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B Infinite at big bang?

  1. Jul 29, 2015 #1
    Following on from this thread. If the universe is infinite and was at the time of the big bang does that not mean that the size and contents was infinite? In other words at the BB there was already infinite amount of galaxies.

    I had understood that at, or just after, the BB the mass of the universe was in a very small space.

    If so, then these two theories, the BB and the infinite universe, seem mutually exclusive, and we can discount one of them if we have convincing evidence for the other. Or is our confidence in both of these so poor that we cannot discount either?

    Or am I missing something?
     
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  3. Jul 29, 2015 #2
    BBT is a theory applicable to the observable universe, which is not infinite.
    The entire universe may or may not be infinite and as far as I know, BBT doesn't have a lot to say about it.
     
    Last edited: Jul 29, 2015
  4. Jul 29, 2015 #3

    Bandersnatch

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    Just to expand a bit on rootone's post: Whenever you hear about the universe at some very early time in its history being the size of <whatever>, it means that what we see now as the observable universe was compressed in such a volume. It does not mean the totality of the universe, or even anything at all that's currently farther than the observable radius.
     
  5. Jul 29, 2015 #4

    phinds

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    There would have been an infinite amount of matter/energy, but there would not have been any galaxies. They came later.

    The OBSERVABLE universe, as has been pointed out, was in a very small space. "the universe" may or may not have been.

    Yes, you're missing several things. See above.

    EDIT: By the way, most of us have similar confusion when we first start learning this stuff. It IS confusing.
     
  6. Jul 29, 2015 #5

    Chronos

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    Yes, BBT tells us nothing about the size of the universe, only the observable universe. Unless otherwise specifically stated, cosmologists use these terms interchangably because 'everybody knows' we are only causally connected to the observable universe. It is entirely likely we will never know with any certainty if there is more to it than our causal patch
     
  7. Jul 31, 2015 #6

    bcrowell

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    I'm not sure from what you wrote whether you're under the impression that it's been established that the universe is infinite or not. As discussed in the thread you linked to, the spatial curvature of the universe is within error bars of zero, so we don't know whether it's positive or negative, and therefore we don't know whether it's infinite or finite.

    No, the big bang was not an explosion that occurred in one region of space. It occurred everywhere at once.

    Yes, you're missing the fact that the big bang was not an explosion that occurred in one region of space.
     
  8. Jul 31, 2015 #7

    ohwilleke

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    I would be inclined to say that the statement "the universe is infinite" when using the word "universe" to refer to something beyond the observable universe aka our causal patch, is a misleading use of the term "universe" that confuses people by using it in a manner different from its common usage in science. For the statement to be non-misleading one would need a specifier such as "the universe, including but not limited to the observable universe", although I'm sure that one could come up with something less clunky. While "universe" may have originally been a term meaning absolute everything that exists, has existed or will exist whether or not it can be observed, decades of usage have confined it to the narrower meaning of events causally connected to the Big Bang.

    The only affirmative evidence for the existence of anything outside the finite universe narrowly defined as is customary is evidence of "Dark Flow" which is apparently doubtful empirically, although a recent post on LQC at Physics Forums discusses another subtle other way that something pre-Inflation and perhaps pre-Big Bang could be observable in the cosmic microwave background.
     
  9. Jul 31, 2015 #8

    bapowell

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    What about superhorizon CMB temperature and polarization correlations?
     
  10. Aug 1, 2015 #9

    bcrowell

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    AFAIK it's the other way around. The common usage in science is that "universe" refers to the entire universe. Using it to refer to the observable universe is a common solecism among laypeople.
     
  11. Aug 1, 2015 #10

    marcus

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    Right. Judging from my experience with astro courses at UC Berkeley, in an academic setting you distinguish carefully between the universe (which is what the standard model cosmos represents) and the currently observable portion of it---the "observable universe".

    I would expect that anyone who has taken a college course in astronomy that included cosmology, or had much contact with professional astronomers, would be used to making the distinction. The "observable universe" is not the universe. As you suggest, laypeople get confused about this, and it's really unfortunate because it leads to a lot of misunderstanding.

    In comoving distance terms, the radius of the observable region is currently around 46 billion LY and it is increasing. As I recall according to the standard (LCDM) cosmic model it is expected to level out at around 63 billion LY. That is in comoving terms---it will include matter/galaxies that are NOW 63 billion LY from us. The distance then will be much greater. Our "causal patch" (as some people like to say) is steadily expanding day by day to include more matter.

    The standard cosmic model does not have a fixed boundary, it does not refer to today's "causal patch". One couldn't do physics if one were artificially limited to this moment's "causal patch". It would be arbitrary, absurd, and unnecessarily complicated. The differential equation that models the universe, models a whole universe.
     
