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Shape of the known universe

  1. Apr 24, 2013 #1
    can someone help me - i am new to forum. i do not have a physics background - medical doctor. i have been looking on the net to answer a simple question - what is the accepted shape of the known universe? if the big bang is correct, then is it accepted that the universe is basically is the expanding outside layer of a sphere? - like the peel of an apple/orange. if so then what becomes of the inside of this sphere? is it a vacuum or it is it nothing at all as in it does not exist - can light travel through it. can a spacecraft fly through it? if the inside space does not exist then is it actually possible to see parts of the universe on the opposite side of the sphere.
    also, if the universe is the outside layer of a sphere then is it true that if you travel in one direction continuously then you will end back in the same position?

    these questions have been bothering me for ages and i can't find any answers or maybe there are but i cannot understand them.

    can someone help me

  2. jcsd
  3. Apr 24, 2013 #2


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    The universe as a whole is of totally unknown shape and size. It might be infinite or it might be finite and unbounded. It is not at all likely to be finite and bounded because that would imply an edge, which gets to be ridiculous.

    The "known" or "observable" universe is a sphere of about 50 billion light years radius centered on your left eyeball.

    This might help: www.phinds.com/balloonanalogy
  4. Apr 24, 2013 #3


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    The observable universe is illusionary. It is a giant sphere centered on the observer, irrespective or where or when that observer happens to be. Where is irrelevant, when determines the size of that sphere.
  5. Apr 24, 2013 #4
    If you mean the observable universe (i.e. the universe that it is close enough for light that has been travelling since the big bang to reach us), that can be considered to be a sphere as others have said.

    If you mean the whole universe there is no clear evidence in support of any particular shape.

    No it could equally well be like the outside of a (ring) doughnut, or an infinite rubber sheet, or even a rubber disk or a swiss cheese!

    There is no "inside", it does not exist within our universe. The sphere/balloon/whatever is only an analogy, and in this analogy the whole of our 3-dimensional universe is contained on the surface of the sphere (or doughnut, rubber sheet, cheese etc.)

    Not by any known mechanism, or any that is considered outside science fiction (this is generally equivalent to the SciFi notiion of hyperspace).

    If it is like the outside of a sphere (or some other shapes), then yes this would be conceptually true. However we know that if it is like the outside of a sphere then it must be a REALLY big sphere - at least 95billion light years across (because we can see that far*) and probably much bigger. What's worse is that we believe that the universe is still expanding (in the sense that the sphere, if it is a sphere, is growing) at an increasing rate which means that however fast you travel the end goal (of the point from which you started) moves further and further away.


    * note that it is possible that when we look that far away we are actually seeing "all the way round" but just can't tell because we are looking so far back in time that we don't recognise what we are looking at. I wouldn't bet on it though.
  6. Apr 24, 2013 #5


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    Well, you could give some feedback. Quite a few people answered, did the response work for you? Did part of it work and part leave you still puzzled?

    I wasn't sure what you meant by the word "known". Cosmologists have a standard model of the U which gives a remarkably good fit to a vast amount of data. But only a PORTION of the standard model universe is currently being observed. They assume the rest is essentially the same---that matter (stars galaxies gas-clouds...) is pretty much uniformly distributed throughout---including where we can't see. This leads to a simple model, that works according to a simple equation that derives from the 1915 Einstein equation. The fact that the model is simple, and derives from the well-tested GR equation, and fits whatever we can SEE sort of justifies using it.

    But we can't say that we KNOW that it's right, and we only observe a portion of the whole, it's only the best most reliable model we have so far.

    And the classical version of the model blows up right at the start of expansion---so that is kind of embarrassing. People are working on improving it so it won't fail right at the very start. (quantum gravity, quantum cosmology, more advanced models, which still have to be tested.)

    So you could, for example, be asking what is the shape of the standard model universe?

    That would evoke responses involving 2D analogies to 3D spatial geometry. A common 2D analogy is the infinitely thin surface of an expanding sphere. the famous "balloon analogy". Or the infinite rubber sheet stretching out in all directions. Those are 2D analogs of what 3D space might be like.

    Or you could, instead, be asking what is the shape of our currently observable region?
    That would get simpler answers. It is just a 3D ball-shaped volume of a certain radius---just a small portion of the whole expanding thing.

    Maybe I shouldn't say more until (and if) we hear back from you and it becomes clearer what you are curious about.

    BTW MrAnchovy answered concisely and really well! I didn't realize---it turned out that my response was mostly just duplicating part of his post! Saying something similar in more words.
  7. Apr 25, 2013 #6
    Many thanks to all that replied. I am rereading the posts to try and get my head round some of the concepts. My previous thinking like the balloon analogy from phinds but I see that that is not strictly correct.
  8. Apr 25, 2013 #7
    I am no physicist either but I do lurk a lot and read, posting questions too.

