What is the maximum recessive velocity of a galaxy in the observable universe?

[EskWIRED]

The farthest galaxies are receding from us the fastest.

Which one holds the speed record?

And how fast is it going compared to c?

 

[marcus]

The farthest galaxies are receding from us the fastest.

Which one holds the speed record?

And how fast is it going compared to c?

Just want to make sure you realize "receding" does not refer to ordinary motion thru space that we are used to. It is just a pattern whereby distances increase---nobody gets anywhere by it, everybody just gets farther apart from everybody else. Relative positions don't change.

the great majority of the galaxies we can see with a telescope are receding faster than c. I guess you probably know that, just wanted to make sure. all that means is the distances from us to them are increasing faster than c. It doesn't mean they are moving thru space faster than light
they wouldn't be catching up with and passing any photons.

You can look up "most distant galaxy" on google and wikipedia. I think the current redshift maximum is around z = 10. That is for a galaxy.

the ancient matter (from before time when galaxies formed) which we see because it emitted a glow of ancient light which we now pick up as the cosmic microwave background, is currently receding at rate of 3c.

That is the farthest matter that we can see with the instruments we've got now.
It is pre-galaxy, and it the distance to it is increasing at rate which is three times the speed of light.

Galaxies don't recede that fast.

To find what speed corresponds to z = 10, go to this calculator:
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html
and put the wavelength stretch factor S = z+1 in for the upper row of the table. In the box where it says "S_upper". then press calculate. So you add one to the number and get z+1 = 11 and plug 11 into this calculator.
You will see that the galaxy WAS receding at 4c when it emitted the light we are now receiving from it, and that it now IS receding at a bit over 2c, something around 2.18 times c.

The reason you have to add one is that astronomers have the awkward custom of using the number z which is ONE LESS than the actual factor by which the wavelength is expanded. An incoming wave has its wavelength mutlitpled by the stretch factor S=z+1 from what it was when the light was emitted. That is the factor by which distances have been enlarged while the light was in transit, traveling on its way to us.

So anyway, check the calculator out. It's handy for a lot of things. Be sure you know how to put stretch factors you're interested in into the S_upper box, and then look at the top row of the table for your answer. The rest of the table can be interesting too, but your question just concerns the top row, for S=11.

 

[marcus]

Here's wikiP about a mini-galaxy UDFj-39546284
http://en.wikipedia.org/wiki/UDFj-39546284
The redshift z is estimate at around 10.
The actual figure they give is 10.3

Or you could say the maximum CONFIRMED redshift is z = 8.55 and plug in z+1 = 9.55 into the calculator. That is for an object which has been tested and retested and they;ve gone thru all the steps very carefully so its redshift is "spectroscopically confirmed" and everybody agrees and feels confident.

UDFj-39546284 has not yet been "spectroscopically confirmed". I personally feel pretty sure they've got z = 10 but I guess it's more difficult to determine redshift for very very early stars because they consist mainly of hydrogen and helium, so you don't have as many chemical elements to make lines in the spectrum. Once those stars have lived their short hot lives and blown up and scattered heavier elements out into space, then new stars can form which have more elements like oxygen and sodium and iron etc. And those elements glow with their own distinctive patterns of spectral lines. So there is more information in the light. More known wavelengths to study and see how much they have been stretched.

The current distance champion changes from year to year. So you just have to look up "farthest galaxy wikipedia" or "highest redshift wikipedia" and find out the current highest redshift, and then use the calculator to translate into speed.

 

[EskWIRED]

Just want to make sure you realize "receding" does not refer to ordinary motion thru space that we are used to. It is just a pattern whereby distances increase---nobody gets anywhere by it, everybody just gets farther apart from everybody else. Relative positions don't change.

the great majority of the galaxies we can see with a telescope are receding faster than c. I guess you probably know that, just wanted to make sure. all that means is the distances from us to them are increasing faster than c. It doesn't mean they are moving thru space faster than light
they wouldn't be catching up with and passing any photons.

You can look up "most distant galaxy" on google and wikipedia. I think the current redshift maximum is around z = 10. That is for a galaxy.

the ancient matter (from before time when galaxies formed) which we see because it emitted a glow of ancient light which we now pick up as the cosmic microwave background, is currently receding at rate of 3c.

That is the farthest matter that we can see with the instruments we've got now.
It is pre-galaxy, and it the distance to it is increasing at rate which is three times the speed of light.

Galaxies don't recede that fast.

