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Debate with my coworker regarding the inflationary theory

  1. Jan 2, 2006 #1
    Hi, I had a little amateur debate with my coworker regarding the inflationary theory and I was hoping somebody could clear up a few things for me, since other internet resources only made me confused.

    First, assuming the expansion happened only to one region of the early Universe, how big did the inflated Universe become? One source shows it became about 1 meter in radius, but it also says the factor of expansion was about 10 the power of 54. Well, that would put the initial size below the Planck's length of 10 to -43, would it not? But then there were no "regions of space", as space-time continuum didn't exist yet, not to mention the fact that we don't have physics to describe anything smaller than Planck's length, as I understand it. So, what gives?

    Second, how big is our horizon with respect to the inflated Universe? My argument is that the [inflationary] theory is not a result of an observation, it came about as a possible solution to a number of problems with the standard Big Bang model. As such, the magnitude of the expansion needs to be as only big as to sufficiently resolve the horizon, flatness, relics etc. problems, and not a parsec longer. What theoretical necessity would require the size of the inflated universe to be a lot bigger than what we can observe?? But one source says our observable universe is very very tiny compared to the whole inflated universe, which sounds like a rather metaphysical claim to me, and practically unnecessary one, unless it comes as a baggage I have to accept to make the theory consistent.

    Finally, how credible is all this talk about the scalar field and its decay into what we call our matter. Inflatons?? Is the idea pretty standard in today's scientific community? Also, is there any possibility to infer what happened to the other regions of space within the theoretical framework of inflation? Did they collapse or continue to expand at a normal rate? Are they physically inaccessible due to different laws governing them?? Would there be any way to *empirically* differentiate between any two hypotheses attempting to answer any "parallel universe" question?

    In advance, thanks for your help or any reference to credible sources.

  2. jcsd
  3. Jan 3, 2006 #2


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    I know it doesn't answer all of your questions, but here's a recent discussion that includes some helpful links:


    I'll try to address some of your other questions here.

    I'm not quite sure how to answer this question. What do you mean by "happened to only one region of the early universe"? At the epoch of inflation, the physical size of our currently observable universe was, as you say, of order a meter.

    That's right, but most models suppose that the "entire" universe is much, much bigger (we can't say how much bigger) than the observable universe, so the "entire" universe would not have been compactified to less than the Planck length.

    Well, it certainly was originally formulated to resolve the problems you state, but we now have some (admittedly weak) observational support for it. The fact that the initial spectrum of density perturbations should be a Gaussian random field is a prediction of inflation and is now on pretty solid ground. Unfortunately, a Gaussian random field is a pretty generic type of density field, so it's not a very strong prediction. Slightly stronger evidence is the apparent coherence of the oscillations that produce the anisotropies in the cosmic microwave background.

    What we would really like is to observe the background of gravity waves predicted by most inflationary models. A direct detection seems unlikely in the near future, but there is some hope that we might see evidence of its impact on the polarization of the cosmic microwave background. Hopefully, in 10 to 20 years we'll be able to say more.

    I think it's more that there's no reason it shouldn't be larger than we observe. The size of the observable universe has been increasing for quite some time now and it would be strange if we were at the exact moment in cosmic history where the edge was reached.

    Also, there's no a priori reason that the universe should be exactly flat. If the entire universe were the size of our observable one, then the flatness we measure locally would actually correspond to a global flatness. On the other hand, if the curvature scale of the universe were much larger than our observable universe, we could measure it to be locally flat with arbitrary curvature.

    Well, scalar fields are certainly thought to occur in conventional particle physics (e.g. the Higgs field). How plausible is it that a scalar field could have decayed into the contents of the present universe? All I can say is that, of the present set of theories, it's viewed by the community as the most plausible. I'm hesitant to attempt to give it a more absolute level of plausibility

    Inflation predicts that much of the unobservable universe should be the same (on average) as the observable part. How much of the unobservable universe? That depends on the model. What about the rest? Well, at this point, it's anybody's guess.

    At the moment, I don't think there are any ways of testing parallel universe theories like you're describing (if I'm understanding you correctly). I can't say for sure what the future will hold. :smile:

