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

Does the Universe have a Boundary ?

  1. Aug 10, 2007 #1
    Subject: Does the Universe have a Boundary ?

    Since nothing can exist outside of the Universe, how then can the Universe have a boundary in any conventional sense?

    Surely, if time before time is considered potentially unfathomable; in similar vein to speak of a boundary to the Universe, seems illogical.

    Then again, if we accept the premise, then we strike the first of quandaries; for if the Universe is limited, ( re the extent of expansion since the 'Big Bang') then that implies an outer edge along which the Universe is expanding into nothingness.

    No matter how I dissect this matter, it only seems to raise more problems than answers.

    So, I put it into the hands of other posters.

    Does the Universe have a boundary ?

    Can anyone answer this definitively ?

  2. jcsd
  3. Aug 10, 2007 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    this again! It seems to be Public Confusiuon Number One. :yuck:

    We mostly discuss standard bigbang cosmology at this forum, which does NOT imply that space has a boundary.

    the part of space that we are receiving light from is limited by a shifting horizon----beyond that horizon light hasn't had time to reach us yet---but as more light has time to reach us this "observable" part includes more and more galaxies. the whole of space is way bigger than the portion we have so far gotten light from and are able to see

    you shouldn't think of horizons (temporary limits to observation) as physical boundaries.

    as far as we know what is currently outside our sight is the same average picture we see already---same old universe just more of it.

    the whole shebang might have finite spatial volume (but no boundary, analogous to a ring) or it might have infinite volume (and no boundary, analogous to a infinite straight line)

    there are exotic cosmologies (which some String thinkers dream up) where there is some kind of boundary separating us from regions of space with different versions of physics. this is more a fantasy. it is not mainstream cosmology.

    let's forget exotica and focus on the mainstream picture---ordinary bigbang cosmology.
    this does NOT assume the whole universe started off from a point (THAT IS A LIE TOLD BY POPULARIZERS WHO WANT TO SELL BOOKS, get it out of your head)

    in ordinary professional cosmologist's picture the universe during bigbang could have had infinite spatial extent and no boundary (analogous to a line), or it could have had finite extent but no boundary (analogous to a ring). either version works.

    if it was infinite then, it is infinite now
    if it was finite then, it is finite now
    in either case space is so large (compared with the currently observable part) and so challenging to measure that WE CANNOT YET TELL if it is finite or not.

    in neither case does the standard picture have a boundary
    Last edited: Aug 10, 2007
  4. Aug 10, 2007 #3
    I've seen a similar question somewhere before. Just that i'll have to dig it up. Well as what Marcus said space itself has no boundaries, even before the bigbang, space had already existed. Just that its only after the big bang that time and matter started to come in place.

    In fact, when you keep thinking that space is finite. Then thats the reason why that would raise more problems than answers in your question. :wink:
  5. Aug 10, 2007 #4
    What I don't get is the infinite volume version. I thought there is finite number of particles in the universe and also that spacetime came into excistence with the big bang and space expanded thereafter.

    How does the infinite version fit in.

    (I have no problem with a closed, curved universe with the ballon analogy, but infinite volume, don't get it.)
  6. Aug 10, 2007 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Your Holiness, the mainstream majority of working cosmologists have never committed to the idea that there are only a finite number of particles

    the standard LCDM model essentially comes in two flavors, the flat (normally assumed to be spatial infinite and beginning with an infinite bigbang)
    and the positive curved (the finite one you understand). They didn't decide yet which is closer to Nature.

    a certain number of science journalists and popularizers have LIED to the public and imprinted people with the idea that cosmologists say the bigbang occurred at a point and involving only finite number of particles. they have to lie because they want to SELL and they cant sell if the public can't picture what they are saying.

    anyway thats all done now and no use crying about it. We still dont know which is right. MAYBE IT IS FINITE.
    The lie is when some journalist gives people the idea that cosmologists have made up their mind in favor of finite. They have not yet made up their mind.

    finite could turn out to be right, eventually, so in some sense the journalists and the public would be exonorated.
  7. Aug 11, 2007 #6
    does the universe have a boundary?

    does a hypercube (4-cube) have 3-faces and 2 hearts!
  8. Aug 11, 2007 #7
    many thanks, Marcus! I completely didn't know this.
  9. Aug 11, 2007 #8
    Also this thread with EL remarks has just clarified my thinking of infinite space.

