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Infinite universe from a finite start

  1. Dec 9, 2013 #1

    I have a question about the character of the universe today and its early state.

    As I understand it there is no consensus as to whether the universe today (the whole universe, not the observable) is infinite, finite, or finite but looped in on itself. It seems to me to follow that if (and it's a big if) the universe is infinite now, and has been expanding at finite rates for a finite amount of time, then it has always been infinite, even a split second after the big bang.

    What confuses me is that I see many respected physicists and periodicals talking about the very early universe and talking of an "infinitessimally small dense hot point", and confidently stating the size of the universe say a second after the big bang as if this didn't imply that by doing so they are necessarily stating that they do not therefore believe the universe is infinite today - unless I've misunderstood what I've put in the paragraph above.

    Is that right? It seems to me that all we can say about the early universe (in order to avoid necessarily taking a position on the size of the universe today) is that the universe at the big bang was infinitely dense, that the space between points was infinitely small. That doesn't mean it was an infinitely small point however, nor that it couldn't still be a singularity. After all the distance between all points can still start expanding in an infinitely large space just as easily as it can in a single point.

    Do you agree with my understanding? Many thanks.
  2. jcsd
  3. Dec 9, 2013 #2


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    Your understanding is correct and only pop-sci goof-balls on The History Channel say that the entire universe was a point at the "big bang" singularity. Whatever it was (finite or infinite) then, it is the same now. The correct statement is that it was a staggeringly hot, staggeringly dense plasma of unknown side.

    I have heard statements, probably ball-park correct, that the observable universe at that time was the size of a <you pick one --- grapefruit, golf ball, or smaller>

    EDIT: by the way, my "goof-balls" statement was not entirely accurate, but my statement about the History Channel was. That is, some reputable physicists have been known to say bone-headed things when doing pop-sci TV shows on The History Channel, and The Science Channel and others. NEVER trust anything you hear on those shows.
  4. Dec 9, 2013 #3
    Thanks for putting my mind at rest phinds. Yes, I know that you can't provide all the details in pop science, but it would be helpful if they could avoid using terminology that is actually potentially incorrect!

    Yes the golf ball/grapefruit etc comments muddy the water, particularly if they don't specify the *observable* element. Although, I suppose, that that term isn't unproblematic either when talking of the earliest moments of the universe!:-D
  5. Dec 9, 2013 #4


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    Oh, they don't even come close to stopping with "potentially incorrect" they often go straight to "flat out WRONG".

    I think as long as they both specify "the observable universe" AND point out that this is not "the universe", then it doesn't muddy things at all but of course pop-sci usually leaves off both caveats.
  6. Dec 9, 2013 #5
    they could also say the start of the universe is a point in time rather than a point in space.....it was 'everywhere' in space.....but that is perhaps too abstract for a first exposure....
  7. Dec 9, 2013 #6


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    One thing cosmologists are habitually guilty of is using the word universe when they really mean observable universe. The 'unobservable' universe, while it assuredly exists, is generally not of scientific interest because it is ... unobservable. So long as you keep this in mind, the stuff they say about the universe in pop media and on tv is quite sensible most of the time. Ordinarily, when a scientist talks about the UNIVERSE, they make it abundantly clear they are not just talking about the observable universe.
  8. Dec 10, 2013 #7
    I bet they're aware. Sometimes it's hard to build young audiences and make them interested using orthodox/academic methods and the fact that it is not that effective in triggering their interest in most cases led them to exaggerate visuals and verbal representation of the subject in the hope that it will be corrected soon. The downside is the amount of misconception that are instilled fundamentally is alarming.

    Going back. The universe is flat with only a 0.4% margin of error. Suggesting that Ob-Universe is infinite in extent; however, considering it has a finite age, we can only observe a finite volume of the Universe. The Universe is much larger than the volume we can directly observe indefinitely.
  9. Dec 10, 2013 #8
    I suppose it's true that if the "big bang" theory is very easily grasped if you think of a single point inflating and expanding. If it had been presented to the general population as being an unknown volume wherein every point inflated and expanded then I doubt it would have caught the popular imagination. Whether the confusions that can occur from "dumbing down" the facts or not is worth it is debatable I think.

    I'd like to note that I didn't say I'd seen such language solely in pop science or that that was solely what I had been referring to. Although it might be prevalent in pop science you find such imprecision everywhere. Just from a very very quick google:

    University of Oregon: The discovery of an expanding Universe implies the obvious, that the Universe must have an initial starting point, a Creation. A point in the past when the radius of the Universe was zero. Since all the matter in the Universe must have been condensed in a small region, along with all its energy, this moment of Creation is referred to as the Big Bang.

