Far away galaxies moving faster than light?

[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 Equations$$R_{\mu \nu }-\frac{1}{2}g_{\mu \nu }R+g_{\mu \nu }\Lambda =\frac{8 \pi G}{c^{4}}T_{\mu \nu }$$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.

 

[Chalnoth]

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

Easy...and a bit wrong. There's a sense in which it can sort of kinda match reality, but it's still wrong.

 

[Drakkith]

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 a quick easy answer to a very very common question, and most people asking it have no clue about anything in cosmology and it would entail a very long and confusing conversation to explain it to them if they even accepted it at all.

 

[Naty1]

Post #1

What did they mean exactly?

The discussion here is how to interpret superluminal expansion...stuff moving faster than light.

A short answer is that using a particluar cosmological model, the FLRW model and the conventions and assumptions that go into it, we calculate 'superluminal' (faster than light] expansion both at really early cosmological times and continuing to today at really great distances. We cannot directly observe such rapid expansion.

Those willing to learn should pay close attention, very close attention, to Chalnoth's explanations. They are NOT easy to interpret without an understanding of the underlying
conventions and some technicalities a/w cosmology.

Here are some underlying ideas that may help. I post these to illustrate some conceptual difficulties people have [me included] with 'superluminal expansion': [I have posted the 'expert' sources from here in the forums where I have them recorded in my notes.

Locally, nothing moves faster than light. Even the most distant observers measure light at 'c' locally.

In introductory physics, "expansion", say of a heated rod, is measured relative to a practical invariant standard, say an invar. measuring tape. In cosmology, "expansion", refers to something altogether more strange and unfamiliar to practical ordinary life: a change in a metric coefficient a(t) in the expression...

[In cosmology, we have to choose which 'invarient' to use as a basis for our calculations; having chosen, we can't directly observe all results! An implication of this is that 'increasing distances' are NOT well represented in the balloon analogy!]

Marcus:

The Hubble rate is decreasing and will continue to decrease. The current Hubble rate says that largescale distances (like between widely separated galaxies) increase 1/140 of a percent every million years. This percentage is expected to decline towards around 1/160 of a percent.

..The scale factor a[t] is increasing as defined by it's time derivative and that is what most people mean when they say expansion is accelerating.

[so here are two 'rates' with different stories..and that's ok ]Wallace, I believe: [apparently a practicing cosmologist]:

The rate of expansion [velocity] is unimportant; It is the rate of acceleration of the expansion that tells you what happens. So in a contracting universe a distant particle could move away, or in an expanding universe a distant particle could come toward you. You don't intuitively expect this behavior if you think of the universe as a loaf of rising bread filled with raisins!

A curve of constant cosmological time [along which we would like to measure a proper distance’ ] connecting two points in a FRW [model] universe is not a "straight line", i.e. it is not a geodesic.

[If you would like to see a better 'picture' of cosmological distance than the simple balloon analogy , check here:

http://en.wikipedia.org/wiki/Metric_expansion_of_space#Understanding_the_expansion_of_Universe

[Note especially the red and orange line descriptions.]

Comoving and proper distances are not the same concept ...as in special relativity. It is important to the definition of both comoving distance and proper distance in the cosmological sense (as opposed to proper length in special relativity) that all observers have the same cosmological age. For instance, if one measured the distance along a straight line or spacelike geodesic between the two points, observers situated between the two points would have different cosmological ages when the geodesic path crossed their own world lines, so in calculating the distance along this geodesic one would not be correctly measuring comoving distance or cosmological proper distance.

[Observers at both ends would get different answers for spatial separation...because distance is a dynamic variable in GR.]

http://en.wikipedia.org/wiki/Comoving_distance#Uses_of_the_proper_distance Wallace:

All this ‘superluminal’ velocity at great distances tells us is how one of many different possible definitions of distance changes. If you accept the FLRW metric then you have to live with that. Other metrics that use different co-ordinates but make the same physical predictions do not contain any apparent superluminal recession...

