absolute distinction between moving "through" or "with" space. Space itself has no absolute motion. The CMB rest frame is only a convenient choice of rest frame when doing cosmology. It is not an absolute frame.

Interestingly, the movement of galaxy clusters with respect to the CMB changes how the cloud of hot gas within those clusters interacts with the CMB. So we can, in principle, measure the movement of these galaxies with respect to the CMB simply by examining their brightness. In practice this is fantastically difficult because it turns out that the spectrum of this deviation in brightness is exactly the spectrum of the CMB itself, so I'm not sure that we've ever had a confirmed example of the kinetic SZ effect (that's the name of this effect).

http://www.spacedaily.com/reports/D...niverse_formation_into_sharper_focus_999.html

 

[Chalnoth]

http://www.spacedaily.com/reports/D...niverse_formation_into_sharper_focus_999.html

Ahh, interesting. Here is the source paper, in case anybody is interested:
http://arxiv.org/abs/1203.4219

The detection at 3.8 sigma isn't that significant for a new detection, but it does seem fairly likely to be correct.

 

[phinds]

There is no absolute distinction between moving "through" or "with" space. Space itself has no absolute motion.

Point taken. I use "with space" to discuss the motion of distant galaxies that recede due to the "dark energy" accelerating expansion, even though I realize that it is more appropriate to just say that they are getting farther apart, not "moving with space".

 

[Krunchyman]

The fact that galaxies move faster than the speed of light is the reason why we could potentially use hypothetical Alcubierre drives to accelerate objects faster than light speed.

 

[Chalnoth]

The fact that galaxies move faster than the speed of light is the reason why we could potentially use hypothetical Alcubierre drives to accelerate objects faster than light speed.

Doesn't work. No galaxy moves faster than a light ray moving past it. Nothing can.

 

[phinds]

The fact that galaxies move faster than the speed of light is the reason why we could potentially use hypothetical Alcubierre drives to accelerate objects faster than light speed.

Nonsense. You would be well served to study some physics before making such statements.