Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Singularity/Expanding Universe questions

  1. Sep 5, 2007 #1
    1. How can any size be attributed to the Singularity? Given it represents all of existence, what else is it being measured against? Can it somehow be measured against itself internally?
    2. Is the Universe that expanded from the Singularity still a Singularity? The internal structure may have changed, but isn’t it still a single entity that represents all of existence?
    3. Do we know the size of the Universe now (if so, how), and how do we know that it’s not infinite?
    4. If we don’t know how big the Singularity was, or how big the Universe is, how do we know that the Universe expanded from the Singularity? Couldn’t it always have been the same size and it has just changed internally?
    5. How does proving that the observable universe is expanding by observing cosmological redshift prove that the Universe per se is expanding? How do we know what’s happening in parts of the Universe we can’t observe? Couldn’t these parts just as easily be contracting?
     
  2. jcsd
  3. Sep 5, 2007 #2
    Very good questions

    Your questions on singularity are very good indeed.
    The essential point is that "what is", no matter how we may call it, is inevitably self-referential, when seen in its wholeness.
    In other words, and in a simple interpretation, if it moves, it must inevitably move within itself, since there is nothing else outside of it.
    Now, this self-referentiality can be manifested in very many ways.
    One of them, of course, happens through us, that is, through our thinking when focused on "what is".
    Consequently, issues like "infinite versus finite", "measuring against something", and so on, are but particular aspects of self-referentiality. And therefore, they are inevitably bound to break down, or at least become problematic, if pushed too far, that is, if they reach deep enough in self-referentiality.
    And your questions simply do precisely that.
    In other words, the scientific approach is fine, great, immensely important and useful, not to mention an exquisite fun. However, it was never really meant to be answerable to every question. And especially not to questions which push the issues too far, that is, too deep self-referentially, as for instance your questions do.
     
    Last edited: Sep 5, 2007
  4. Sep 5, 2007 #3

    olgranpappy

    User Avatar
    Homework Helper

    1. What Singularity?
    2. No. Sure.
    3. Yes, by observation. We don't.
    4. What singularity? Oh... right. No.
    5. It's expanding on a large scale, there are plenty of blue-shifts too.

    P.S. I'm not a cosmologist. You might want to google "Cosmological Standard Model" or something to that effect.
     
  5. Sep 5, 2007 #4

    olgranpappy

    User Avatar
    Homework Helper

    Perhaps this is a better question for the biology forum--Forum Admins feel free to move the thread as you see fit--but I'll ask it in this thread: What is the sound that a duck makes?
     
  6. Sep 5, 2007 #5

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    1&2: Firstly, a singularity is not really a physical thing, but is simply a term that is used to describe an area where the mathematics of a certain model breaks down. The beginning of the universe is one of these points-- our physical laws break down at the start of the universe.

    3: We know the size of the observable universe, from observations! However, we do not know whether or not the universe is infinite or finite-- although most theorists at the moment favour a finite universe.

    4: Again, a singularity is not a physical entity, and as such has no size. We think that the universe has expanded from a size smaller than it is now, by observing the galaxies around us receding. If it has always been the same size, then how could all galaxies around us be moving away from us? (Keeping in mind the cosmological principle-- we do not reside in a special part of the universe)

    5: Well, the unobservable universe could be contracting, but that would firstly disagree with the cosmological principle, and secondly would make the universe a lot more complicated!
     
  7. Sep 5, 2007 #6

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    The fact that you say this shows that you do not understand the theory properly. As with the OP, you should read up on what is a singularity-- there are a few threads here that you may wish to search for.
     
  8. Sep 5, 2007 #7

    wolram

    User Avatar
    Gold Member

    Cristo, your reply seems a little arrogant, as if you have solved all the riddles of cosmology, understanding a theory is one thing ,but thinking it is right is another, can you tell if string theory is right ,or quantum loop gravity, or the standard model, i hope you do not take this the wrong way but AFAIK no knows what a singularity is yet.
     
  9. Sep 5, 2007 #8

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    Well I apologise if my post sounded arrogant; it was not meant to be. My point was, however, that a singularity is not a physical entity. It is merely a mathematical tool that tells us that there is a problem with the physical law at that point. So, the fact that the big bang model has a singularity at the start simply tells us that we do not know whow to model the beginning of the universe (as we do not have a theory of quantum gravity). It is expected that when we do have such a theory, these singularities will disappear. So, the important question is regarding how to get rid of these singularities and not in what one is.
     
  10. Sep 5, 2007 #9

    wolram

    User Avatar
    Gold Member

    Thankyou cristo, your reply is so honest.
     
  11. Sep 5, 2007 #10

    EnumaElish

    User Avatar
    Science Advisor
    Homework Helper

    Finite with respect to what? (Is Möbius strip finite?)
     
