Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Distances to "young" galaxies

  1. Jun 14, 2017 #1
    I’m joining this forum just to see if someone can answer this question in a way that I can understand, because it’s driving me nuts.

    I read about galaxies observed by the Hubble Space Telescope that were formed “shortly” after the Big Bang. For example, the article may say that they are 12 billion light years away (and we are therefore seeing them as they were roughly 1.7 billion years after the Big Bang, assuming the BB happened 13.7 billion years ago).

    However, our observable horizon in any direction is 47 billion light years – or 94 billion light years in diameter – due to hyper expansion of space.

    Why aren’t these galaxies we can see that were formed roughly 1.7 billion years after the BB referred to as being more like 42-43 billion light years away?

    Beyond that, we don’t have any idea how big the universe is since we will never see most objects beyond our observation horizon because they are moving away from us faster then the speed of light. How is it we can even see objects that were formed so soon after the BB?

    Obviously, I’m confused.

    Thanks to anyone who can help me understand this, or point me to some helpful resources.
     
  2. jcsd
  3. Jun 14, 2017 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Probably because the author of the news doesn't understand cosmology. Every time I see something like that in a Wikipedia article I fix it, but it is like fighting windmills.
    We only see the galaxies within the observable universe. For every universe age, there is a given "distance then" where we see the objects now. For the cosmic microwave background (CMB), this "distance then" is about 42 million light years (and the corresponding "distance now" is about 46 billion light years).

    Our distance to matter that emitted the CMB we see today always increased faster than the speed of light. We still see the light because the relative expansion slowed down. Initially the distance between us and the emitted light increased, but after a few billion years the expansion of the universe was slow enough for the light to come closer to us. The Ant on a rubber band is a nice analogy. Initially its distance to the goal increases, but eventually it reaches the end.
     
  4. Jun 14, 2017 #3

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    ... assuming that the expansion is linear. :rolleyes:
     
  5. Jun 14, 2017 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    With the accelerated expansion there are places where the light will never reach us even in the very distant future, but that is not important to understand the past history of the universe.
     
  6. Jun 14, 2017 #5

    phinds

    User Avatar
    Gold Member
    2016 Award

    To expand slightly on mfb's answer, the light emitted by distant galaxies was emitted, say, 12 billion years ago. The distance of that galaxy from us when the light was emitted was, say, 5 billion light years (that's just a made up number but it's to show that the distance was not 12 billion light years). Subsequent to that the galaxy moved away from us and is now over 40 billion LY away. The emitted light did not reach us in 5 billion years even though it started out 5 billion LY away from us because the space through which it moved was stretching, so it took 12 billion years. Light emitted "now" (I put "now" in quotes because it gets complicated) when that galaxy is 40+ LY away from us will reach us in, say 200 billion years (again, I'm making up numbers but this is the right idea) and the galaxy will be, say, 500 billion LY away from us.

    The ant on the rubber band analogy that mfb pointed you to is helpful.
     
  7. Jun 14, 2017 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Light emitted now by anything more than ~16 billion light years away will never reach us, assuming the cosmological constant is indeed constant.
     
  8. Jun 15, 2017 #7

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Just for clarification, the 16Gly figure cited by mfb is roughly the proper ['now'] distance to a galaxy with a current redshift of about z=1.8. Such a galaxy is crossing our cosmological event horizon and no photons emitted from this time forward will ever reach earth. This, however, does not preclude light from galaxies with a current redshift exceeding z=1.8 from reaching earth. For example, we still observe cmb photons which have a current redshift of about z=1090 - which corresponds to a proper distance ~ 45Gly. A galaxy currently at redshift z=11, which is about the maximum observed redshift for any known galaxy corresponds to a proper distance of ~ 32 Gly. So, obviously the universe was expanding more rapidly at high z, which explains why we can still observe stuff like cmb photons and z=11 galaxies. This is why proper distance is a confusing way to discuss cosmological distances. See Expanding Confusion, https://arxiv.org/abs/astro-ph/0310808, by Davis and Lineweaver for further details.
     
  9. Jun 20, 2017 #8
    Thanks for all your responses.

    So I’m not missing anything – the articles are incorrect and these young galaxies are much further away than the 12b to 13b ly’s stated.

    I’ve been ruminating on the second question, and here it is – I hope with more clarity.

    Start with this:

    - the further away an object is from us, the older it is since we are looking back in time.

    - our observable horizon is ~ 47b ly’s. Most objects further away from us than that we will never see since they are moving away from us faster than the speed of light. (I know that our observable horizon is expanding and that the light from some “closer” objects beyond the 47b ly’s will eventually be encompassed by our horizon, and eventually make it’s way to us)

    - So, we don’t know how big the universe is – it may be infinite.

    - A galaxy that is, say, 13b ly’s old is almost as old as the universe itself.

    Such a galaxy is most likely well beyond our 47b ly horizon, so my question is – how can we even see it?

    As mfb said “With the accelerated expansion there are places where the light will never reach us even in the very distant future”. Surely a galaxy almost as old as the universe is one of these “places”.

    This makes my head hurt.
     
  10. Jun 20, 2017 #9

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Milky Way or the Andromeda galaxy could very well be as old as the universe, and yet you'd have no trouble seeing them, nor would they be beyond the observable universe.

    What is usually reported, is galaxies which formed early in the history of the universe. As such, these are the youngest, not oldest galaxies. You'll always see those galaxies near the edge of the observable universe, since that's where the early light was emitted.
    In terms of 'images' of galaxies that 'exist' at some distance or another, there are no galaxies beyond the observable universe - there were no galaxies back then.
     
  11. Jun 20, 2017 #10
    Yeah, I misspoke - I meant very young galaxies we can see in the distant past.
     
  12. Jun 20, 2017 #11

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Ok, then such a galaxy is not beyond the observable universe.
     
  13. Jun 20, 2017 #12

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    As stated above already, "moving away from us faster than the speed of light" is not where the horizon is. We see things where the distance was always increasing faster than c for every time in the past.
    Right.
    Right.
    Why do you think so? Some are inside, some are outside.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted