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Couldn't the universe be finite if Omega =1?

  1. Jun 6, 2014 #1
    I am not a physicist or a cosmologist, just a science layman who has been doing a lot of reading and thinking. I have been reading a lot in popular literature that if Omega =1, then the universe must also be infinite. Do you think this is just an over-generalization intended for the general public? I can see expanding space as becoming infinite in volume when it reaches infinite time, but unless it was infinite to begin with, how could it have become infinite within a finite amount of time? Furthermore, since Omega can equal 1 with a finite amount of mass, it seems that the universe could have begun with a finite amount of mass. If were the case, how could it be flat, infinite, homogenous, and isotropic without an average density near zero?

    The only way I can see the universe as currently being flat, infinite, homogenous, and isotropic while having infinite mass is if it was infinite in mass and space before expansion began. But that idea gives me some headaches, too.

    Wouldn't infinite mass require a quantum fluctuation of infinite magnitude, something highly improbable?

    An what about Mach's principle? If the universe had infinite mass, wouldn't all matter have infinite inertia?

    Lastly, couldn't the volume of finite flat space be expanding in the direction of time, so that it is currently finite, but is infinite at t=∞? Isn't that the more reasonable idea? if that is what has been meant by an infinite universe all along, wouldn't it be a good idea to clarify this to the general public?

    I am very interested in hearing your thoughts on this. If I have missed something, please forgive me and fill in the gaps in my knowledge.
  2. jcsd
  3. Jun 6, 2014 #2

    Simon Bridge

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    Welcome to PF;
    The cosmological ##\Omega## belongs to the FLRW model of the Universe.

    In order for the Universe to be finite, it must have some curvature - so if you keep going in one direction you end up back where you started.

    ##\Omega=1## means zero curvature.
    So the pop-science guys are correct that it means an infinite flat universe.
    Although - I am a bit uncomfortable with this parameter being used to draw global conclusions.
    You have already noticed that it is not a good idea to get your science from pop-science shows.

    You intuition about being infinite in space and mass before expansion is correct.
    In that situation, the expansion is understood in terms of density. The big bang would, therefore, be a rapid expansion from a hot dense state, not a small-volume one.

    The primal energy density need not have been infinite so you don't need infinite fluctuations. Besides, the improbability of our Universe coming into being is irrelevant - put simply: we do not know how many "trials" there were, so we don't know the overall odds. We do know that the probability of getting the Universe we are in is currently 1: you are looking at it.

    Mach's principle just asserts that local laws are influenced by the large scale structure of the Universe. The important word to notice here is "structure". It not how much mass there is but how it is distributed.
    ifaik, there are not that many people taking Mach's principle, in it's simple forms, seriously - except maybe ans an exercise.
    i.e. we understand the centrifugal effect in terms of non-inertial vs inertial reference frames - just like we do with gravity - and not in terms of a cosmological force where all the matter in the Universe pulls your arms out when you spin.

    If the Universe is finite and flat, then it must have an edge ... which means it is not isotropic. The laws of physics near the edge would lose their symmetry in the direction that you run out of Universe. But certainly you could postulate a Universe like that - you could say that we don't see the edge because it is retreating from us faster than the speed of light or something.

    But no - that is not what was meant by an infinite Universe all along, and it is not a more reasonable idea in the sense that it runs foul of Occam's Razor.

    Aside: I am being a bit loose with the terminology here - but I think you'll get the idea even if I'm making some topologists twitch a bit.
    Last edited: Jun 6, 2014
  4. Jun 7, 2014 #3
    Hi Simon,

    Thanks very much for the friendly welcome.

    Good to know that I am not the only one who feels some discomfort with this.

    It's good to hear you confirming my reasoning regarding this. Are there compelling observational reasons for assuming infinite space and mass prior to expansion if space has zero curvature? Or are they simply philosophical, or just an integral part of the model?

    As an alternative scenario, what if we start out with finite space and mass prior to expansion, so that we begin with a primeval singularity that actually is smaller than a proton, as it is often described to have been in popular literature? As long as the expansion is exactly enough to prevent eventual recollapse due to gravity, then in this scenario Omega would equal one and we would have a finite universe with zero curvature, correct? Or if it is more than enough to prevent recollapse, we have a negatively curved finite universe, correct?