    On top of what was said so well I'd like to mention few things to ponder about our Universe and possible realities and I know I'll get corrected as needed (these ideas are not mine, just may way of understanding/describing them):
    - what we consider to be constants today might not be so in the cosmological past (including the speed of light in vacuum)
    - we don't know real distances, times and speeds, as we have no absolute measuring devices, we have great models like SR & GR which helps us describe nature logically, but relatively, and they do their job quite beautifully for practical purposes
    - some scientists claim Universe got created out of nothing. Sure, one has first to define what nothing means... It is that which is left when you take everything we know out of existence, like mass, energy, light, gravitation, in short, everything existing, what is left though is not something I'd define as nothing, but it can be perceived that way, since, it's not existing... It's quantum fluctuations, which give rise to virtual particles (which come in pairs or particle and anti-particle) popping in and out of existence constantly and everywhere (not just out there in open space, but within your physical body as well), but all this (annihilation of particles) happens so fast that we cannot observe it, that is why we say virtual particles. Though, on the event horizon of a Black Hole (BH) such particles can become real and we get Hawking radiation (BH splits pairs into two real parts, where one part gets sucked into BH and the other radiates out). So, is it possible that the whole Universe was born out of that (which started out probably due to some kind of symmetry breaking within quantum fluctuations)
    - based on previous line we can see that there can be many, many Universes out there, if they are physically separated from our observable Universe (meaning that light from that Universe is not reaching ours)
    - with having infinity as most possible truth for Universe, then we can also have infinite numbers of Universes just like ours, and infinite number of you who is reading this ;)
    - having said all that there are theories which say our Universe is a sophisticated computer simulation. I don't buy it, but none can deny it either. On this concept what I wonder most is, what if we make such a computer simulation and let it run from Big Bang onwards, would self-aware life (as ours) spontaneously arise within it? And if you extend this thought: if it would then there is no free-will, right?

    What else I find fascinating: well entanglement of particles and double-slit experiment where consciousness affects the outcome (some say these two things are the same)

    And personally concept of infinity is what is most interesting to me. First, is it just a concept or it can be physically real? But let us say it is real, then do you see what that single thing can infer? Well, to me it means that there is no limit to anything... So, imagine conscious being evolved way beyond ours... Humans reach IQ up to 200, but in everlasting existence do we have a limit? Won't go on to not get deleted, but my email is open for more discussion if anyone interested.
    Last edited: Apr 25, 2013
  9. Apr 25, 2013 #8
    Last edited: Apr 25, 2013
  10. May 5, 2013 #9
    The 4 dimensional sphere can be a useful mental picture for some purposes. When using this mental picture I envision it in polar co-ordinates. 3 special dimensions exist as the surface. The radial dimension is time. This provides very convenient answers to your questions of what is inside the sphere (the past) and outside the sphere (the future). It also conveniently explains why the universe is expanding and why it is nonsensical to ask what happened before the big bang.

    Like all mental pictures this one has it's limits and if you try to apply it where it is not applicable you will come to wrong conclusions. If you want a better understanding I would highly recommend a series of cosmology lectures presented by Lenard Susskind available on you-tube. Here's a link to number 1. Links to #2, #3, etc can be found from there.
    Last edited by a moderator: Sep 25, 2014
  11. May 8, 2013 #10
    Hmmm, if the universe starts from a single point and all the "stuff" is moving away from each other.......how can there not be a center. It seems that is a definition of a center point? Well at least in 3D space.

    Edit and if the universe can bounce or be modeled back to a singularity has does every thing get back to a point that is not the center.
    Last edited: May 8, 2013
  12. May 8, 2013 #11


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    As has been pointed out in this forum approximately 4,789 times, IT DOESN'T
  13. May 8, 2013 #12
    Thanks for pointing at out for the 4,790th time. I will read up on the big bang. I guess I misunderstood what singularity means and implies
  14. May 8, 2013 #13
    try here

    https://www.physicsforums.com/showthread.php?t=506991 [Broken]
    Last edited by a moderator: May 6, 2017
  15. May 8, 2013 #14
    Thank you, I did understand. is there is a finite mass to the universe? an analogy might explain why I am confused....an ocean is homogenous and isotropic. there is a point in the ocean that all the mass would be balanced. Is the universe not the same?
  16. May 8, 2013 #15


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    It is not known whether the universe is finite or infinite. The mass will be whichever the universe is.
  17. May 8, 2013 #16


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    The big bang singuarity is different from a black hole, which does have a center. Bear in mind that the term singularity actually just means our equations fail to yield sensible results beyond a certain limit. It should not be construed as suggesting a dimensionally finite point. The universe could well have been infinite right from the start, which is the only way it could be infinite now. The more relevant question is just how big is the universe? That is unknown, and possibly unknowable. We know how big the observable part is - that's easy. It is merely the speed of light times the age of the universe. Cosmologists have deduced the entire universe is much larger than the part that is observable. Using 'circles in the sky' studies it has been concluded the size of the universe is no less the 24 giga parsecs - see http://arxiv.org/abs/astro-ph/0310233 for discussion.
  18. May 8, 2013 #17
    Very good your ocean analogy is a good one.
    In order for the universe to beconsidered homogeneous and isotropic, we need a large enough distance scale. Greater than 100Mpc.
    All energy/mass distribution at that scale must also be homogeneous and isotropic.
    There is a finite value for the observable Universe. Theres no reason for that not to be the case beyond the observable, for a finite universe. If its infinite then it follows to have infinite energy/mass.
    Last edited: May 8, 2013
  19. May 8, 2013 #18
    BTW I thought I read that the WMAP data had a flat universe within a very small margin of error.
  20. May 8, 2013 #19


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    Yes, it does. What's your point?
  21. May 8, 2013 #20
    the title is of the thread _shape of the known universe_. I was answering the implied question

    edit - I did not see anyone who actually stated the shape as flat.
    Last edited: May 8, 2013
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