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Infinite? finite?

  1. May 19, 2010 #1
    So this has bugged me for a long time. I'm a physicist in training, and have very little knowledge of the cosmos, but:

    I've heard for a long time the notion that the universe is infinite. For a long time, this troubled me, because I really couldn't conceptualize how something could be infinite, and at the same time, how we would truly even be able to measure that something is infinite.

    Then I hear that the universe is expanding.... Something that is infinite, cannot expand.

    Finally, I constantly hear big numbers, of calculations of how many galaxies there are in our universe, and how many protons and how many stars, etc etc. If our universe was infinite, there would be an infinite number of stars/galaxies/protons.

    I'm not high enough in mathematics just yet to really grasp it, but my first thought of this is that it's "mathematically" infinite. That is, for all our calculational purposes, we just assume it is. That's just a wild guess though

    If someone could explain this, that would be fantastic.
     
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  3. May 19, 2010 #2
    Have you considered the idea that perhaps our universe (like an inflating balloon) is finite, expanding into an infinite expanse of emptiness? However, this raises the problem of a "leading edge" to the universe. What would that look like?

    Regarding the counts of stars and galaxies and protons: these colossal numbers cited in books are most certainly estimates. They are also based on how much we can see or observe. Even if the universe were infinite and there were an infinite amount of stars, there would only be a limited amount that we could actually see and observe and count, because light travels at a finite speed. There would be some stars, "infinitely far" away, that we could never see, because light from there would never get here.

    To assume the universe is mathematically infinite is reasonable, most likely because it is just so huge (in comparison to everything else we know).
     
  4. May 19, 2010 #3

    tiny-tim

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    Hi Jake4! :wink:

    I don't think the universe is considered to be infinite.

    Unbounded, yes, but only in the way that the Earth's surface is unbounded (ie, you can't fall off it :wink:).

    So it's like an expanding balloon. :smile:
     
  5. May 19, 2010 #4
    That makes more sense to me.

    Although the thought of it expanding in an infinite space, obviously makes a person question that space, how it can truly be infinite, and what laws exist there.

    I feel like when going up in scale you run into more problems than going down. Going down in scale, you can eventually find the most fundamental (theoretically) as string theory proposes. However when going up in scale, there always has to be something preceding, and all encompassing.

    For some reason MIB comes to mind. Our universe is just a ball on a necklace on the collar of a dog : P

    Its an intimdating idea though, that is very unsettling.


    When string theory and m theory (not an expert, and I'm not sure which one it falls into) says that our universe is just a soap bubble among soap bubbles, I immediately wonder, 'in who's tub?'
     
  6. May 19, 2010 #5

    Redbelly98

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    Moderator's note: This thread is asking about "the current status of physics as practiced by the scientific community", and that is within forum guidelines. Questions about the guidelines themselves should be asked in Forum Feedback.

    EDIT: This post is in response to some now-deleted, off-topic posts.
     
    Last edited: May 20, 2010
  7. May 20, 2010 #6

    marcus

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    I think SpaceTiger would disagree strongly. I remember arguing with him about this. He was a cosmology PhD student at Princeton at the time.
    He pointed out that the favored view among cosmologists was the infinite version of the standard (LCDM) model.

    This was true a couple of years ago and I believe still is. I was trying to get a hearing for the minority view that she might be spatially finite, like a 2D balloon surface except 3D.
    I never suggested that the finite volume version was favored! Only that it was possible, it hadn't yet been excluded by the data.

    When I compare the WMAP7 report with the WMAP5 report that preceded it, I see that they are giving LESS credence to the finite volume idea. It has NOT made headway.

    We don't know, of course, but they overwhelmingly prefer to work with the spatial infinite (zero overall curvature) version.

    So that means infinite amount of matter, infinite number of galaxies. Because matter is approx. uniformly distributed throughout infinite volume.

    It is important to realize: There is nothing mathematically or logically problematical about this.