  12. Aug 1, 2015 #11

    marcus

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    Ohwilleke you astonish me! Where do you get your idea of "common usage"? Can you link to some professional cosmology literature where the authors don't make the distinction between observable universe and the standard LCDM cosmos?

    In my experience which is of course limited---not a professional researcher just a longtime interested cosmology watcher---I don't know of any professional who confuses them. The common usage in cosmology for the observable universe is "observable universe". One is careful to make the distinction. Can you link to some professional literature with a different usage?

    ==quote==
    For the statement to be non-misleading one would need a specifier such as "the universe, including but not limited to the observable universe", although I'm sure that one could come up with something less clunky.
    ==endquote==
    Sorry. You have your shoes on the wrong feet. We already have a non-clunky "specifier". It is the word "observable". Universe refers to the whole thing. Observable universe refers to the observable region. There are lower-bound estimates of how much larger the whole universe is. One can infer that it is much larger than the currently observable region, based on upper-bound measurements of the spatial curvature.


    ==quote==
    decades of usage have confined it to the narrower meaning of events causally connected to the Big Bang.
    ==endquote==
    I don't understand. What do you mean by "Big Bang"? Do you picture it as only involving space and material that gave rise to our currently observable portion. Brian Powell had a (rather brief) comment about that.
     
  13. Aug 1, 2015 #12

    bcrowell

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    You're comparing two things:

    (a) "everything that exists, has existed or will exist whether or not it can be observed"

    (b) "events causally connected to the Big Bang"

    But these are the same thing, contrary to your assertion that b is a subset of a. There aren't events that aren't causally connected to the big bang.
     
  14. Aug 1, 2015 #13

    marcus

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    Chronos, I can't make logical sense of this statement. When I try to give a definite meaning to your words it seems to be a non sequitur.

    "our causal patch" has to mean something definite, so let's say it means our causal patch as of August 2015.

    In order to "know with certainty" that that set of matter etc. is NOT the whole shebang all we need to do is WAIT. If for example the CMB continues to shine it will be coming from more distant matter. if the CMB sky continues to look about the same, apart from increasing redshift, that will indicate our observable universe is larger than the 2015 version and includes new matter that will be having a causal effect on us for the first time. This will prove there is in fact "more to the universe" than our August 2015 observable region or, as you say, "causal patch".

    Conversely if there is NOT more than August 2015 observable region, then in time we will discover that to be the case, e.g. as when the CMB abruptly goes out. :oldbiggrin:
     
    Last edited: Aug 1, 2015
  15. Aug 1, 2015 #14

    Chronos

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    I use the term in the customary sense: "A causal patch is a region of spacetime connected within the relativistic framework of causality (causal light cones)." [wiki]. As you know, at a redshift beyond about z=1.7 [ a proper distance now of around 15 Gly], photons emitted by objects today will never reach us. Given the CMB is at a proper distance now of around 45 Gly, there are vast expanses over which photons have never and will never be able to reach us, no matter how long we wait. Which means what you see is what you get in the distant universe. No hitherto unviewable regions of deep space will ever suddenly pop into view. We are causally disconnected from such regions.
     
  16. Aug 1, 2015 #15

    marcus

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    Matter that is today at 47 Gly will (hitherto unviewable) gradually come into view. Matter that is today 50 Gly from us will (hitherto unviewable by us) gradually come into view.
    this will take a very long time but according to the standard LCDM cosmic model it will happen. I can find the future times for you.

    It sounds from what you are saying that you did not realize this, which would be surprising (knowing you) after so much discussion. It is simply untrue that "what we see (now) is all we eventually get" of the distant universe. But maybe I don't understand what you are saying.

    There certainly is a limit to the projected increase (in comoving distance terms ) of the observable amount of matter, the observable region . But 50 Gly is nowhere near that limit!
     
    Last edited: Aug 1, 2015
  17. Aug 2, 2015 #16

    Chronos

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    How does that square with the Davis and Lineweaver claim we will never view photons emitted today at a distance in excess of z=1.7? I concede I may be confused if that claim has been rubutted.
     
    Last edited: Aug 2, 2015
  18. Aug 2, 2015 #17

    Bandersnatch

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    Take a look at the spacetime diagram from the L&D paper:
    Capture.PNG
    At present, the distance to the event horizon (where it intersects with the 'now' line) is 17.3 Gly. This means that, as you say, light emitted from those galaxies will eventually reach us at t=∞, while nothing beyond ever will.

    However, the even horizon extends to our past as well. Light signals that were emitted in the past whose lightcones lie within the event horizon will also reach us.
    For example, if you look at light emitted at recombination by objects that are now at 63 Gly, their light will be now passing the 17.3 Gly point marking the present event horizon distance, will join the light emitted there presently, and reach us at infinity.