    I believe one great but often unnoted aspect of PF are the archives and the search function. There so many great explanations and thoughts accumulated here!
  10. Aug 18, 2007 #9
    No.. Nothing never ends

    As simpel as this sounds it is the answer.... How can nothing end.... what would be on the other side... of the boundry? :!!)
    Last edited: Aug 18, 2007
  11. Aug 19, 2007 #10


    User Avatar
    Science Advisor
    Gold Member

    Pardon the interruption, this is nonsense. Put your facts on the line before asserting conclusions . . . Oh, I see you have no undisputed facts . . . science does not work that way.
  12. Aug 19, 2007 #11


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi Bob, welcome to PF! I see this is your first post. I think you may be essentially right---or at least in harmony with the mainstream scientists who work in cosmology.

    I'm a "science-watcher", not an authority. So I just report what's happening in the lines of research I watch and speak my opinion. But when I think over all the mathematical models that quantitative cosmologists have tried to fit to the data and match the universe, they NONE of them had any boundary except at places where they broke down.

    In other words, as far as I know, nobody wants to have a boundary on their model of space or spacetime either. the only time they get a boundary-like limit is where something blows up

    some models, if you evolve them back in time they keep on fitting the data reasonably well for a while and then at a certain point they go haywire and fail to compute sensible answers. that is not a clean boundary, but it does represent a LIMIT TO THE MODEL. so it is in a way like a boundary.

    a breakdown of a mathematical model is called a singularity. like the function f(x) = 1/x has a singularity at x = 0 because it goes off to infinity when you try to divide by zero. the usual model of spacetime around a black hole has a singularity at the pit of the black hole-----so people are trying to fix that by developing models of a black hole which don't break down and might correspond better to what nature actually does. but so far it is an unresolved problem.

    So I think you are RIGHT in the sense of being in accord with conventional quantitative working cosmologists. they don't think of spacetime as having boundaries------in any case not clean well-defined ones.
    And I think the REASON they don't put a clean well-defined boundary in their models is essentially the reason that you gave!
    they simply cannot imagine what could be on the other side!

    but I must hasten to stress that I might be wrong about this. this is simply my opinion. We have lots of differences of opinion here, for the most part courteous and civil.
    Last edited: Aug 19, 2007
  13. Aug 19, 2007 #12
    This is because it hasn't been answered to the satisfaction of the many members here. If you are a no boundry proponent, then it behooves you to ask ... what have I missed that makes this so hard for others to understand? Obviously the explanations aren't cutting it, and for good reason IMO. When a big Bang advocate uses phrases like "when the universe was the size of a grapefruit", others cannot accept these advocates wish to say that the universe has no boundry, when they just use one to describe it.
    Actually it is all we discuss. All other threads are locked.

    The CMB is the end of the line .... is it not? Although we don't know if or when the last microwave enters our picture. We simply don't know.

    Have to agree here, but thats not whats being discussed.

    You don't know that, it is inferred, which in turn begs the question.

    And the whole shebang may have a boundry. Why did you leave that possibilty out?

    This does not exclude the possibility.

    These popularizers are not telling lies. They just don't know of any way to explain a no boundry universe ... noone does. Which once again begs the question.

    Could have had? Well which is it? Or could it be none of the above?

    Exactly! We just don't know.

    In this case you choose to ignore the obvious, by putting up boundries that clearly are not there. Pun intended
  14. Aug 19, 2007 #13
    More questions

    Wow! (Words Over Wires) Ok, here is my formula 0=0 or =o= ...lighten up. The bigger question, to me, is how small can something be? What if you cut a string (as in String theory) and what is a brane (M-theory) made of? What is Atomic Expansion vs. Gravity? The biggest thing in the universe may be the ego.:!!)
  15. Aug 19, 2007 #14


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    That is where the popularizers mislead you. they should never have said "when the universe was the size of a grapefruit"

    Personally I don't like grapefruit, but it would be OK if someone says "when the OBSERVABLE universe was the size of a grapefruit". Then there is no boundary, only an horizon. That means when all the stuff which we have so far gotten light from, which is just a part of the whole, could have been contained in a volume the size of a grapefruit.

    Or, if they are not talking about the observable but about the whole thing---which we can only make inferences about by measuring curvature and by modeling what we are able to see----if they are talking about the whole thing then they shouldn't say grapefruit

    because spatially the whole thing is normally assumed to be either infinite (and a grapefruit is not infinite) or else something like S3 which we don't yet have a non-techie word for in English. The technical word is "three-sphere"-----or something else but those are the two leading pictures

    Observationally the CMB is not the end of the line. Primordial neutrino background is almost certainly there and waiting to be observed when people get neutrino detectors that work in the right range.

    The CNB neutrinos come from the first second. The CMB comes from when the universe was 380,000 years old.

    If CNB neutrinos are NOT observed when detectors are built that should see them, then there will be a huge uproar in cosmology :biggrin:. Not seeing them would have to cause a revolution.

    If they are seen, then people can study the CNB spectrum and map it etc just like they study the CMB now and learn more stuff.

    In your post you oftentimes say "We simply don't know."

    That is par for the course. Think about it. NO SCIENTIFIC THEORY IS EVER PROVEN TO BE TRUE. All that happens is that theories get continually tested and eventually they predict something that isn't observed and get proven false.

    then an improvement replacement is devised which can withstand all the other tests plus that additional one.

    it is a kind of darwinian thing---or a "last man standing" except it never ends.

    survival of the best fit.

    When I talk at Cosmology forum, I do the best I can to report the STANDARD COSMOLOGY CONSENSUS which is based on 1915 GENERAL RELATIVITY.
    And I try to leave room for some improvements to General Relativity which are under development----quantum gravity.

    Do I BELIEVE 1915 general relativity? Of course not! It has singularities, places where it breaks down. Therefore it can't be right and will eventually be replaced. (I guess if I have a belief it is in this process of testing and replacement where the laws and models are gradually improved.)

    I dont have time to continually be telling you my philosophy of science, which includes a deep sense of the limitations of human knowledge. I'm very interested in what are the best models so far and I want to talk about that, not philosophy.

    So when people come and say "what can we know?" :zzz: "how can we be sure, we are mere finite animals on a little planet?" I just find it boring and mostly ignore it. Skepticism doesnt tell me anything I dont know already.

    The amazing thing is how well the models work.

    What we get a lot of at PF is INCOMPETENT skeptics who are unable to see how well the standard LCDM model works because of a FAILURE OF IMAGINATION. Either they cant imagine a big bang with infinite spatial extent, or they cant imagine a three-sphere, or some mathematical thing just boggles them.

    My attitude is I don't care if someone rejects the LCDM----I dont believe in the standard model myself. What I dont accept is when someone rejects the LCDM because of a damn GRAPEFRUIT.

    We have an amazingly good provisional model that matches all kinds of data better than we had any right to expect, so I don't like to see it rejected for the wrong reasons or out of simple ignorance.