    One additional thought along these lines. Suppose we start out with infinite space and infinite mass. But the expansion is not enough to prevent recollapse, so that Omega is greater than one. How do we compress infinite space into finite space? And how do we fit infinite mass into finite space? If that seems irreconcilable, then perhaps we would need to assume starting out with finite mass and space if Omega is greater than one. But if we can assume they are finite if Omega is greater than one, then what's to prevent us from assuming they are finite if omega is equal to or less than one?
  5. Jun 7, 2014 #4
    How about exotic geometries like a toroidal universe? It would be flat but finite.
  6. Jun 7, 2014 #5
    Yes, I agree that flat does not mean infinite. A Picard Horn, and a Poincaré dodecahedral space would also have flat local geometry and no locally curved space. But what my post is about, is that as long as we start out with finite matter and space before expansion, it seems logical to me that we should have a finite universe even with zero global curvature.

    What do you think?
  7. Jun 7, 2014 #6
    Yes, a finite universe will remain finite.
  8. Jun 7, 2014 #7


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    A Picard Horn is negatively curved, a Poincare dodecahedron is positive.

    There is no reason whatsoever to impose those assumptions. Many interesting and intuitively appealing cosmologies have been considered in the past, but the universe has ignored our preference and stuck with the simplest cosmology imaginable - flat, infinite, and perpetually expanding. I think we lack the authority to overturn this decision. :wink:
  9. Jun 7, 2014 #8
    Isn't truly infinite an impossibility? I think we say that even for a true singularity we draw the line at the Planck length? Perhaps we should do the same for the size of the complete U eg 10^35 times O.U.?
  10. Jun 7, 2014 #9

    Simon Bridge

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    This is where I was being a bit sloppy with the terminology ;)

    The trick is to verify that empirically - otherwise this is just a "no true Scotsman" argument.

    Nope - that is not what the "plank length" means.
    Do you know of any publication that draws such a line?

    Why would you pick that number? Why not OUx10^36 or OUx10^34?

    Basically - such arguments run foul of Occam's Razor.
    There is no need to make such an assumption.

    What we are talking about here is not so much the way the Universe is or is not, but how we choose the model we use to describe the Universe. Sure you could use a model which is flat and finite ... but those describe exactly the same stuff as the flat-infinite ones with harder maths. Therefore we choose to use the ones with easier maths.
  11. Jun 7, 2014 #10

    Simon Bridge

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    Back to the OP:
    Yes. There are good scientific reason for using an infinite-flat model for the Universe. Oversimplifying: it is the model with the easiest maths which also agrees the most with what we can see. There are a lot of models that also agree with what we see - they have harder maths.

    i.e. you could have a toroidal geometry. Go look it up :)

    Also see: https://www.physicsforums.com/showthread.php?t=10542

    Lets be plain: all science involves taking a philosophical position - you can look it up under "philosophy of science". What we are doing in these responses is telling you what's what in terms of that position. The position itself is not up for debate in these forums - that kind of discussion never gets anywhere. However, it is not "just" anything - the position taken here is a position which has been, and continues to be, immensely useful and relevant to understanding how Nature works where the other historically competing positions have not.

    There are a great many alternative scenarios - possibly infinitely many. The difficulty is not in coming up with alternatives but in choosing between them.

    If you mean that omega=1 need not necessarily, by itself, mean an infinite Universe, even in the FLRW model, you are correct. The pop science show was, indeed, not being entirely accurate in it's depictions - charitably: it was making a bunch of assumptions without stating them.

    Well spotted. You will come to realize that this is a bit like realizing that a politician may be lying or that your lawyer may just be looking out for his own interests ahead of yours.

    Outside the realms of pop-science shows: there are good reasons for using an infinite flat model even though it is not the only model that fits observation.

