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
