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I Infinite versus finite space

  1. Oct 18, 2017 #76
    Ok thank you. I get the part about string theorists. SO...the end point of my whole post was that I was watching a show where the cosmologist said at like a trillionth of a second after the big bang, the universe was approximately one centimeter across. I don't know if he meant the big bang era or the inflationary era. What struck me as odd was that as I mentioned, I thought the "rulers" themselves were expanding. So in what way can something be a centimeter across. Centimeter is only a centimeter in reference to a larger measurement. Or was THIS and example of a cosmologist just trying to explain something about relative size in terms the layman might relate to?
  2. Oct 18, 2017 #77


    Staff: Mentor

    I meant "criticism" in the sense of presenting a counter-argument, not in the sense of expressing a judgment of value.

    I don't think your implied analogy here is valid. A computer programmer has to meet the requirements of his users, because the users are the reason he's writing the program in the first place. But science does not have to meet the requirements of people who don't understand science (except in the sense, irrelevant to this discussion, that scientific knowledge underlies our technology). If scientific theories that work happen to be expressed in terms that are hard for lay people to understand, that's just a fact about science that has to be accepted and dealt with; and if pop science presentations distort the science so that they don't provide a proper understanding of it, that's also a fact that just has to be accepted and dealt with.

    We're not "writing off" the non-expert; we're just pointing out that he's a non-expert who doesn't properly understand the science, and that reading pop science won't fix that condition. You have to look at the actual science--the textbooks and peer-reviewed papers that describe, in precise technical language and math, what the best current scientific theories actually say. Again, that's just a fact about science that has to be accepted and dealt with; complaining that a non-expert is being "written off" when this unpleasant fact is pointed out to him doesn't change the fact.
  3. Oct 18, 2017 #78


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    That is NOT a description of the universe. It is, and he most likely did not know this, a description of the OBSERVABLE universe which is a totally different thing. This is the kind of crap you get in pop science.
  4. Oct 18, 2017 #79


    Staff: Mentor

    Strictly speaking, he meant neither. Those times quoted by cosmologists are not actually times in the current best model; they are times in an idealized model in which there is no inflation and the "big bang" is indeed an idealized "initial singularity" (which does not appear at all in the current best model). Basically, what the cosmologists are doing is taking the temperature of the universe at some point in time in the actual best current model, looking at the idealized model to see at what time after the "initial singularity" that temperature occurs, and then giving that time as though it was an actual time in the actual best current model.

    (The comment by @phinds regarding what the "size" given actually refers to is also valid.)

    Yes, this means cosmologists routinely use confusing and misleading language (at least it's misleading if you're trying to understand what's actually going on). And yes, these are actual cosmologists, actual experts in the field. This is a good illustration of why you cannot trust pop science sources, even when they are written by actual scientists.
  5. Oct 18, 2017 #80


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    Staff: Mentor

    It depends on the type of rulers. Are they made out of freely floating particles in space? Then they expand. Are they made out of a solid material (neglecting that there was nothing solid back then)? Then they do not.
    The length of a second is defined based on fundamental quantities (here: in cesium atoms - they didn't exist either back then, but let's ignore practical details), and a centimeter is based on the length time travels in a defined length of time. That allows a measurement of a centimeter no matter where you are. The observable universe today had a diameter of something like a centimeter back then. The precise value depends on the model and the question which point in time exactly you consider.

    In this analogy the race car drivers are professional astronomers.
    You are someone who has seen descriptions of car races made for people who have never seen them.
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