Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does inflation lead to a smaller universe at 380000 years?

  1. May 4, 2012 #1
    Why does inflation mean that the universe was much smaller at 380000 years after the big bang than we would predict from the Big Bang model alone? What would we expect the two sizes to be?
  2. jcsd
  3. May 4, 2012 #2


    User Avatar
    Science Advisor

    Inflation simply means the universe is expanding as a function of time. Go backwards in time and it gets smaller.

    Try a Google search to answer to the other question.
  4. May 5, 2012 #3


    User Avatar
    Science Advisor

    Well, I don't know about predicting a different size, but a universe that has an early period of inflation definitely takes longer to expand from the same seed size to the same late-time size. Not much longer, but some.

    The reason is that with the classical big bang, where the only energy density that matters at very early times is radiation, the expansion rate is slowing down dramatically early-on. So the picture is one where at early times things are very hot, very dense, and with an incredibly fast expansion rate that is rapidly slowing.

    With inflation, the opposite is happening. The dominant energy density is a form of dark energy that causes a very rapidly-accelerated expansion. So at very, very early times, the rate of expansion was actually quite slow, and was sped up to a high rate of expansion later. Because the early expansion was actually slower, it takes more time under an inflationary model for the same seed to expand the same amount.

    However, the difference in expansion time is basically a tiny fraction of a second, so for all intents and purposes it does not matter. But it is an interesting conceptual statement.
  5. May 22, 2012 #4
    Many thanks Chalnoth and mathman. Really what puzzled me was a sentence in one of Marcus Chown's books where he says that if inflation did occur then 380000 years after the Big Bang the universe would have been far smaller than if we assume expansion alone (with no inflation). I'm still not really sure what he means here.
  6. May 22, 2012 #5


    User Avatar
    Science Advisor
    Gold Member

    Inflation occured around E-36 seconds after the big event, not 380000 years.
  7. May 23, 2012 #6
    380,000 years is when the CMb was released. I think Chown has it wrong. This is an image thats in my text book and a in a lot of text books:
    youll see that the casual patch is much smaller at the onset of inflation , once inflaiton ends you have the same sizes as in standard big bang cosmology.
  8. May 23, 2012 #7
    It sounds like a typo, if it said "if inflation had not occurred ..." that would make sense, the region would have been many orders of magnitude smaller. Or perhaps it's the other way round, because of inflation, the material from which we receive the CMBR was much smaller (pre-inflation) than it would have been without inflation.
  9. May 23, 2012 #8


    User Avatar
    Science Advisor

    That's not right. Inflation refers to a very specific kind of expansion, namely one that is exponential (accelerating), that occurred early on the history of the universe (prior to BBN and CMB.)
  10. May 23, 2012 #9


    User Avatar
    Science Advisor

    During inflation, the Hubble scale barely grows (during de Sitter expansion, i.e. a CC, it does not grow at all.) So the Hubble scale, which marks the boundary of the observable universe, is roughly the about same just after inflation as it was just before. Had the universe otherwise been expanding through a radiation dominated epoch instead of inflating, the Hubble scale would have been growing. So, I think the key point here is that Chown appears to be referring to size of the *observable* universe.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook