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A universe infinite in size and matter

  1. Oct 19, 2003 #1
    What evidence exists for:
    1) a universe infinite in size and matter
    2) a universe infinite in size but not matter
    3) a universe finite in both.

    thnks
     
    Last edited by a moderator: Feb 6, 2013
  2. jcsd
  3. Oct 19, 2003 #2
    i think the second of those options is impossible; how could an infinite universe be finite in matter?

    i think the last option has the most evidence to support it. if any of the big bang models are correct, the universe must be finite in size (and matter, i think). the universe can be finite and still have no boundaries. i recommend you read stephen hawking's a brief history of time.
     
  4. Oct 19, 2003 #3

    jcsd

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    1) Lots and lots

    2) none

    3) not much

    Fuego many big bang models do deal with infinite universes and it is indeed one of these models that ic currently thought to be correct.

    Hawking's book in terms of cosmology is out-dated.
     
  5. Oct 19, 2003 #4

    jimmy p

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    The problem i feel with answering a post like this is that there are too many theories and viable opininons to prevent any decent information. What you read in someones report could be completely denied in another scientists opinion.

    I would say from what i know, that hard to comprehend as it is, the universe is of finite size AND mass. Scientists have predicted size of the universe by using expansion rates that they have worked out. I also remember reading that they had calculated the mass of the universe, but im not sure how reliable that is...

    Ok, for infinite size, scientists say, if the universe is of finite size, what is outside of it? I know that a lot of mathematicians say infinity does not exist, then what is the highest you can count to? It goes on forever...makes sense that the universe should (bad analogy, but hey!)

    I dont think there is any evidence or discussion about infinite size, but finite matter because if you think about it, how can a figure be placed on the value of infinity?

    Not sure if this helped but its what i think so
     
  6. Oct 19, 2003 #5

    jcsd

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    Jimmy P, nearly all cosmological evidence points to a universe that is infinite in size.

    Infinity certainly does exist to mathematicans, IIRC it is an axiom in set theory.
     
  7. Oct 19, 2003 #6

    jimmy p

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    Wouldnt that depend on which theory you prefer to believe? I thought that a lot of people who trust the big bang theory would say that the universe has to be of finite size, and is expanding. im prob wrong, im just some punk A2 student lol. :smile:

    Frankly my brain hurts when trying to comprehend universal physics. I prefer to believe that the universe is finite size but we dont know what is outside of it. But then again if the universe is everything, how can there be an outside so it must be infinite. But if the universe is expanding, what is it filling? ARGH!!! brain overload!!!
     
  8. Oct 19, 2003 #7

    jcsd

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    Well it would depend whether you thought the model with lots of observational evidence to back it up or the model with little observational evidence to back it up(and indeed lots of observational evidence agansit it as the two models are obviously mutually exclusive) was correct. Big bang theory produces models that are both infite and finite (in space) and which is correct can only be decided by verifying the difefrent predictions they make.

    The unievrse isn't expanding into anything in eiher models, it's just expanding.
     
  9. Oct 19, 2003 #8

    jimmy p

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    *pop* *splat splat splat* my brain has just exploded and landed in pieces on the carpet.

    Which model do you believe? Im sorry but this is where is slip up! in some models the universe is infinite but expanding (in general)... surely there has to be a range. Wouldnt that model debunk any theories that the universe will get too big and collapse in on itself (circle of life) which is reasonably popular?? Wouldnt this mean that the universe was ALWAYS infinite because if it started as nothing it would have had size as it expanded? hmmmm...*ping* more university options open up!!
     
  10. Oct 19, 2003 #9

    jcsd

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    The unievrse is genrally beleived by cosmolgists to be spatially infinite and I'm with them on this one as given the results from WMAP it's difficult to say otherwise (dodecahedrons and toruses excepted).

    Yes an a spatially infite universe will contiune to expand for ever which rules out the idea of a big crunch which is generally not thought to be a realistc scenario for the final stage of the universe these days.
     
  11. Oct 19, 2003 #10

    jimmy p

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    So does this mean that the big-bang theory is becoming less popular? Does that mean that the universe is ageless? ( i was gonna say timeless but that would rule out a dimension wouldnt it?)
     
  12. Oct 19, 2003 #11

    Eh

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    All of that same evidence is also consistent with a finite universe. With the exception of a very small finite universe, this is not something that can be tested and so is more of a philosophical question.
     
  13. Oct 20, 2003 #12

    Nereid

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    The big bang thoery is still consistent with the observational data, and that's perhaps the strongest thing you can say about a theory. Of course, there are a number of free parameters, but once you plug those in (as derived from observation) all is well. The big bang theory is NOT a "theory of everything", as physicists like to describe a consistent theory which accounts for all four fundamental forces (gravitation, strong, weak, electromagnetism).
     
  14. Oct 20, 2003 #13

    jimmy p

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    But surely as soon as the big-bang happened and the first particle was created, the universe had size and mass. Garn this is deep stuff...any UK lecturers take note!! (im looking to apply to uni to do physics!!!)

    So how does an infinitely sized universe start?
     
  15. Oct 20, 2003 #14

    jcsd

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    No so, a universe wih a flatness of ~1 is almost certainly infite the other solutions are non-trivial. Also an accelrating universe in standard cosmological models is infinite.

    I'll say it for the last time the universe in current standard cosmology is infinite.
     
  16. Oct 20, 2003 #15

    jcsd

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    The big bang model or it's current correction; inflation is the standard and it can generate spatially infite universes.
     
  17. Oct 20, 2003 #16

    Eh

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    That doesn't matter. A flat universe and finite universe that is multiply connected is still consistent with the evidence, regardless of any human preference for a simpler shape. As well, it is also possible that the universe is spherical but appears flat on small scales, if large enough as a whole.

    The acceleration has nothing to do with the size of the universe. A finite universe with a cosmological constant can still expand forever.

    You can say it as many times as you want, but a finite universe is still consistent with the evidence.
     
  18. Oct 20, 2003 #17

    jcsd

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    I said an accelerating expanding universe, off the top of my head I don't think that there is a standard cosmological model of an accelerating unievrse with finite space. I'd have to check this out, but the non-triviality of the flatish but finite universes comes from the fact that they require specfic arrangements of matter.

    You are correct that a universe may appear flat but m,ay actually be spherical but this cannot fit into the standard big bang model as it would require expansion at a rate far beyond what is observed.
     
  19. Oct 20, 2003 #18

    jimmy p

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    So what you are suggesting is that the universe IS expanding, but not uniformly all round, which would have caused it to be spherical. What theories suggest that the universe is flat (sounds a lot like 'the earth is flat' business to me) and what evidence supposet that then?

    Im sorry if i have been living in a box for a long time but a guy has to ask questions doesnt he?
     
  20. Oct 20, 2003 #19

    Eh

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    Slap a cosmological constant with a high enough value and even a closed universe will expand forever - I think.

    I don't think they do. Space (not talking about spacetime) can be curved even in the absence of matter. Of course a universe with a hole in its shape seems less natural than a sphere or infinite plane, but that is just an arbitrary human judgment.

    Are you sure that wouldn't depend on the intital conditions? If we assume the universe begins with zero volume, I think any size of the current universe is a problem for the standard hot big bang model.
     
  21. Oct 20, 2003 #20

    Eh

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    It starts out infinite. If you run the clock back to the early universe, the density gets much higher. In the classic singularity this density becomes infinite, but is so at each point throughout an infinite volume of space. A finite universe gets smaller as the density increases.
     
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