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A silly doubt

  1. Jun 29, 2011 #1
    I read in a book by M. Kaku:

    "To describe the universe, cosmologists sometimes use the example
    of looking down from the top of the Empire State Building, which
    soars more than a hundred floors above Manhattan. As you look down
    from the top, you can barely see the street level. If the base of the Empire State Building represents the big bang, then, looking down
    from the top, the distant galaxies would be located on the tenth floor.
    The distant quasars seen by Earth telescopes would be on the seventh
    floor. The cosmic background measured by the WMAP satellite would
    be just half an inch above the street."

    I am confused by one thing:
    If the light of big bang reached here now, how come we came so far before light arrived here as even our galaxy originated from big bang?

    Please explain.
    Thank you.
  2. jcsd
  3. Jun 29, 2011 #2
    The light has been travelling at speed c since the putative big bang. Matter has not. So its not surprising that the light got here first. Its currently believed that light was unable to excape through the dense matter of the early universe for the first 300,000 years. As to the red-shift: although light always travels at c in a vacuum it loses energy (momentum) through gravitational interaction. So after escaping its 'cage' it still had to fight most of the mass of the universe to get here. It is gravity, not 'slow light', which is responsible for the red-shift.
  4. Jun 29, 2011 #3
    The same condition can be attributed to any point in space/time. Effectively this is the temporal edge, and wherever you are in the U you are always sat on the temporal edge.
  5. Jun 29, 2011 #4
    I don't think I am the right one to exactly pinpoint the answer to your doubt. But I hope you aren't thinking that the big-bang is an explosion in the same sense as a bomb explosion.
    The big bang occured everywhere not at any special point in space.
    I would suggest you to read the FAQs of the cosmology section. I don't have any great physics or higher mathematics background but still could gain something from the FAQs. Those are written very lucidly.
    https://www.physicsforums.com/showthread.php?t=506991 [Broken]
    Last edited by a moderator: May 5, 2017
  6. Jun 29, 2011 #5


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    The gravity thingy is irrelevant. The matter that eventually resulted in planet earth was separated from CMB photons we currently observe faster than 'c' due to expansion / inflation. The matter comprising our galactic neighborhood also emitted CMB photons, but, before the CMB photons we currently observe. It is obviously illogical to propose we can ever observe photons emitted by our ancestral matter. Cosmo Novice is correct, we do reside at the temporal edge of the universe - as does every other observer in the universe.
    Last edited: Jun 29, 2011
  7. Sep 24, 2011 #6
    "Bomb explosion" was my analogy of big bang, can't think of it any other way. Even the link you gave me didn't help much. Anyways I was reading the book just out of curiosity, and now realize that things are far too complicated than they appear so won't probe into it anymore. Thank you all for helping.
    Last edited by a moderator: May 5, 2017
  8. Sep 25, 2011 #7
    I hope not. That might be a recipe for an easy life, but if you want to understand physics then I would advise to carry on probing into anything that seems wrong until you find out what the problem is.

    Anyway, the answer to your question involves the coordinate systems allowed by general relativity. Cosmologists prefer to use a time coordinate equal to the time experienced by objects, but then objects can move faster than c according to the coordinates - but not faster than light, which moves faster still.

    If you choose to use more 'special relativistic' coordinates where the speed of light is a limit then [STRIKE]the cosmologists will get angry with you[/STRIKE] your time no longer matches the proper time of objects (as in the twin paradox), so the cosmic background is no longer half an inch above the street
  9. Sep 26, 2011 #8


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    That's the problem with the common view of the big bang. Instead of an explosion, imagine the universe as a baloon that we all live on the surface of. At the beginning of the universe the baloon was not inflated. As time went on the balloon expanded and everything on the surface experienced an increase in distance from everything else. The further apart two objects are on the balloon, the faster the distance between them increases as the balloon expands. If you have a balloon and a marker you can do this at home by simply putting a bunch of dots on part of the balloon and blowing it up. The dots at the opposite edges of your dotted area get further apart faster than dots nearer to each other.

    That is just a way to visualize it though, as that is not exactly what happens. At the beginning of the universe everything was "closer together" meaning that the distance between everything was smaller. Over time the distance between everything increased. There was not an "explosion" like is commonly thought. Interestingly, the term "Big Bang" was coined by an opponent of the theory that the universe came from a finite time in the past.

    See this article for more info: http://en.wikipedia.org/wiki/Big_bang
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