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How far can we see?

  1. Mar 30, 2009 #1
    I was wondering, since the universe is expanding, what is the maximum distance that we will ever be able to see any object?

    Also, how long do we have before the sky becomes nearly black?
     
  2. jcsd
  3. Mar 30, 2009 #2

    mgb_phys

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    At the moment 46.5 billion light years*
    The 'furthest' object you can see is the cosmic microwave background. This is the point at which light and matter separated in the early universe so you can't see further back than this because there was no light.
    So ironically the 'edge of the universe' (if you like) isn't black but white (at least in the microwave band).

    The limit isn't so much distance as time - as we look further out, we are looking further back in time.
    There is a distance before the first stars formed where you could consider it black in the visible.

    *The rate that this is increasing depends on the cosmological model of the day so is a little complicated to work out.
     
  4. Mar 30, 2009 #3

    marcus

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    Our galaxy is not expanding. We are surrounded by stars and they are not thinning out.
    For the sky to become starless, we would have to wait until most of the stars in our neighborhood burn out (and the formation of new stars ends)

    So your question has nothing to do with expansion.

    The way to answer it would be to look up wikipedia facts about stellar lifetimes.

    Actually our galaxy is scheduled to merge with the large Andromeda galaxy. That should be pretty exciting. The sun and planets might accidentally be ejected from the main merger bulk by some fluke, some largescale gravitational slingshot effect. If not we might find ourselves surrounded by more stars than before.

    I forget how long before the merger happens. Maybe something on the order of a billion years. Definitely something to look forward to.

    Anyway, expansion is real enough, but it affects the distances to remote galaxies, not distances like a few thousand lightyears! So the sky will remain beautiful at least until the combined stars of Milky and Andromeda burn themselves out.
     
  5. Mar 30, 2009 #4

    mgb_phys

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    Sorry, I took that to mean - how far out do we have to look for everything to be black.
     
  6. Mar 30, 2009 #5

    marcus

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    I'm curious. What distance do you mean? The distance away the object was back THEN when the light left it and started towards us?

    Or the distance away the object is NOW on the day when the light gets here and scoots down the telescope?

    For hands-on definiteness you should take an example? Like suppose you see a galaxy today that has redshift z = 4. How far away was it when the light left? How far is it now?
    To find out, google "wright calculator" and type in 4 and press the button.

    the distance then is what Ned Wright calls angular size distance.
    the distance now is what he calls comoving radial distance.

    Check it out.
    http://www.astro.ucla.edu/~wright/CosmoCalc.html
     
  7. Mar 30, 2009 #6

    marcus

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    I thought your response was informed and helpful--I wouldn't contradict what you said, just add more information of a different sort. AFAICS everything you said was correct.
     
  8. Mar 31, 2009 #7
    Thanks for the replies! What I mean by asking how far is the distance that is the maximum that an object can be visible is, how far is the point at which space expands faster than light between us and an object? Making it impossible for the objects light to ever reach us.
     
  9. Apr 1, 2009 #8

    marcus

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    Have a look at the Lineweaver SciAm article in my signature----where it says "princeton.edu".

    There is a fine point here. Most of the galaxies we can see today were receding faster than light (the distance to the galaxy was increasing at a rate greater than c) at the time that the light left them and began its journey to us. And nevertheless the light still got here!
     
  10. Apr 1, 2009 #9
    Thanks for the help marcus:)
     
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