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Light Years And Expanding Universe

  1. Jun 7, 2003 #1
    How is it that we are able to see galaxies that are billions of light years away considering that we are presumably travelling from the same origin? Would that somehow mean that the distance between the two galaxies was growing at a rate greater than the speed of light? If not, why not?

    The same question holds for the cosmic background radiation--or is it since the universe was expanding long before the creation of this radiation, we are only seeing the radiation created on the outside boundaries?

    Is the answer simply that there was a huge "head-start" between matter prior to the creation of light? If that's the case, then what existed prior to it? Were these particles travelling at the speed of light?

    Perhaps this seems like a silly question, but it's one for which I have yet to get a good answer from my Astronomy101 teacher. :)
  2. jcsd
  3. Jun 7, 2003 #2
    When the first stars started emitting light the universe was already a huge size. And the distance between the star that emitted the light we see and our place in the universe since then increased considerably.

    The cosmic microwave background radiation is suposed to be caused shortly after the big bang, when the universe became less dense, so that photons (from matter - anti-matter annihilation) could travel freely. These photons are supposed to travel anywhere and in any direction.

    The answer is: we don't know. But some theories developed assume there were scalar fields before the Big Bang happened.
    The BB is the effect of large scale changes in the scalar field.

    See for example http://moriond.in2p3.fr/J00/" [Broken] for a lecture on inflation and the creation of matter in the universe.

    There exists no silly questions, only not asking the question is silly.
    Last edited by a moderator: May 1, 2017
  4. Jun 7, 2003 #3


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    Well, first, we aren't travelling from the same origin. The Big Bang created spacetime and was characterized by a hugely rapid expansion of space at first. There was no matter at that time, just a soup of energy and fundamental particles. After about 300,000 years of expansion, that energy/plasma (which was everywhere with spacetime) cooled enough for those fundamental to glom together and form atoms of matter. At that point, the universe became transparant to light. Then over time, those atoms grouped together into stars and galaxies, etc. And of course, space kept expanding that whole time.

    Once there were galaxies, the expansion of space was not faster than light (it is currently measured by the Hubble Constant...something like 60-70 kps/Mpc). Once stars/galaxies formed (already millions/billions of light years apart from each other), their light could travel to other stars/galaxies. About 13.7 billion years have passed since the Big Bang, so light has had almost that long to travel...so we can see things that are up to about 13 billion light years away in every direction. The whole universe is bigger than that. And don't forget the finite speed of light means that the farther away you look, the older the image is.

    The CBR is that original hot energy of the universe that keeps cooling as the universe expands. The CBR is, and has always been, everywhere. The Big Bang was not an explosion from a central point into empty space. It happened everywhere in the universe simultaneously (it's just that the points within the universe have been getting farther apart since the Big Bang). There are no outside boundaries.

    So, I guess the point is that the stars and galaxies formed far apart from each other and it take light a certain amount of time to get from one place to another. The universe has only existed for a certain time, so we can only see light that has been able to reach us within the alloted time frame.
  5. Jun 7, 2003 #4


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    The standard big bang picture is that light and also some forms of matter were there essentially from the start, but the light was interacting with the matter.

    The sun, for example, is not transparent because it is too hot and dense for a ray of light to get thru without being scattered. There was light in the early universe but it was continually interacting and being scattered----a kind of opaqueness. So we do not see that early light.

    It is estimated that the universe became transparent around 300 thousand years into its life. The CMB photons began their journey then----it is sometimes called the moment of "last scattering".

    The universe had become less dense and cooler, so that light could travel without constantly being absorbed and reradiated by stuff, i.e. being scattered.

    I do not think of it as a "head-start", but there was this opaque period of 300 thousand years at the beginning.

    Early, at the very hot time, speeds of particles were relativistic. Because high temperature corresponds to high speeds of moving around---high kinetic energy.

    Please look at the lightcones drawn in Ned Wright's cosmology
    tutorial....you are learning "astronomy 101" and he teaches
    "cosmology 101" at UCLA


    you must scroll down about 40 percent to find the pearshaped

    the side of the lightcone, in red, can be seen as the path
    in a spacetime diagram, of a photon coming to us from
    far in the past, like a CMB photon,
    and traveling thru a space which expands as the photon
    travels on its way to us

    keep on asking questions
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