Are galaxies moving faster than the speed of light?

[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.