To find what speed corresponds to z = 10, go to this calculator:
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html
and put the wavelength stretch factor S = z+1 in for the upper row of the table. In the box where it says "S_upper". then press calculate. So you add one to the number and get z+1 = 11 and plug 11 into this calculator.
You will see that the galaxy WAS receding at 4c when it emitted the light we are now receiving from it, and that it now IS receding at a bit over 2c, something around 2.18 times c.

The reason you have to add one is that astronomers have the awkward custom of using the number z which is ONE LESS than the actual factor by which the wavelength is expanded. An incoming wave has its wavelength mutlitpled by the stretch factor S=z+1 from what it was when the light was emitted. That is the factor by which distances have been enlarged while the light was in transit, traveling on its way to us.

So anyway, check the calculator out. It's handy for a lot of things. Be sure you know how to put stretch factors you're interested in into the S_upper box, and then look at the top row of the table for your answer. The rest of the table can be interesting too, but your question just concerns the top row, for S=11.

Yes, I realize that the universe is expanding, and that the distances we are discussing are increased by that phenomenon. Indeed, you seem to have discerned what I was really asking about. I was wondering how fast the cosmic background radiation was receding from us, but I couldn't even quite imagine measuring something like that, so I asked about galaxies.

I don't at all understand how we can "see" superluminal expansion. I understand that there must have been superluminal expansion to account for the distances we see - but I did NOT know that the velocities are currently (according to our observations and our version of "currently") superluminal. I most certainly did not know that "the great majority of the galaxies we can see with a telescope are receding faster than c."

So to be sure that I understand, I take it that we can image objects in various phases of the expansion of the universe. if we look at close-up objects, we see them recede at a rate close to the current (and accellerating) rate if expansion. If we look a little farther away, we see objects as they were when the universe was not expanding quite so fast. And if we look very, very far, we see things as they were in ancient epochs of the universe, way back when the universe was undergoing superluminal expansion?

I was not aware that photons existed during superluminal expansion. I thought that by the time of the CMB radiation, superluminal expansion had ceased.

Have I got this all wrong?

 

[George Jones]

I don't know if this helps or confuses:

I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55.


Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

O                                B        A        C
*                                *        *        *


*                    *     *     *
O                    B     A     C

The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, $t_e$. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, $t$. Notice that space has "expanded" between times $t_e$ and $t$.

Suppose that at time $t_e$: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time $t$, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because A's recession speed at time $t_e$ is greater than c, the pulse fired towards us has actually moved away from us between times $t_e$ and $t$.

Now, suppose that the distance from us to galaxy B at time $t$ is the same as the distance to galaxy C at time $t_e$. Even though the distances are the same, the recession speed of B at time $t$ is less than than the recession speed of C at time $t_e$ because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times $t_e$ and $t$.

Since A's recession speed at time $t_e$ is greater than c, and galaxy C is farther than A, galaxy C's recession speed at time $t_e$ also is greater than c. If, however, the Hubble constant decreases enough between times $t_e$ and $t$, then B's recession speed at time $t$ can be less than c. If this is the case, then at time $t$ (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times $t_e$ and $t$.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time $t_e$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time $t$, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

 

[Mordred]

I don't at all understand how we can "see" superluminal expansion. I understand that there must have been superluminal expansion to account for the distances we see - but I did NOT know that the velocities are currently (according to our observations and our version of "currently") superluminal. I most certainly did not know that "the great majority of the galaxies we can see with a telescope are receding faster than c."

So to be sure that I understand, I take it that we can image objects in various phases of the expansion of the universe. if we look at close-up objects, we see them recede at a rate close to the current (and accellerating) rate if expansion. If we look a little farther away, we see objects as they were when the universe was not expanding quite so fast. And if we look very, very far, we see things as they were in ancient epochs of the universe, way back when the universe was undergoing superluminal expansion?

I was not aware that photons existed during superluminal expansion. I thought that by the time of the CMB radiation, superluminal expansion had ceased.

Have I got this all wrong?

to add to George Jones reply

Vrec=D*H_o in simple form the greater the distance, the greater the recessive velocity.

what this means is that Vrec is an observer dependent scalar value, from Earth we see the recessive velocity increasing the further you look. However if we were to teleport instantly to a location where we see the recessive velocity at 3c. The expansion rate would be the same as it is near Earth.
If you think about that, it essentally means that light has no problem tranversing local distances.
as the local expansion is far far slower than the speed of light. Now as that photon approaches Earth, the distance from Earth also decreases, therefore so does its recessive velocity.

There is a point however that light can never reach us, If I recall correct we will be able to see it around 17.3 billion Gly. I would have to double check that on the calculator Marcus mentioned.