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Looking Back in Time

  1. May 21, 2009 #1
    I guess I must be missing something obvious, but I've had this on my mind for a long time and can't see the error in my thinking.

    Light travels at a fast, but finite speed, so we see things not as they are now, but as they were when the light left the object.
    If follows from that, that the further an object is away, the further back in time we're looking at it.
    So far, so good.

    But we often hear astonomers talking about seeing back ever close to the origin of the universe - to the big bang.
    And this is what bothers me. Please fault me reasoning for me:

    Any event only happens once - and the instant and the instance of that event is never repeated.
    An "event ripple" propogates outwards from that event at light speed, but if you miss that event - if it passes you before you started looking - you'll NEVER SEE IT.
    So, for example, consider the the sun, which is 8 light minutes from Earth.
    If it exploded NOW, and you slept for the next NINE minutes - you'd miss it forever (of course, you'd be dead so the point is practically moot).

    Now unless the universe expanded faster than light speed for some of its history, the "event ripple" of the Big Bang will always be beyond anything in the universe, and so no one/thing will ever see it - and no one/thing ever did. In fact, it defines the limits of the universe - and if you're in it - you can't see it.

    Where did I go wrong?

  2. jcsd
  3. May 27, 2009 #2
    At the event of the Big Bang, the universe "exploded", but it only exploded in a sense that it started to expand. It expanded quickly at high speeds and as it expanded it cooled down. Perhaps at some point the universe expanded at a speed close to the speed of light, but I'm unsure of it actually expanded at a rate that matches the speed of light.

    I might not have answered your question, but maybe it helped =P
    Last edited: May 27, 2009
  4. May 27, 2009 #3


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    As long as you are talking about the standard cosmology model that professionals in the field use, then you can be sure. This model derives from General Relativity and fits the observational data, so it is a pretty good bet.

    In that model there is no outer boundary and on one speed that the universe expands. Individual distances expand, at a rate proportional to distance. So larger distances expand more rapidly.

    Naturally some distances will be expanding exactly at the speed of light, and others will be expanding faster. Distances to some known objects are expanding several times faster than c. It all depends on how large the distance is that you are looking at.

    We get this same misconception several times a week. You are thinking of expansion cosmology as an "explosion" of matter from a central point outwards into empty space. That is not how the professionals picture it.
    "Big Bang" is just some bad terminology that caught on with the public. Journalists liked the term. But it is not descriptive. The words "Big Bang" actually lead people into misunderstanding and confusion.

    If it were an actual physical explosion outwards into empty space then of course we wouldn't see early universe stuff, because the light would have outraced the matter. Light from early universe processes would be long gone. Everybody who thinks that expansion is a physical explosion seems to eventually realize this and is puzzled by it---so we get a stream of folks all asking this same question.

    I wonder what the solution to this problem is. We have to come up with an image which can grow in the public's imagination and fight against the explosion misconception, hopefully overcoming it. The experts are working with a mathematical model of expanding distances, what we need is an intuitive picture that corresponds closer to their actual model--but something you can evoke with words.
    The best thing I know of so far is the balloon model---a reduced dimension simplified toy model where the 2D surface of the balloon takes the place of regular 3D space.

    That has the obvious disadvantage that although people can grasp the balloon model, once they understand that they still have trouble going up one dimension and getting from the 2D picture to 3D.

    But if you want, you can try. Google "wright balloon model" and watch the animation. It has the basic elements. The white whirling things are galaxies. The colored wrigglers are photons of light---they travel thru space (whereas the galaxies don't move, they stay at the same latitude longitude position on the balloon and just get farther apart.)
    All existence is imagined to be concentrated on the surface that you are watching. You see the galaxies standing still while the photons of light travel between them, from one galaxy towards another. And you can see the wrigglers wavelengths get larger---that is called redshift.

    There is a lot to watch and understand just in that simple 3 minute video. A lot of stuff that is important in real 3D cosmology. (Only in a 2D toy version.)

    If you watch the animation enough you will see the answer to your puzzle. The photons always travel at the same speed---like one millimeter per second in the toy model. And since the photons only travel across the face of the balloon, they never "get away". A photon that starts out in the early universe is still there after distances have expanded by some factor (and its wavelength you can see graphically has expanded by the same factor that distances have.)

    Imagine a person living in one of the galaxies. He always has some incoming light that originated in the early universe. Look at the animation---you will see this---it's evident. So naturally, if you take the toy picture up to the analogous 3D situation, we can and do observe light from the early universe---the oldest is from around year 380,000. Earlier light is blocked out by glare. But still year 380,000 is pretty old!

    Check it out and see if it clears up the puzzle for you! :biggrin:
    Last edited: May 27, 2009
  5. May 28, 2009 #4
    Thanks for your help and your patience, Marcus. That pretty much does it for me.
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