    Jake, I am talking the standard professional cosmo picture here. What Redbelly reminds us is the main topic here. You are mistaken. "Expansion" in cosmology means that distances between things are increasing.

    This can happen just as well in an infinite setup as in a finite. There's more for you to ask questions about, and learn about. Like what distance measure do cosmologists use when they state the Hubble expansion law. Lots left to understand. But it is basic that a an infinite continuum can expand (in this sense) and does not need any surrounding space to expand "into".

    You might try reading the balloon model sticky thread. I've always preferred the finite version of the LCDM model to the infinite version. The balloon analogy works better. But we don't know which one nature likes. It doesn't hurt to think things thru using the finite version. After all at any given point, an infinite flat universe looks almost the same as a very very big finite universe.

    Jake, general relativity allows stuff that special rel does not. GR can describe situations where SR does not apply. So don't get hung up on that. Keep asking questions.
     
    Last edited: May 20, 2010
  8. May 20, 2010 #7

    Chronos

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    The observable universe appears temporally finite - around 13.7 billion years - from our perspective. There is a finite amount of space and matter within this observational limit. It is difficult to project what might lay beyond with any confidence.
     
  9. May 20, 2010 #8

    tiny-tim

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    Hi marcus! :smile:
    But what does zero overall curvature (which WMAP tends to support) have to do with spatial infinity?

    An "Asteroids" screen has no boundary and uniform zero curvature, but it's still finite. :wink:
     
  10. May 20, 2010 #9
    Keep in mind, when scientists talk about the Universe, they really talk about the Observable universe, which IS FINITE.

    A non recognized view would be the universe is not only infinite when it comes to spacial dimensions, but also infinite in scale. In other words, no fundamental particle, but a fundamental principle. The universe, just like a fractal, seems to have self similarity between the very big and the very small, sadly such theories are discarded as crackpottery and not even investigated, let alone being funded or supported by the mainstream in any way
     
  11. May 20, 2010 #10

    bapowell

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    Also, to clarify, if the universe is a (3+1)-dimensional expanding sphere (like the balloon analogy) there need not be any higher dimensional space that the sphere is expanding into. Perhaps counter intuitive, but this is important. The geometric properties of spacetime in general relativity are strictly independent of their embedding into a higher dimensional space. While it helps to visualize a balloon by imagining it existing within our 3D space, it by no means needs this ambient space to be a balloon (the topology and geometry of the sphere are encoded in functions defined on its surface that are independent of any embedding).
     
  12. May 20, 2010 #11
    I on the other hand feel the very concepts of "finite" and "infinite" will loose meaning when we finally obtain a more general picture of the Universe.

    An analogy would be in ancient times when someone looked at the "apparent" flat earth and wondered what lay beyond eye-sight. A common-sense approach would either be more "flat" earth or it would just stop somewhere. But once we obtained a better understanding of the earth, the concept of "flat" no longer applied but rather something qualitatively different, "curvature" was required.

    I firmly believe that will be the case with our understanding of the Universe one day: Something different that finite or infinite may be needed to more precisely describe it.

    So IMHO, it is neither finite nor infinite. I base this belief on the many phenomena in the world today that exhibit "jump-discontinuities" in their behavior and feel their origins lay in the very makeup of the Universe which is likewise so.
     
  13. May 20, 2010 #12

    marcus

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    Not true in my experience. Cosmologists when they want to talk about the observable U will actually say observable.

    The standard model (LCDM) is not a model of the observable U. It is a model of the universe. The observable portion is defined as all the stuff out to a certain distance (called the particle horizon) within the U as a whole.

    In some popular books and magazine articles you can find cases where the author is a scientist, but confuses U with observable portion. It is just carelessness. The distinction is carefully made in professional communication.
     
  14. May 20, 2010 #13

    marcus

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    Tiny,
    Essentially nobody uses toroidal models (à la Asteroids or PacMan game). It is certainly a mathematical possibility. But the simpler mathematical assumption is flat infinite.

    Out of a thousand papers you might find a few that consider the toroidal possibility and try to rule it out below a certain size, by determining that no periodicities have been observed out to a certain range. I think you know the literature pretty well so you may have seen the two papers by Cornish, Spergel, Starkman. But I think you are also probably aware that the huge bulk use a version of LCDM that is flat infinite--don't consider the donut or toroidal possibility. (No evidence of it, would just add complication.)
     
  15. May 20, 2010 #14
    Keeping in mind dark matter and energy are totally undetectable (conveniently) I'd say for example that the expansion of the universe applies only to the observable part. I mean, how do we know that the expansion is not something local, and for that expansion there isn't a neighbor region that is contracting and it's all temporary? Besides assuming things based on other assumptions and hypothetical entities??

    Last time I checked, cosmology hadn't made the leap from "theoretical" to "practical" ;)

    I'd rather remain skeptical towards anyone making claims about the universe past the observable horizon. If history of science has taught me anything it's that people were ALWAYS wrong when it comes to the universe, why do we think we got it this time? It's only a matter of time before we come up with something better, it has always happens and history tends to repeat itself :)
     
  16. May 20, 2010 #15

    bapowell

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    In what way are dark matter/energy undetectable if we've...ummm....detected their existence?
     
  17. May 20, 2010 #16
    google is our friend this time :)

    the dark bunch was needed to fill some holes in the standard model and provide support for it at a time it was failing, it did a good job i'd say
     
  18. May 20, 2010 #17

    bapowell

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    Are you serious?? You didn't answer my question. In what way are dark matter and dark energy 'undetectable'? Do we not see plenty of observational evidence for dark matter?

    I don't see how the scientific community has handled dark matter/dark energy is any different from the way any scientific endeavor is typically conducted. You observe a phenomenon that is inconsistent with your current model (galaxy rotation curves, eg). You postulate a solution (non-baryonic weakly interacting particles, eg), and determine what the observable effects of this solution are. You compare these effects with observation (WMAP, LSS, Lya, galaxy rotation curves, etc). As an added bonus, potential extensions to the standard model of particle physics offer good dark matter candidates.

    There is nothing wrong with hypothesizing the solution to a problem. That is how science works. Nobody is claiming to understand dark energy -- it's a popular 'unknown' in modern cosmology. One hypothesis, that dark energy is quantum vacuum energy, is well motivated. It of course needs to be tested more thoroughly -- we're working on it.

    These ideas were not hastily assembled to 'fill some holes'. This is the way science is done, and progress is certainly being made. I think I can safely say that the choice to place the word 'dark' in front of the words 'matter' and 'energy' was the worst choice scientists ever made. For some reason laypeople who lack any real understanding of the science have taken this as a sign of ignorance or as a 'cover up' on the part of scientists.
     
  19. May 20, 2010 #18
    If you cannot see or measure something, it is undetectable, it is that simple

    You can make a bunch of fancy experiments and interpret the results in a way that suits you, but it is still all based on hypothesis and even if the whole world is forced to conform to that so they can get good grades at school - that does not make it any more real

    IMO theoretical cosmology has long lost it's way, and thats the reason we have almost zero innovation in actual space exploration, the rockets today use the same engines they used 40 years ago, despite the enormous advancement in all other areas
     
  20. May 20, 2010 #19

    bapowell

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    But dark energy/matter influence their environments. Surely measurements of these effects constitute measurements of dark energy/matter.

    This isn't the way that science works. Nobody is doing this when it comes to dark matter/energy. If you feel they are, please give specific examples.
     
    Last edited: May 20, 2010
  21. May 20, 2010 #20

    bapowell

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    But cosmology has nothing to do with rocket science. What are you talking about? In what way does your opinion matter if you do not understand cosmology?
     
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