    If you understand the conventional mainstream model and then go off on your own and find some other model you like better thats fine. And about saying "we don't know" all the time---OF COURSE we don't know, just don't preach us sermons about it

    For me, the hottest thing in cosmology right now is the fact that Reuter has presented a model that gets just the right amount of inflation in the early universe without having to concoct any exotic "inflaton" field or arbitrary "slow-roll" potential. he gets the amount that people decided was needed to fit the data and he gets it without adding any JUNK.

    he is also able to explain some other things. It is a minimalist approach that somehow gets a lot out of meager assumptions. He goes beyond 1915 GR, but only in a way that was well-established by people like Feynman in other branches of physics a long time ago. So only in a very careful conservative way. If you want to learn about cosmology beyond the conventional consensus LCDM, then my suggestion would be to forget this boring boundary business and look at stuff like Reuter that is happening in the field.
    Last edited: Aug 19, 2007
  16. Aug 19, 2007 #15


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Well you sound cheerful this morning! Hello.
    The usual answer is the planck length.
    (this is not about string or M----it goes back to a paper of max planck in 1899)

    the estimate is still standing after more than 100 years.

    essentially the argument is something like this (maybe someone else will provide an alternative) the idea of length has the idea of being able
    to MEASURE a length in it. in a practical operational sense it doesnt mean anything unless you can compare two lengths and say bigger and smaller.

    the planck length is 1.6 x 10-35 meters and there are quantum mechanical (1926) reasons why nobody has been able to propose a method for comparing lengths smaller than that. even in a thought-experiment it doesnt work.

    it has to do with the inherent fuzziness of the world and if you try to fight fuzziness by making things more massive or energetic (like accelerators use higher energy to probe microscopic structure) then you get to a point where you are concentrating too much mass (or energy) into too small a space and the fabric of space will not support it and it goes *glup*.

    measuring smaller than planck length gets caught between the rock and the hard place----between the fuzzy indeterminacy of the one hand and the black hole rupture strength on the other.

    Planck is interesting because he calculated the planck length back in 1899 many years before there was any proper sense of quantum mechanics or Heisenfuzz uncertainty. he had an intuition about it.
    Last edited: Aug 19, 2007
  17. Aug 19, 2007 #16


    User Avatar
    Gold Member

    even earlier natural units

    Google “Natural units before Planck” and you will find George Stoney used similar units as early as 1874. This was mentioned last week by one of the speakers at the IGC Inaugural conference at Penn State, I forget who.
    They differ from Planck units by powers of alpha, the fine structure constant, approximately equal to 137, and were deduced from the electron charge even before Planck's constant was known.
    Jim Graber
  18. Aug 20, 2007 #17
    Finitude or infinity of space is a thing of the mind. The human mind is not capable of thinking in a different way and that is why we tend to set a mental boundary to the universe and try to visualize what lies 'outside'.
    But for all we know, there need not be an outside to everything we see.
  19. Aug 20, 2007 #18

    exactly- they should emphasize that they are only talking about the matter we can currently observe within the Hubble Volume being within a small volume-

    I think alot of this confusion would go away if the popular literature would simply say that the universe was aprobably already infinite at the time of the big bang- and was in an infinitely dense state that expanded- this gif illustrates it perfectly:

  20. Aug 20, 2007 #19


    User Avatar
    Gold Member

    setAI said :
    This is not the case with the 'standard' FLRW model, where a(0) = 0 gives an infinite energy density and no spatial extent at all. Have you got a reference for the model you are drawing on ?
  21. Aug 21, 2007 #20


    User Avatar
    Science Advisor

    Misconception upon misconception....

    The 'standard' Hot Big Bang model says nothing at all about t=0 and certainly nothing about a=0. The standard Hot Big Bang model starts with a hot dense infinite universe that undergoes a brief period of inflation, followed by many other interesting phases of evolution.

    The greatest misconception about the Big Bang that people have is that the theory is something that is focused on explaining some moment of creation. In fact it is a theory that describes the evolution of the Universe from a dense hot state to cooler less dense states as it expands and says nothing about how and why the moment of creation occurred or indeed whether there was a particular moment when it happened. This seems somewhat inadequate which is why people are now looking at extensions to the present theory that deals with the thorny pre-inflation issues, but this is not standard Big Bang theory, these are extensions to it.

    The Big Bang is a terrible sign for the theory it signifies but history has stuck us with it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook