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Thread: Why we still receive light from very distant galaxies?

  1. #1
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    Why we still receive light from very distant galaxies?

    Please forgive me if this simple question is too obvious, I'm just learning
    Suppose I see a very distant galaxy XYZ, for example 11 billion light years away. That means light took that time to arrive here and was emitted when the universe was only 2.7 billion years old.
    However, by that time this galaxy must have been much closer to our milky way and thus to us. Of course we werent here to see it, but light should have arrived much earlier than today since it didn't have to travel that long.
    So, since inflation stopped quite some time ago, and thus faster than light expansion no longer applies, ancient XYZ's light must already have reached us billions of years ago, and we should only be able to see more recent emissions.
    Why that is not the case?
    Thanks

  2. #2
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    Welcome Dan,

    You can think of it as the photon and our galaxy were racing away from the distant galaxy we observed, and it took 11 billion years for the photon to make up all the initial distance. The analogy I just gave is simple, and if you try to apply it for precision cosmology it is *way* too simple... I simplified the issue of the universe expanding by pretending that the two galaxies had some constant relative velocity. It is more complicated to work out them math for space expanding.
    Forming opinions as we speak

  3. #3
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    Quote Originally Posted by DanRozenfarb View Post
    Please forgive me if this simple question is too obvious, I'm just learning
    Suppose I see a very distant galaxy XYZ, for example 11 billion light years away. That means light took that time to arrive here and was emitted when the universe was only 2.7 billion years old.
    However, by that time this galaxy must have been much closer to our milky way and thus to us. Of course we werent here to see it, but light should have arrived much earlier than today since it didn't have to travel that long.
    So, since inflation stopped quite some time ago, and thus faster than light expansion no longer applies, ancient XYZ's light must already have reached us billions of years ago, and we should only be able to see more recent emissions.
    Why that is not the case?
    Thanks
    There is nothing to prevent distant galaxies moving away from us faster than light, it doesn't contradict relativity. We can even see some of them today. The initial (post inflation) expansion was much faster than it is now which is why the light from objects that were very close to us (only millions of light years away) has taken billions of years to reach us. Here is a very good article which explains the most common misunderstandings of the Big Bang theory in understandable language. It is well worth the read.
    Last edited by loglo; 2012-May-05 at 12:07 AM. Reason: syntax

  4. #4
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    Dan,

    The light we see now from galaxy XYZ was indeed emitted
    11 billion years ago. Light which was emitted by XYZ earlier
    than that reached Earth at some time in the past. Light which
    was emitted by XYZ later than the light we are seeing now
    will reach Earth some time in the future.

    As you say, XYZ was much closer to our current location when
    it emitted the light we see now than the 11 billion light-year
    distance the light travelled to reach Earth. And XYZ is now
    much more than 11 billion light-years away, since we have
    been moving farther apart the whole time.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  5. #5
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    Quote Originally Posted by DanRozenfarb View Post
    Please forgive me if this simple question is too obvious, I'm just learning
    Suppose I see a very distant galaxy XYZ, for example 11 billion light years away. That means light took that time to arrive here and was emitted when the universe was only 2.7 billion years old.
    However, by that time this galaxy must have been much closer to our milky way and thus to us. Of course we werent here to see it, but light should have arrived much earlier than today since it didn't have to travel that long.
    So, since inflation stopped quite some time ago, and thus faster than light expansion no longer applies, ancient XYZ's light must already have reached us billions of years ago, and we should only be able to see more recent emissions.
    Why that is not the case?
    Thanks
    Inflation stopped but the universe still expands and that rate is increasing.

    So this is a terminology thing.

    "Inflation" is usually used when talking about the early universe. We are talking the first 1x10-32 second and it has been "expanding" ever since.
    Both terms refer to space getting larger.

    graph.gif

    The size of the visible universe changed was HUGE. 40 orders of magnitude. But at that point it still is only like 1m across. At 1 second it was 1x1020m across.

    Expansion is still happening and seems to be getting faster so the above image should look something like this.
    inflationexpansion.png

    Where the red is how the rate of expansion started increasing a few billion years ago.

    Remember the image you are looking at is logarithmic so you can see the detail where it is needed.

  6. #6
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    May 2012
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    Thank you all for your responses. I'll take a deeper look at the subject and study your suggestions, and come back here if still in doubt.
    Also thank to you all I now feel encouraged to ask some other questions that are lurking in my head.

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