  12. Sep 5, 2007 #11

    vld

    User Avatar

    The good thing in cristo's comment is that he is trying to separate "flies from the soup", i.e the singularity notion from those having physical meaning. Indeed, there are theorems saynig that a sufficiently massive object will collapse to a point. Then how can we deal with a physical object transforming into a mathematical abstraction and vice versa? The "vice versa" is referring to the universe born from a point. Perhaps using the right terminology might help avoiding confusion.
     
  13. Sep 5, 2007 #12

    wolram

    User Avatar
    Gold Member

    Terminology, a human idea, i must remember that, but will remembering it make it right?
     
  14. Sep 5, 2007 #13

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, sure it is.
     
  15. Sep 5, 2007 #14

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    Do we know the size of the universe now? Not really, it is usually assumed that it is infinite, and that the observable universe is a finite region of this infinite (or at least very large) region. Observations have not so far contradicted this idea. Experiments like Circles in the Sky have not found any verifiable indication of the universe having a finite size. (For a while it was thought that the universe might have a closed decahedral structure based on these experiments, but it was a false alarm).

    Most tests seem to agree with a "flat" universe, which if one accepts General Relativity, implies an infinite one. (I'd have to dig for specific references on this issue, hopefully this point will not be seen as something that needs to be argued about and hence the references won't be needed).

    So the current model is that the universe is infinite, and was "born infinite", but that the part we can actually see, the obserable universe, started at a point.

    It should be obvious that it's very difficult to ascribe any definite notion of existence or non-existence to the parts of the universe we can't see. I haven't to my own satisfaction resolved the issue of whether things we can't see (the non-observable parts of the universe) could leave a "signature" that we could analyze.

    A related comment. If we accept that the universe is infinite now, for the time being, then it was also infinite at the time of the big bang, even though what we can actually see, the observable universe was a point at the time of the big bang.

    See for instance Nasa's WMAP cosmology page for a reference for this often overlooked point, which I'll quote in part:

    So the current cosmological models have our observable universe starting "at a point" - they do not, however, have the entire universe (assumed to be infinite) starting at a point. I hope that helps.
     
  16. Sep 5, 2007 #15
    Thanks for the replies. Just wanted to let you know I’m still here. I don’t have time to respond at present, but I am reading.
     
  17. Sep 5, 2007 #16

    olgranpappy

    User Avatar
    Homework Helper

    With respect to the usual definition of distance between two points; there is a maximum value for the distance between any two points on the Mobius strip.

    Now consider a torus. It too is finite. For the same reasons.

    Of course, there are no "walls" on a torus, but that doesn't mean it is infinite, as we have just seen. A region enclosed by "walls" too is finite. For the same reasons. Thus we have seen that having or not having "walls" doesn't correspond at all to being or not being finite...
     
  18. Sep 5, 2007 #17
    cyclic universe theory

    okay, i'm new here, but i have come up with a theory related to the big bang, a variation, if you will, which i would really like to share, for the purpose of receiving critizisms, maybe somebody will point out that another person has already suggested this theory, whatever, but i wanted to bounce this idea off of people more knowledgeable and interested than most of my friends.

    essentially:
    over billions of years, maybe a trillion or more, all matter in the universe will have been absorbed by either many supermassive black holes, or possibly just one, if the black holes can impact and absorb each other. this last black hole will collapse under it's own gravity, perhaps condensing into the theorized singularity in the big bang theory. that point will then explode as per the big bang, and create a new universe. essentially, what if ours is not the first universe?

    also, this would offer the idea that the term 'universe' could be used to describe a period of time, and not just a 'place'.

    i have a few more related in-depth ideas, but before sharing them, i figured i'd go ahead and pose this question for discussion.

    and if anyone would like to contact me about this for further discussion outside of this message board, i'm at imtheknife@gmail.com
     
  19. Sep 5, 2007 #18
    I have had similar (if not the same) thoughts. It seems many people have. Don’t want to derail my own thread with wild speculations but this a wacky thought I’ve had recently that I’ve never heard of before (maybe because it’s too stupid :-). Instead of the current model where space it universally expanding and matter isn’t, How about matter is universally shrinking and space isn’t? Seems however that the redshift in light is created by it being “stretched” by space expansion, so no cigar.
     
  20. Sep 6, 2007 #19

    olgranpappy

    User Avatar
    Homework Helper

    Pauli would have liked this, because he could have quipped that this is not even a theory.
     
  21. Sep 6, 2007 #20

    vld

    User Avatar

    Better to say, in order to get it right, one has to be able to distinguish between the mathematical and physical objects. A zero-size point is obviously a mathematical object plagued with infinities. It can be succesfully used for idealisations when describing a physical object but it seems that some people take it as if it WERE a real physical object (e.g., a singularity). Terminology is a secondary thing.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Singularity/Expanding Universe questions
Loading...