    University of Michigan: At the point of this event all of the matter and energy of space was contained at one point.

    I do take your point that popularising difficult conceptions necessarily involves some dumbing down, but when does it stop? Something like the history channel, yes that's for people with a passing interest only. But it seems to extend much further than this - surely you'd expect it to stop before college level. There was no starting proviso given on those university sites that they were talking solely about the observable universe before the parts I quoted. I googled extensively before asking my initial question - favouring sites beyond those of pure pop science - because I heavily suspected what phinds confirmed for me. But finding information that bridged the gap between "cosmology for dummies" and a full-blown peer-reviewed thesis was almost impossible.

    Ironically, in writing this post I found this on Harvard's site http://www.cfa.harvard.edu/seuforum/faq.htm#e1 which would also have confirmed what I thought, so it's clear that confusion created by dumbing down is a real problem. After all it's not *that* complicated - when you're presenting to an audience which is beyond absolute beginner - to specify that you are talking about the observable universe and actively avoiding something that is just wrong, or debatable, like saying the whole of everything started at a single point.

    To contrast what those what those university sites wrote, the wikipedia entry for "big bang" says "At this time, the Universe was in an extremely hot and dense state and began expanding rapidly." So, although this wouldn't have answered my question, at least it doesn't appear to confirm the opposite since it avoids talking about the size of the universe simply commenting on its density and heat.

    Although I have an adult interest in science my background is in languages - perhaps this is why I struggle when language is used imprecisely in relation to science. You might - and do - teach children "i before e except after c" when teaching spelling, but you would never say to an adult "there is no need ever to worry about whether a word is spelled with ie or ei since you must always use "ei" after the letter c and "ie" in all other instances" since this oversimplification has exceptions. You just wouldn't do it.

    Rant over - at least there are sites like this to check internet facts on!:smile:
    Last edited: Dec 10, 2013
  10. Dec 10, 2013 #9
    Planck length equivalent to infinitesimally large size(?)

    Hi - I too have wondered how something a finite size can expand to be possibly infinite.

    In Brian Greene's book, "The Elegant Universe" he states that the smallest size anything can be is the Planck length (I can't remember exactly what this is, but it's about the size of an electron).

    Therefore, the early universe can't have been smaller than this. he also states that mathematic.ally, one can't tell the difference between something the Planck length and something infitessimally large.

    I might be jumping to conclusions here, but does this mean the universe can be infinitesimally small and infinitesimally large at the same time? This would negate the need for inflation!

    Perhaps I should call this theory the Tardis theory in honour of Dr Who's 50 th anniversary!!
  11. Dec 10, 2013 #10


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    It doesn't -- you're missing the point of the above discussion. If the universe was initially infinite, it stayed infinite. If it was initially finite, it stayed finite.
  12. Dec 10, 2013 #11
    I'm sorry, but I do understand English and that was exactly what the initial contributor had stated!!

    I took a course in undergraduate level physics a few years ago which may now be a little out of date, but clearly stated that the universe came from a single point, and that we do not know whether or not it is infinite in size now.
  13. Dec 10, 2013 #12


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    No reputable cosmologist would say that the universe started at a single point in space. In TIME, yes, but space no. I don't dispute that you read that in a textbook, I'm just saying that if you did the author was either simplifying or didn't know what he was talking about.

    EDIT: I should add that the "single point in time" is not really proven. It IS a consequence of the math of the big bang theory, but that is disputed by some of the "multiverse" theories (which have no supporting evidence but which are discussed by reputable physicists)
  14. Dec 10, 2013 #13
    I'm sorry, but I didn't. I said I couldn't see how it could possibly be true, to which phinds replied that it indeed wasn't and this was an oversimplification sometimes provided erroneously by pop science.

    See the Q and A here http://www.cfa.harvard.edu/seuforum/questions/ in particular the section "Did the Universe expand from a point? If so, doesn't the universe have to have an edge?"
    Last edited: Dec 11, 2013
  15. Dec 11, 2013 #14
    Just to extend. 'In time' not to get confused with 'time came first' but imagine it as a second observer. According to the model. It is simply a different aspects of the same entity. They exist simultaneously.
  16. Dec 11, 2013 #15
    Hi - seem to have e generated a lot of buzz by my comments!! One could quibble about the language for ages, but the question is: do I take it on good authority that current thinking has changed In the last few years, as the course I took was not out of a book but was given by Dr Dan Hooper, a leading cosmologist and expert on dark matter. I'd be grateful if someone could enlighten me on this.
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