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Thread: The Visible Horizon- Astrophysics Question

  1. #1
    Join Date
    Feb 2003
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    The Visible Horizon- Astrophysics Question

    All right, here we go... please correct me if I have any of my facts wrong, as the info I'm basing my query on comes from many different sources.
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    The question was (drumroll), if we can see (or "detect" is a better word, I think, since you can't "see" the cosmic background radiation) objects/radiation from the beginning of the universe, how is our visible universe expanding? How can new objects be moving into our visible horizon if we can already "see" objects and areas of space whose light has taken 14 billion years or more to reach us, especially since our visible universe's horizon is expanding at the speed of light?

    My conclusion was that the objects and parts of the universe whose light takes longer than the age of the universe to reach us must have been moving at faster than the speed of light at some point, as our visible horizon is expanding at the speed of light. I asked my astronomy professor (Dr. Tom Abel at PSU, you may have heard of him) about this and he said that yes, some parts of the universe can at one point move faster than c, but that we could never detect them because of that. I was already familiar with that idea, the concept of exclusively superluminal particles (tachyons, I believe). However, it didn't really answer my question, as we obviously are detecting some of those once-superluminal areas as they move into our visible horizon - that's why they are moving into our horizon.

    Slowing down from above lightspeed is impossible (well, highly improbable, I'm a fan of quantum mechanics ) just as accelerating past lightspeed is impossible. So how could these areas of space have done just that?
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    Anyone got an answer, or a comment? If the explanation to this is Inflationary Theory, just tell me...

  2. #2
    Join Date
    Jun 2003
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    8,753
    I anm not an astronomer, so you probably know more about these things than I do, but I don't see the problem.

    The 'visible' universe is expanding, because of the expansion of space.

    The first 300,000 years or so of the universe is opaque, because it is too dense; but the most distant objects we will ever be able to see will be objects 300,000 years after the big bang;
    because we are moving forward in time and space is expanding, these first visible objects will be more and more distant, and occupy the surface of an expanding sphere, so will become more and more numerous.

    None of the objects that we can ever hope to see have slowed down from superluminal speeds, as far as I can figure out.

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