Marcus:

The properties such as "cosmological time", , "spatial distance", "time", "preferred coordinate systems", "preferred metrics (even with cross terms)", "expansion", "Hubble flows" etc are by and large properties of a particular solution to the Einstein equation….. With a particular solution, a specific metric, there may well be a preferred time, an idea of being at rest with respect to Hubble flow... These things are not absolute, but depend on one's choice of metric---and hopefully the metric will be a reasonably good fit to observation. Operationally, comoving distances cannot be directly measured by a single Earth-bound observer.

Marcus: [FTL is faster than light or 'superluminal'...

...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.

So what are we to make of all this?

Going back to the OP question:

What did they mean exactly?

[Chalnoth: If I get anything wrong here please correct!]

What they mean 'exactly' is that within the framework of the 'standard cosmological' FLRW [Friedman, Lemaitre, Robsertson, Walker] model of the universe, where a bunch of 'non intuitive' [to laymen] conventions must be used, we have some explanations and understandings of what's happening in the universe. The FLRW model is the standard used from just after the big bang up to the present and into the forseeable future. It's the model underlying the diagram I linked to above.

The FLRW model is only an approximation of our universe, so why use it: because it contains exact solutions to the Einstein Field Equations. Nobody knows how to solve 'exact' cosmological models. Nobody can solve them for a solar system of galaxy. FLRW doesn't even work at solar or galactic scales because the assumptions of homogeanous and isotropic characteristics [things are uniform at really,really, large scale] are poor approximations at such 'small' scales.

It's the 'metric expansion' of this model that I'm pretty sure underlies Chalnoth's posted comments.

You can get any 'distance' [metric expansion' measure you want...each measure will vary by convention and model. Cosmologists use the 'FLRW metric' as their standard distance measure. In the linked picture note that the orange line, the 'metric distance' today between the Earth and the quasar has a particular curve. [In the balloon analogy, it is not a great circle of the baloon surface..such as the one an airplane would travel!] Why that orange curve: Because it follows constant cosmological time [it's parellel to the purple grid lines of constant cosmological time in the linked illustration] a convenient but coordinate dependent convention.

So if it is the same cosmological time at the distant galaxy and Earth in the model, you can see why such a distance can never be directly observed from earth! [because observation is limited by light speed and that takes an elapsed time.]

If you have read this post this far, you may also be interested in the best discussion I have found on this subject in these forums. It provides great explanations, but its LONG.

Does Space expand? [2007]
https://www.physicsforums.com/showthread.php?t=162727&highlight=current+flow

Stick with the 2007 posts up to about post #100 or so...have fun

 

[Naty1]

I missed at least one concept I wanted to mention in the above post.

Part of a quote from Marcus in the previous post:

...With a particular solution, a specific metric, there may well be a preferred time, an idea of being at rest with respect to Hubble flow...

Wikipedia explains the convention of cosmological distance and time this way:

... "comoving" observers ...move along with the Hubble flow. A comoving observer is the only observer that will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted.

Operationally, comoving distances cannot be directly measured by a single Earth-bound observer. The comoving time coordinate is the elapsed time since the Big Bang according to a clock of a comoving observer and is a measure of cosmological time. The comoving spatial coordinates tell us where an event occurs while cosmological time tells us when an event occurs. Together, they form a complete coordinate system, giving us both the location and time of an event.

http://en.wikipedia.org/wiki/Comoving_distance

I think what this means is that when you move with the Hubble flow, spatial coordinates [which are not invarient] remain fixed so distances between them remain that way as well...expansion is 'stopped' in time and a fixed distance can be calculated. ...time then becomes 'constant' to permit an unchanging distance measure snapshot. I believe this is the 'preferred' time referenced by Marcus. Note that such a frame is not possible on earth, so we cannot make such an observation of such 'distance'...such an inability is illustrated in the linked diagram in the previous post if you look carefully. It's a calculated result rather than an observational one.

 

[Lino]

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.

Thanks Chalnoth.