    I understand that many of my responses are vague, but that's mainly because there are a lot of inflationary models, each of which will give differently quantitative (and in some cases, qualitative) answers to your questions.
    Last edited by a moderator: Apr 21, 2017
  4. Jan 4, 2006 #3
    Thanks for the link, ST. You were right, it didn't really address my questions, but one thing became clear to me - there are no clear cut answers to these questions. There are a lot speculative theories on the subject and the jury is still out there, although no matter what flavor of inflation it is, it still seems to be the best explanation for the BB problems they've got. I just want to understand the general picture of the theory, without math, but in a conceptually consistent manner. With that in mind, let me get to your comments.
    Well, as I understand it, in order to resolve the horizon problem and the absence of "exotic" particles today predicted by BB, only one "spot" of the very early universe expanded. It's like you'd pick a spot on a piece of rubber and stretch it drastically. The density of all the "stuff" in that spot will go to zero. And that's what I meant by "inflation happened to only one region of the early universe". One region got inflated "instantaneously", resulting in the density of "exotic particles" and the expansion pressure from BB reaching virtually zero in the inflated region. Once the scalar field [in the inflated region] reached a threshold point, due to the size of inflation, the energy from the field decayed into the particles we observe today. Our observable Universe is a subset of the inflated region. My questions were, can we deduce what happened to the other regions (not affected by the inflation) and how big did the inflated region become? I hear some say, without any convincing rationale, that the inflated region is HUGE, and I don't understand why it needs to be huge, a priori or otherwise. To me, the inflation has to be only as big as to solve the horizon, relics, and flatness problem and no bigger. In other words, why say the expansion happened by a factor of 10 to the 10 million (as one source said) and produce a humongous inflated region, way beyond the observable universe, when all you need is a much less factor that produced simply what we observe today (given it solves the BB problems). Am I making sense or not?
    I see, let me rephrase to make sure I got you right. The "entire" universe is the sum of all those causally unrelated regions besides the inflated region I mentioned earlier. The universe as a whole was above the Planck's length when one region "exploded". But at the moment of that explosion, that region itself was way below the Planck's length. So, when the region expanded by a factor of ~ 10 to the 54, the resulting region was only about a meter in radius. Is that accurate? It also implies that the separation of forces causing the inflation ( as I understand it) didn't create the internal pressure causing the expansion of the whole universe affected by the separation. It created a field in the universe that made a tiny spot of it to blow up, to put it a layman's terms. Right?
    Ok, "exact" would sound like a quite a coincidence, but how bigger? And speaking of coincidences, I understand the curvature of the inflated universe couldn't be any different than 1 part in 10 to the 59! Otherwise, it would have either collapsed or flown apart. So, why not have another coincidence? Let's invoke the Anthropic principle again, which seems to be the default explanation these days for anything that could have been otherwise, but that's another topic :)
    I'm not sure I follow you here because I'm not clear on what you mean by "entire universe". To me, as mentioned earlier, the "entire" includes the regions that were not in the Inflated spot. Those regions are hypothesized to be other "bubble" universes, parallel universes, or what have you. My earlier question was, can we deduce anything about those universes, besides simply assigning them some ontological status.
    This answers my earlier question, I guess. We don't know how much bigger than we can see, eh?
    You made the picture clearer nevertheless. Thanks ! :smile:
  5. Jan 4, 2006 #4


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    There are many speculative theories on the subject and the jury is still out. Inflation remains the best explanation for the BB problem. I just want to understand the big picture. Without math, that is impossible. GR metrics is the place to start.
  6. Apr 1, 2006 #5
    I would be grateful if Pavel and or ST or someone else would react on my late reaction.
    Inflation, to me, seems to be brought up as a means to solve some of the problems of (a) the big-bang theory. The biggest problem is that, for most but not all physicists, the BB has a begin. So, for them, it is an act of creation or Intelligent Design. In such a case one has to explain that our observable universe grew out from zero to about one meter in about 10-43 second. Related problems such as horizon and flatness are then to be solved To avoid those problems (or maybe better to not “create” them), I think it is better to see a big-bang as a (phase) transition in an infinitely long process without begin or end (e.g. Steinhard’s approach).
    Some further remarks.
    1) I must admit that I am only a physicist who is very interested, who reads a lot of the literature on cosmology and discuss this with old friends, but still is an amateur in this area. I really try to understand as much as possible and don’t want to give up commonsense and intuition.
    2) Fully I agree with Pavel that there is no sensible dimension in physics below the Plancklenght i.e. 10-35 meter.
    3) The highest density which can occur (I suppose even in a singularity) is the density of a supposed Planck-particle. .
    4) If all of the 1080 baryons of our now observable universe where compressed at the event of the BB to that Planck-density, then one can easily calculate that if there where a ball with all our observable universe compressed in it, its radius would have been
    10-14 meter. So if inflation were to exist then it value could not have been greater than 1015 and certainly not 1052.
    5) In case of a BB phase transformation event at a Planck-density situation there could not have been a horizon problem. I wonder if a phase transformation started even at some orders lower density, horizon and flatness problems would arise. Nothing is known for the speed of light in such a dense environment.
    6) I think you are right to be unhappy with all the current ‘answers’ (even with those interpretations of the WMAP 3 results of March -3 2006) with respect to inflation and maybe we must wait until gravity experiments from our background become available.
    7) In the meantime I hope that answers from theorists and/or even philosophy can attribute to finding the right direction of investigation.
  7. Apr 1, 2006 #6
    Sorry, in my reaction some faults were introduced by the system, such as 10^15 became 1015 etc, but I am sure the reader will immediately see how things must be read.
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