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WorldWeird: Help me, cosmic sciences types!

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  1.  (7090.1)
    I'm having some trouble wrapping my head around the relation between the age of the universe, the distance light travels to get to us, and how the universe has been expanding.

    http://www.scientificamerican.com/article.cfm?id=most-distant-grb

    A violent explosion picked up by a NASA satellite earlier this year is the oldest object ever seen by astronomers, its light having been emitted some 13 billion years ago. At that time the universe was roughly 5 percent of its present age and the big bang was a fairly recent occurrence, having taken place just 600 million years earlier.


    The universe was much smaller 13 billion years ago. Why would we be able to see that light now instead of having those photons pass our location in space a long time ago? I mean (hopefully making my question clear), I'm assuming that our point in space would have been a lot closer to the point of origin of that light back then than it is now. Right?

    So why would that light arrive after 13 billion years rather than, say, two billion years in a much smaller universe, passing the point in space that would eventually hold us?

    Is any of this question making any sense? Because I feel like I'm tripping balls every time I contemplate space/time.
    • CommentAuthorLani
    • CommentTimeOct 29th 2009
     (7090.2)
    Well, although the universe was smaller then, the star started off many billions of light-years away, so it would still take a long time for the light to reach us. And when it does, the light won't even be in the original wavelength - In those billions of years that passed while the light was traveling toward us, the star was traveling further and further, due to the universe expanding, leading to the redshift phenomenon (essential the Doppler effect, but with light). Does that answer your question?
  3.  (7090.3)
    Urrrrgggggghhhhhh....my head hurts.
  4.  (7090.4)
    So this is the cosmic version of that nightmare you have were your running down the corridor but the end of the corridor keeps pulling away from you and you can never reach the end no matter how fast you run?
    Maybe there is a connection?
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      CommentAuthorAriana
    • CommentTimeOct 29th 2009 edited
     (7090.5)
    So why would that light arrive after 13 billion years rather than, say, two billion years in a much smaller universe, passing the point in space that would eventually hold us?


    Okay, you're imagining that our planet and the light are both like cars driving away from a starting line. That the explosion is the point of origin, and both us and the light are moving away from it at the same time. And because light travels faster than we do, it should have passed us.

    Instead, you need to think about two people holding either end of a REALLY stretchy rubber band. REAAAALLLY stretchy. And there's a little ant running down the rubber band from one person to the other. Only as the little ant is running, those two people are stepping away from each other -- stretching out the rubber band. So the rubber band is getting longer as the ant runs, right? For every inch the ant thinks it's running, the rubber band has gotten a half-inch "longer".

    Now, the ant IS moving faster than the rubber band is getting longer, so it WILL eventually get to the other end. But what started out looking like would be about a foot of travel-time has turned into twenty feet by the time the ant gets to the other end. So when the ant turns around to see where it started from, it suddenly looks like TWENTY feet! And, where it started out looking like a five-minute trip, because there kept being more and more and more distance, it turned out to be twenty minutes.

    That help?

    (Yes, I am well aware of the inaccuracies in this metaphor. Dear lord. It's to help with visualization, not to use as an equation. Pffft.)
    • CommentAuthorKradlum
    • CommentTimeOct 29th 2009
     (7090.6)
    Wow! Ariana is not just a web guru, she's also an astro-physicist with the unusual ability to make astro-physics understandable to the layperson! Once I digested the metaphor I think it makes perfect sense.
    • CommentAuthorFan
    • CommentTimeOct 29th 2009
     (7090.7)
    There's an illustration at Understanding the expansion of space, which says,

    Spacetime is highly curved at cosmological scales, and as a result the expansion of the universe is inherently general relativistic; it cannot be understood with special relativity alone.


    "General relativity" means/says that space is curved by gravity (whereas special relativity is that light is constant, i.e. that Ariana's ant is moving away from one end of the rubber band at the speed of light, and moving towards the other end at the speed of light, even though the two ends are moving apart).

    Anyway, light which reaches us now, and which originated from an object on the other side of the universe, and which originated when the universe was small/young, is light which has taken a curved path to reach us.
  8.  (7090.8)
    Now, the ant IS moving faster than the rubber band is getting longer, so it WILL eventually get to the other end. But what started out looking like would be about a foot of travel-time has turned into twenty feet by the time the ant gets to the other end. So when the ant turns around to see where it started from, it suddenly looks like TWENTY feet! And, where it started out looking like a five-minute trip, because there kept being more and more and more distance, it turned out to be twenty minutes.


    Ah, makes perfect sense now.

    Thanks everyone!
  9.  (7090.9)
    I like the ant metaphor. Makes a bit of sense to me that way, too.
    And it's cute. And memorable.

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