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The Scale of the Universe

  1. Jul 23, 2010 #1
  2. jcsd
  3. Jul 23, 2010 #2


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    Cool, a nice variation on the old Powers of Ten movie (itself an adaptation of the neat 1957 book Cosmic View which can be viewed here). I did notice one inaccuracy, though--due to the expansion of space, the size of the observable universe (i.e. the maximum distance some object, at rest relative to the microwave background, could be from us today and for it still be possible to us to today be receiving light from that object when it was younger) is much larger than 14 billion light years--see wikipedia's Observable universe article.

    edit: Also, no idea where they got the claim that the size of the whole universe is 93 billion light years! If space is flat it could be infinite. And saying "we're probably not at the center of the universe" in that final image would seem to be meaningless since even a finite universe isn't expected to have any outer boundaries to define a center, unless they're talking about something like the idea of "domain walls" in inflationary theory (beyond which space would continue, but certain basic fields would be in different ground states and thus the high-level laws of physics would appear different)
    Last edited: Jul 23, 2010
  4. Jul 23, 2010 #3
    Domain walls also do not have a center.
  5. Jul 23, 2010 #4
    i dont believ thats what they meant. they were pointimg toward the center of the screen and SOME dumbass might believe they were saying "WE ARE THE CENTER OF THE UNIVERSE!!! WORSHIP ME!!!" :rolleyes: just as some still beleive the Earth is only 6500 years old... (i got into a fist fight on my construction site over this.)
  6. Jul 23, 2010 #5


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    Diagrams like the one on this page seem to suggest the structure of the different domains is something like bubbles in foam, and each bubble has a center:


    Why do you say they wouldn't have centers? Relative to whatever cosmological coordinate system we find most natural, can't we find the distance between any point inside the domain and any point on the domain wall? There might be different ways to define the "center" of an irregular shape, but we could pick some simple definition, like the point inside the domain that has the largest distance to the closest point on the domain wall...
  7. Jul 24, 2010 #6
    It's not that easy...

    Your picture looks like there was some universal time, common for all domains. This is not correct. In domains time flows "inside", that means near the wall the time is near 0 and in the center it is a big number. Actually, the point in the center of a domain corresponds to time infinity.

    Domain wall is not something static, it rather moves at the speed of light for any observer.

    Also, each domain does expand, so each point of it is equivalent.

    Maybe it is possible, but I think the line connecting two points from different domains would have to go through singular points, so it would be hard to define its length.

    Lol, I read it so long ago, so I don't remember where I saw all of this.
  8. Jul 24, 2010 #7


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    Surely the rate that time "flows" can only be defined relative to some coordinate system? If so, what coordinate system are you using? Can you provide a link or other source that backs up what you're saying here?
    Why does expansion imply "each point of it is equivalent"? As an analogy, successive spacelike cross-sections of a future light cone give a light sphere which is also expanding at the speed of light, but relative to any given inertial frame the light sphere does have a center (although there is no frame-independent definition of the center)
  9. Jul 25, 2010 #8
    Domains expand in such a way, so each point is equivalent. There's no preferred point. There's no "closer" and "further" from any particular point to a domain wall. In fact, from the inside of domain point of view, the domain wall is in your past. You would have to go back in time to touch it. Digging deeper - the domain wall is the Big Bang itself, except in this theory the initial state of Universe has finite nonzero size.

    You assume that you are in special relativity flat space. Domains expand in different way, more like cosmological inflation.
    Our Universe was once compressed in one point, but we don't have any center today, have we?

    Sorry, I can't. It was too long ago. Perhaps it's time for me to shut up.
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