    I've found this version of the above graph at physics.stackexchange:
    Capture.PNG
    It's got the advantage of using a time scale more intuitive for non-cosmologists, and including lines of constant recession velocity.
    More details here: http://physics.stackexchange.com/questions/60519/can-space-expand-with-unlimited-speed
    One can trace a signal emitted at any time in the past, and observe how it passes the consecutive regions of superluminal recession rates.

    Keep in mind that both graphs use co-moving distances, so the object presently at 63 Gly whose light we'll be able to observe at t=∞ emitted the signal at much closer proper distance (at the time) and will have receeded rather quite farther than that (∞) by the time it reaches us.
     
  19. Aug 2, 2015 #18

    marcus

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    Nice! Such a perfect job of patient clear explaining that I should know better than to say a word--just distracts from the flow of understanding. But can't resist pasting in Jorrie's default table you get when you open his table&graph-making calculator, with the particle horizon column added. It shows that 63 Gly (comoving) being approached as a limit, in the particle horizon column in the last row. The scale factor in that last row is a = 100 so comoving (nearly) 63 corresponds to proper (nearly) 6300 Gly at that time, which is what you see.
    [tex]{\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}[/tex] [tex]{\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&D_{par}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&0.0004&0.0006&45.332&0.042&0.057&0.001&3.15&66.18\\ \hline 0.003&339.773&0.0025&0.0040&44.184&0.130&0.179&0.006&3.07&32.87\\ \hline 0.009&105.913&0.0153&0.0235&42.012&0.397&0.552&0.040&2.92&16.90\\ \hline 0.030&33.015&0.0902&0.1363&38.052&1.153&1.652&0.249&2.64&8.45\\ \hline 0.097&10.291&0.5223&0.7851&30.918&3.004&4.606&1.491&2.15&3.83\\ \hline 0.312&3.208&2.9777&4.3736&18.248&5.688&10.827&8.733&1.27&1.30\\ \hline 1.000&1.000&13.7872&14.3999&0.000&0.000&16.472&46.279&0.00&0.00\\ \hline 3.208&0.312&32.8849&17.1849&11.118&35.666&17.225&184.083&0.77&2.08\\ \hline 7.580&0.132&47.7251&17.2911&14.219&107.786&17.291&458.476&0.99&6.23\\ \hline 17.911&0.056&62.5981&17.2993&15.536&278.256&17.299&1106.893&1.08&16.08\\ \hline 42.321&0.024&77.4737&17.2998&16.093&681.061&17.300&2639.026&1.12&39.37\\ \hline 100.000&0.010&92.3494&17.2999&16.328&1632.838&17.300&6259.262&1.13&94.38\\ \hline \end{array}}[/tex]
    http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html
    Open the "column definition and selection" menu to add the particle horizon column to the default table.
     
    Last edited: Aug 2, 2015
  20. Aug 2, 2015 #19
    Whoah, if that is the case then yes I have fundamentally misunderstood the big bang thing.

    As it says here,
    I had understood that the mass of the universe was in a single point at the BB and that since then it has been moving away from that point, in all directions, and coalescing into galaxies.

    Are you saying that at the BB all matter came into existence in many different places; essentially where it is now, if not for gravity moving matter around, and also expansion?

    If the universe is infinite then matter appeared everywhere instantly in an infinitely sized universe; then the matter coalesced into galaxies.

    If the universe is finite then matter appeared everywhere instantly in an enormous finitely sized space; then the matter coalesced into galaxies.
     
  21. Aug 2, 2015 #20

    Bandersnatch

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    That's correct.
    Imagining that everything remains where it is at all times (relative positions don't change), and only distances grow with time is a good way to visualise BB. (It might help ease oneself into understanding co-moving coordinates later on)
    With the caveat that the expansion means that back then everything was closer together, and if you extrapolate backwards far enough the 'closer together' becomes 'infinitely closer together' - so even in an infinite universe you can say that everything was infinitely close (which is unlikely to be a real state in the history of the universe - this particular prediction is likely unphysical).

    In my opinion it is really better not to look at BB from 'the point of origin' onwards to the present time and beyond - it brings about singularities, energy popping up from who knows where and other questions that have more to do with personal preference and speculation that with describing what the model does.

    I think it is better to start with now, and then look backwards or forwards in time, and when the model ends up with singularities just stop using it.
    This way you don't worry about where stuff came from, but can appreciate the very simple predictions: that earlier in time everything was just where it is now (relative positions), filling the whole of available space, however large, but all closer together, and in the future it will get further apart.

    Then the question of whether the universe is finite or infinite becomes secondary - the model works for both cases.
     
    Last edited: Aug 2, 2015
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