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Questions on the Inflation Debate

  1. Nov 24, 2012 #1
    The commonly accepted age of the universe is 13.7 billion years. I assume that assumes an inflationary epoch. Since inflation is in debate, what’s the age without it? And since expansion is accelerating, I assume expansion was slower 10 (or 20) billion years ago. Could it be infinitely old? (IE no bang?) Maybe Ned Wright’s calculator needs an inflation switch.

    Another odd thing I notice is that discussions of inflation usually include an after size. Steinhardt’s Inflation Debate says the size after is a dime. Wikipedia says 100 million light years. Big difference. It seems odd to me to say a size since inflation requires a “flat shape”. Doesn’t that mean there’s no edge?

    The Inflation Debate
  2. jcsd
  3. Nov 24, 2012 #2
    I'm actually in the middle of looking for a *different* Sean Carroll blog post in response to your question, but I found the lackluster endorsement of inflation (generally) in this post a bit surprising: Do You Think Inflation Probably Happened?
  4. Nov 24, 2012 #3
    This is the post i was looking for: The Eternally Existing, Self-Reproducing, Frequently Puzzling Inflationary Universe

    I know this is slightly tangential to the OP but the points Sean Carroll mentions against inflation (I also recall a similar feature article in Scientific American a few years ago) came off to me as fairly potent. The gist of it seemed to be, "Well the reasons for the whole idea (of inflation), in the first place, basically suck.....but thinking of (and dealing with) other models would require a lot of work, so.....you know...whatever." :smile: Here's some meat:
    Certainly doesn't sound reassuring...
  5. Nov 24, 2012 #4


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    One thing is relatively clear, whatever happened in the very early universe must have looked a lot like inflation. Inflation flounders when you attempt to define initial conditions, but, so does virtually every other model. The Planck mission should shed some light on matters, but, will not necessarily be convincing evidence either way. LIGO, or some similar gravitational wave detector, is probably our best hope for the immediate future. The gravity wave signature of inflation would be uniquely different than just about any other option.
  6. Nov 25, 2012 #5
    That's been my understanding as well.
  7. Nov 26, 2012 #6
    My understanding is that LIGO has virtualy no chance of detecting primordial gravaitational waves and so will not make any ruling on inflation either way.
    To answer the original question the age of the observable unvierse is the same with or without inflation (it was only supposed to last a tiny fraction of a second anyway).
    Its also my understanding that Planck has an outiside chance of detecting the B mode polarisation but its not designed for that so we'd have to get pretty lucky for this.
    What we really need is dedicated B mode polarisation such as CORE or EPIC. A space based gravitational wave observatory suchg as LISA is porbably still not going to be powerful enough to detect primordial gravitational waves. But a next gen mission such as the BIg Bang Observer hopefully will be. I think they are looking for a 1000 times more sensitivbity than LISa and dont forget LISa is currently not funded, so this is likely a long way off, read more on BBO here:
    http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37836/1/05-2157.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  8. Nov 26, 2012 #7


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    Yes, to further emphasize Phil's important point: 13.7 billion years is the age of the observable universe, i.e. about the age of the CMB. Since the CMB is created after inflation, one can effectively think of inflation as preceding the big bang. After inflation ends, the universe goes through nucleosynthesis, decoupling, etc just as in the standard big bang model without inflation.
  9. Nov 26, 2012 #8
    Forgive me if I've got this wrong but, I thought that the CMB was 380k years (or 780k years - I can't remember which number is right ) younger than the universe? Are you just saying that as a proportion of its life this 380k years is insignificant?


    Last edited: Nov 26, 2012
  10. Nov 26, 2012 #9
    380k years is the right number. That's the amount of time that's estimated to elapse from the beginning of the expansion of the universe to the release fo the CMB.
    The CMb is the earliest thing we can see with light. Hence Bapowell's point that the observable universe begins with the CMB and hence inflation happened before the big bang.
    However i think some clarification is helpful. People may mean different things when they say big bang.
    1 the beginning of the observable universe - thats the CMB
    2 the beginning of the expansion of space time, thats estimated to be 380k years before the CMb was emitted.
    3 the beginning of all space and time - this is a prediction from general relativity that I think is less and less believed. Most would argue that both quantum gravity theories such AS LQC and other approaches such as eternal inflation imply there was no such ultimate beginning 13.7 bio years.
    Perhap[s there is some ultimate beginning in these models but even if there is they wil be way beyond any conceivable observation we could makee.

    One important point to note there are two potential ways to observe before the CMB was emitted.
    1 Observe the cosmic neutrino background
    2 observe the gravitational wave background.
    Both of these i think its a safe bet to say are far into the future. Although if PLanck is very lucky it might see a sign in the CMB of an imprint of the gravitational wave in the form of the way the CMB is polarised.
    This a good article to read from NAture magazine which also discusses the inflation debate:
  11. Nov 26, 2012 #10
    Thanks Phil (for the clarification and the article).


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