Far away galaxies moving faster than light?

and9

NO spacetime is neither created nor destroyed. Whoever said that spacetime is being created between two galaxes to accomodate 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

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 dissapear forever from their detection just as we see them.

Chalnoth

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

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

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.

Radrook

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

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, lets 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 in to 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

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

Chalnoth

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.

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.

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.

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.

Mark M

That is what cosmology holds, that the universe is expanding. This is because of Hubble's Law - there is a linear relationship between the proper distance to a galaxy, and its redshift. Photons emitted from distant galaxies traverse more expanding space than do closer galaxies, and are redshifted more - the expanding space stretches their wavelength. The gravitational pull of the galaxies makes the expansion of space absolutely negligible inside of them.

Flustered

Galaxies would not be elliptical or irregular if they were not moving.

Mark M

Why do you conclude that? Galaxies rotate, and are filled by large amounts of matter and dark matter that have gravitational pulls far stronger than the expansion of the universe.

Chalnoth

Perhaps you mean that galaxies would not collide if they were not moving? This is strictly true, but the statement that "galaxies are not moving" must be understood in context. A few points:

1. It is not an absolute statement. It is a relative statement. Galaxies are not moving relative to the CMB.
2. It is not an exact statement, but an approximate one. Individual galaxies can have quite a bit of motion with respect to the CMB, sometimes as high as 1000km/s. But when you take a large collection of galaxies, say, over a few hundred million light years, the average motions with respect to the CMB nearly cancel.