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Fastest Receding Galaxy?

  1. Jul 4, 2013 #1
    The farthest galaxies are receding from us the fastest.

    Which one holds the speed record?

    And how fast is it going compared to c?
  2. jcsd
  3. Jul 4, 2013 #2


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    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 :biggrin:
    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:
    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.
    Last edited: Jul 4, 2013
  4. Jul 4, 2013 #3


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    Here's wikiP about a mini-galaxy 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.
  5. Jul 4, 2013 #4

    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?
    Last edited: Jul 4, 2013
  6. Jul 4, 2013 #5

    George Jones

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    I don't know if this helps or confuses:

  7. Jul 4, 2013 #6
    to add to George Jones reply

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

    what this means is that Vrec is an observer dependant 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.
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