    I believe this is a complete answer to the question you stated in post #1.
    Further reading:
  12. Jun 7, 2014 #11
    Please note that I said the local geometry would be flat, not the global topology. Added later: Oh, I see your point. Because of the negative curvature, one end of the horn is finitely curved but the other is open. I am curious though. Why does the Wikipedia article say it has finite volume if one end of it is open?

    I am not trying to impose assumptions on anyone, and agree that as a science layman, I am certainly no authority on the subject. I am nothing more than a self-study student. But sometimes (though rarely) students can ask penetrating questions. I am just trying to see through and discern some of the over-generalizations I have heard in the pop-sci media. An infinite universe has a lot of philosophical and theological implications. If a universe that matches observations need not be infinite, that's pretty important to know, for me at least.
    Last edited: Jun 7, 2014
  13. Jun 7, 2014 #12
    I am not asking these questions because I the kind of fellow who relishes controversy. I am just someone who is trying to discern the truth after hearing a lot of over-generalizations in popular science media. I agree that the Standard Model is called just that because of it's utility. Another model that I cannot help but wonder if it has a bright future is Carmeli's Cosmological Relativity, since it explains observations so well, and without the need for dark matter, dark energy or a cosmological constant. (It does require a fifth dimension, however!) What do you think of it's potential?

    Yes, it is. Thanks very much for your generosity in taking the time to answer it, Simon.
    Last edited: Jun 7, 2014
  14. Jun 8, 2014 #13

    Simon Bridge

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    ... he means there is no reason to impose those assumptions on a cosmological model, not a person.
    It is a standard turn of phrase in science discussions - the word "impose" is taken in it's mathematical context.

    The idea is that the assumptions we include in a scientific model should come from someplace other than inside our own heads.

    I think that is a topic for another thread :)

    Also see:
    Looks like a non-starter.
    Last edited: Jun 8, 2014
  15. Jun 8, 2014 #14
    Infinities do result in some paradoxes, such as Hilbert's paradox of the grand hotel. It seems similar to expanding infinite space. Is it a solution to the problem of expanding infinite space, or rather a show-stopper? With a universe of both infinite space and infinite mass, the volume of the mass is infinite, but it still must be smaller than the still larger infinite volume of space. Even if the same metric is used to measure the volume! What if we subtract the infinite volume of space from the infinite volume of matter? We end up with another infinite number, though the "size" of it ought to have some relationship to the density of space!

    Another paradox associated with infinities is Gabriel's Horn. Though infinite, it's volume can be shown to be finite. Does anyone know, is this why a Picard's horn global topology of the universe would have finite volume?
  16. Jun 8, 2014 #15

    Simon Bridge

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    Cantor can teach you how to handle infinities - hint: not the way you are doing.
  17. Jun 8, 2014 #16


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    Really strange comment. Are you imagining the universe to be a relatively small clump of matter expanding into a preexisting empty space?? :eek: Because that's not the case - not at all! The matter uniformly fills all of space, at all times, whether infinite or not. Both the matter and the space expand together. They are always the same size!
  18. Jun 8, 2014 #17
    Simon, a British cosmologist (Penrose?) came up with this size 10^35 or was it 36?
    Anyway it was just an example number for some thing extremely big but less than infinity.

    Also regarding the smallest possible size for anything I thought that I had read this was called the plank length, so the only thing smaller would have to be a true S. (I thought just by logic)
    Last edited: Jun 8, 2014
  19. Jun 8, 2014 #18
    No. I am imagining both space and mass to be infinite to begin with, and the average density decreasing as space expands. At any point in time, you could subtract the infinite volume of mass from the infinite volume of space, and the difference would be infinite, too. So you would end up with one infinity being larger than the other, though they are both measured by the same metric.
  20. Jun 8, 2014 #19
    What is the "volume of mass" and how do you subtract infinite volumes?
  21. Jun 8, 2014 #20


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    Ok, well that doesn't make any sense either. :smile: As I said, the standard FRW cosmologies describe the behavior of a continuous distribution of matter. Matter fills all space uniformly in these cosmologies, and there is no meaningful way to split things into "mass volume" and "empty space volume". And if you could, the metric certainly would be different.
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