Geometry of the universe

  1. Astronomers say that there is neither an edge or center to the universe, yet we live in a finite universe. I understand how our universe can be finite yet have no edges due to the curvature of the universe, but I can't understand how it doesn't have a center. Space must be being curved around something right? is it possible that a supermassive (and I mean SUPERmassive) black hole could lie at the center of the universe curving the entire visible universe around it? just a thought
     
  2. jcsd
  3. DaveC426913

    DaveC426913 16,239
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    Compare the 3-dimensional curvature of the universe to the 2 dimensional curvature of a ball.

    If an ant lived its entire life roaming on the surface of a ball, it would experience a universe that was of finite extent yet had no boundary - and its universe (which is the surface of the balloon) has no centre. No spot on the surface of the balloon (the ant's entire universe) can be considered any more a centre than any other spot.
     
  4. marcus

    marcus 24,521
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    Jackson, be careful to distinguish between "entire visible universe" and the universe.
    Astronomers don't say the universe is finite. They don't know whether finite or infinite.

    They say the visible universe, or socalled observable universe HAS center (our solar system) and they say it IS FINITE. The radius as of today is estimated approximately 46 billion lightyears, with us at the center naturally.

    That's the OBSERVABLE universe. But when they talk about the UNIVERSE that is a completely different thing. Not to be confused with the observable portion. Like what Dave just said: what you can see of the earth's surface, out to your horizon, is different from the entire surface of the earth.
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    About cosmology. It is a mathematical science which means finding the simplest math model which best fits the data. Adjusting the model to match the past observations and seeing if it then predicts future observations. The present model has been amazingly successful. That doesn't mean it's RIGHT. As soon as anyone can offer a simpler model with a better fit, people will go for it. The current model is just the best the best we can do for the present with the math tools and the data which we've got. But it does not mean that the current model is RIGHT.

    And it is true that the current model has no center and no edge or boundary. And the matter is approximately evenly distributed (largescale average) throughout the space.
    And the largescale average curvature is either zero or very close to zero.
    This geometry is described by the 1915 law of evolving geometry called Gen. Rel. (GR).
    It is both a law of gravity and a law of how geometry interacts with matter and changes over time. (The expansion you hear about is according to this law and has been checked.)
    This takes a while to understand.
    Lets be a little more careful. Astronomers do NOT say the U is finite. They say the observable portion is finite. Which it obviously is.

    About the whole U, we don't KNOW, and astronomers do not claim to know. They can tell you about the math models they use, which fit the data remarkably well and have a remarkable track record for predicting accurately.

    The good cosmic models we have are a great human achievement and it's right to be proud of them and want to describe their features (like no center, no boundary or edge, approx even distribution of matter, expansion, a calculable history of structure formation, the microwave background....) But ultimately we don't know some basic things.

    We don't know whether the finite version of the model is best or whether the infinite version is best. They give almost the same numbers so the fit about equally well. So we can't distinguish! More observation is needed, in order to tell. And so on. Things like that.
    They really shouldn't say they KNOW and I don't think they do say that typically. There is usually some modest reservation stuck in, if you read carefully. Like "according to our theory" or "according to our model" blah blah X Y and Z. And of course the theory or model can be eventually proven wrong and improved 5 or 10 years down the road.

    Anyway that's my take on it, as an interested bystander. Keep asking questions. It's fun.
     
  5. How can the universe, either visible or entire, be any larger than 14 billion light years in diameter if it is only 14 billion years old? Since nothing can travel faster than the speed of light, wouldn't that create a boundary around 14 billion light years? and thats in the universe is expanding at the speed of light which I don't beleive it is.

    Thank you for you post by the way it was very helpful
     
  6. marcus

    marcus 24,521
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    I'm glad it was helpful!
    About this:*The radius as of today is estimated approximately 46 billion lightyears.*

    That is, as of today, if you could freeze the expansion process, and measure by timing a radar blip or light flash. It would take 46 billion years to make the oneway trip.

    that's the idea of the today distance or (technically called) proper distance.

    It is that far because the distances have grown during the almost 14 billion years that the light was in transit.

    the material that emitted the glow which we now receive as microwave was about 1100 times closer when it emitted the light, than it is today as the light is arriving to us.

    It was about 41 million LY THEN (proper distance) and it is about 46 billion LY NOW.

    Light travel time (say 13.7 or as you say 14 billion years) rarely corresponds in any simple way to these distances since the expansion has occurred at widely varying rates over the course of history.
    So one tends not to use light travel time as a gauge of distance, except in popular accounts aimed at general public.
     
    Last edited: Dec 23, 2011
  7. DaveC426913

    DaveC426913 16,239
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    The speed of light is a local limit; the expansion of the universe is not limited to the speed of light. There are many threads on this.
     
  8. so the universe can expand without moving any matter faster than the speed of light relative to any other body? I still can't understand how if there has been only 14 billion years to travel, how anything could be farther than 14 billion light years away without breaking the laws of special relativity
     
  9. marcus

    marcus 24,521
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    SR has limited applicability it is a theory about a non-expanding spacetime geometry which has no curvature. This is not realistic. But SR is very useful in any situation where the curvature is almost zero and the expansion is so slight as to be negligible.

    The trick is to know when NOT to apply SR and when NOT to expect it to give reliable answers. It is after all only an approximation to the real geometry.

    SR works best over relatively short distance and brief timespans, away from very massive objects.

    GR is apt to work better on large (like intergalactic) distances and long (cosmological) time scales. But all mathematical models are only approximations and must have their limits of applicability. Even GR fails if pushed too far, as to the center of a hole or back in time to the very start of the bang. There is always room for improvement of the models.
    ===================

    A nice thing is that GR RETAINS ALL THE GOOD STUFF OF SR, because it has SR built in to its local coordinate patches---ready to serve as a useful local approximation. So when one moves up to the more general theory one does not have to GIVE UP anything. One just gets the same SR stuff but in a more complete context with the additional ability to describe curvature, gravity, expansion of distances etc.
     
    Last edited: Dec 23, 2011
  10. Hi all

    Just for clarification when you talk about the radius, Are you talking center point to the perimeter or circumradius.

    thanks

    kevin
     
  11. phinds

    phinds 8,784
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    Circumradius? What makes you think that concept is pertinent here?
     
  12. sorry
     
  13. Drakkith

    Staff: Mentor

    The radius from our vantage point here on Earth, to the furthest visible distance.
     
  14. Thank you Drakkith for the clarification

    kevin
     
  15. DaveC426913

    DaveC426913 16,239
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    Just to be clear here. This "centre of the observable universe" is a subjective thing. If we flew to Mars, or A. Centauri, or Andromeda, we would still be at the centre of the OU - it would be just centred on wherever we are.

    It is exactly equivalent to standing on the Earth and observing that the Earth's observable surface has a radius (the horizon) and we are at the centre. But that would be the case no matter where on the Earth we stand. It does not mean that any particular point on the Earth is special compared to any other.
     
  16. If the Universe has no edges then how can it have a center?
     
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