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Size of the universe

  1. Apr 13, 2010 #1
    Hello, I am quite new to this forum, but have been lurking around reading your posts for some time now.

    I have one question that has bothered me for some time. Scientists have been observing the universe with advanced technology, and seem quite certain that their observations indicate the age of the universe is approximately 13.73 billion years old, and that the "diameter" of the observable universe is 93 billion light years. My question is how is this possible if supposedly nothing can move faster than the speed of light? I've read somewhere that the universe itself expands with a speed greater than c, and I've also stumbled upon tachyons. Which also supposedly moves faster than light. I was wondering which "things" are limited to the speed c, and why is it this way?

    This does not make any sense to me. I've watched numerous documentaries where physicists say that nothing can move faster than light. Have they not studied the universe's expansion, or is this just a matter of course?

    I hope some of you can shed some light on this topic for me, I haven't found any good explanation to this yet..
  2. jcsd
  3. Apr 13, 2010 #2


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    This is quite a common question. My suggestion is to read this article: http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf [Broken]
    and come back with any further questions you may have.
    Last edited by a moderator: May 4, 2017
  4. Apr 13, 2010 #3


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    I'll give you a nutshell answer:

    Nothing can travel locally faster than the speed of light, but there is no such constraint on points billions of light years apart. It is not the points that are moving faster than c, it is the space between them that is expanding. Locally, those points are relatively stationary.
  5. Apr 13, 2010 #4
    Thanks! That article was very helpful.

    But I'm still wondering about the tachyons, if they even exist why can they travel faster than c without receding speed
  6. Apr 13, 2010 #5


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    Tachyons are hypothetical particles; there is no reason to believe they exist. The point is that relativity does not forbid particles travelling faster than c, but it does forbid them slowing down to c. Also, they would travel backward in time.
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