Is it true? Is the universe flat?

Discussion in 'Physics & Math' started by camilus, Dec 6, 2010.

  1. camilus the villain with x-ray glasses Registered Senior Member

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    So a few months ago I saw a documentary that talked about an experiment that suggested the universe is flat. They did that cosmic light triangle in space and measused the sum of the angles almost exactly 180 degrees.

    But I recently saw a lecture where the lecturer was claiming that considering any theory of a non-flat universe was redundant. That it was "proven" that the universe is flat.

    Is this really the case? where can I find the most reliable info on this?
     
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  3. mathman Valued Senior Member

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  5. arfa brane call me arf Valued Senior Member

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    Parts of the universe are definitely not flat. The part you're standing on, for instance.

    If the 'entire' universe was flat it wouldn't have any non-flat (that is, curved) regions, but it does. So a simple test of the proposition: "the universe is flat", is the above.
     
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  7. James R Just this guy, you know? Staff Member

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    It's certainly not redundant. Astronomers have been furiously trying to measure the average mass density of the universe for decades now. One reason to do that is precisely to determine whether the universe is flat or not. Flat is the knife-edge, by the way. On either side of that (positive or negative curvature) we get either a big crunch some time in the future or else infinite expansion. (Flat has infinite expansion too but at an ever-decreasing rate. All of this ignores the effects of dark energy.)
     
  8. AlexG Like nailing Jello to a tree Valued Senior Member

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    Inflation theory explains why the universe appears flat.

    If the early universe deviated from flatness by as much as .0000000009, the current universe would be so far from flat that there would be no question. (nor any intelligent life to ask the question)

    OTOH, in an inflationary universe, omega (the measure of flatness) is forced to a value of 1.
     
  9. camilus the villain with x-ray glasses Registered Senior Member

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    Inflation explains why it APPEARS flat, but is it truely flat? I think it's possible for the universe to have inflated and expanded so dramatically that it appears flat to our "puny" measurements. Like the ant of the ballon, it just appears flat but it certainly isnt.

    I dont agree with Einstein, an infinite and flat universe scares the hell out of me. Something homeomorphic to a hypersphere always appealed to me so much more.
     
  10. AlexG Like nailing Jello to a tree Valued Senior Member

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    How flat is flat? If omega is equal to .999999999999999999999999, is it flat? To what degree of accuracy will you hold the universe to?
     
  11. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    One must be careful to distinguish the large scale structure from the small scale structure. Local overdensities don't effect the shape of the universe on large scales. In this case, flat should be read as "two parallel light rays will travel infinitely far, remaining parallel throughout their journey". This is an ideal statement, in that we don't consider what would happen if those light rays accidentally hit a galaxy somewhere along their trip.
     
  12. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    What do the error bars look like?
     
  13. camilus the villain with x-ray glasses Registered Senior Member

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    so you're saying all they measured was the local geometry, where these light beams traveled through what appeared to be R^3, but that is not necessarily a reflection of the global geometry?
     
    Last edited: Dec 6, 2010
  14. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Yep, which is probably the case. Omega is slightly smaller than 1, as I recall. I can't remember the map between omega and R, the radius of curvature, but the figure that I've heard (and don't take my word for it) is 70 million light years. In other words, our little patch is 14 million light years in radius, and the universe itself (i.e., the things we could never hope to see) is like 70 million light years in radius.

    Again, I'm not confident about these numbers at all, I just wanted to give you an idea of scales.
     
  15. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Philosophically, though, I think a flat universe is MUCH more interesting.

    For example, consider this: If the universe is flat, it is infinite. If it is infinite, then all random events occur with probability 1. That means that, not only do you exist here, you have an exact copy somewhere else in the universe, safely (hopefully) outside of your light cone. Not only do you have an exact copy, but there's also an infinite number of exact copies of you.
     
  16. AlexG Like nailing Jello to a tree Valued Senior Member

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    If you squint hard, you can see them.
     
  17. camilus the villain with x-ray glasses Registered Senior Member

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    what would an infinite universe mean in terms of the amount of energy in the universe?
     
  18. AlexG Like nailing Jello to a tree Valued Senior Member

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    It would mean an infinite amount. Once you introduce infinte, amounts become nonsensical.
     
  19. arfa brane call me arf Valued Senior Member

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    "Cosmological measurements are made in a finite region, so we have to consider two types of asymptotically Euclidean metrics, connected ones and disconnected ones."
    --S Hawking

    I suppose if the universe can be highly curved in small regions, but flat overall you have to consider what happens to the local curvature--if the density of matter is a certain value cosmologically (the Friedmann model)? I'm not pretending to understand the above except tangentially. (he he)

    --taking a punt on what a disconnected metric is, I think Hawking means the metric of a compact disconnected region which is a singularity. He goes on to state that these "won't affect scattering calculations because they aren't connected to infinity, but they will affect measurements that are made in a finite region."
     
    Last edited: Dec 6, 2010
  20. camilus the villain with x-ray glasses Registered Senior Member

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    Thats exactly the answer I was looking for. Now what would that imply in terms of the first law of thermodynamics??

    (as I always say, as long as you're asking intelligent questions you are on the right track.

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  21. RJBeery Natural Philosopher Valued Senior Member

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    Is that right? I'm not saying it isn't but this is surprising to me. What is the connection between the mass of everything, the force of the big bang, and the pull of gravity such that they are perfectly balanced in a flat universe?
    Could an infinite, flat universe contain only a finite amount of mass? Also, could a flat universe be finite? Or is "curvature back on itself" the only way for us to envision universal boundaries?
     
  22. camilus the villain with x-ray glasses Registered Senior Member

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    I dont think it is perfectly balanced, the dark energy is accelerating the expansion. So its not static or in a "steady state"

    I dont think a flat universe could be finite. It would leave too many paradoxical questions, like the "edge" of the universe, and what is beyond the edge.. Therefore I believe the most theoretically ideal structure would have it curve back onto itself, therefore making finite and compact, but still unbounded and thus no "edge" whatsoever.
     
  23. Fraggle Rocker Staff Member

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    Whoa. That seems to assume a relatively consistent density of matter throughout the universe. Why does it have to be that way? If density starts out with a specific value at the centerpoint (or any point if there is no center, or if we merely don't know where it is) and attenuates in some appropriate function as we move outward, there could easily be a finite amount of mass or energy.

    Consider as a simplified example a universe that is a narrow tube that's infinite in one direction. And let's talk about matter rather than energy because then I won't have to stop and look up the terminology. In the first centimeter of the tube we find .9 grams of matter. In the second centimeter, .09 grams. In the third, .009 grams. Etc. There is matter everywhere in this universe, but its total mass is only 1.0 grams.

    A similar algorithm can be easily developed for a three-dimensional universe that is infinite in all directions, or even a universe with more than three dimensions. The math is trivial.
     

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