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

How do inflation, and the zero energy flat universe tie into one another?

  1. Jan 26, 2013 #1
    New to cosmology and somewhat confused. I get all the ideas indvidually, that inflation is the exponential growth in the beginning of the universe, than that gravity and matter cancel eachother out to make the zero energy universe, and that a flat universe is the only one that is zero energy and could come from nothing. I'm just confused on how all those ideas kind of relate to one another to form the idea of the "zero energy flat universe that came from nothing."

    This may be elementary for most of you but as I said I'm new to this stuff.
  2. jcsd
  3. Jan 26, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    They are each ideas that a particular kind of model for the Universe has to satisfy in order to be that kind of model. However, your question kinda provides it's own answer: just reverse it -

    Presuppose that the Universe came from nothing - then it must have started out with no energy too (nothing means nothing), so it must end up with zero net energy which makes it a zero-energy universe. The only one that does that (says your question) is a flat one. There - you've tied all the concepts together :)

    There are other ideas about the nature of the Universe too.
  4. Jan 27, 2013 #3
    My understanding is slightly different from what you have written, so I'd be interested to get any other feed back.
    As I understand a closed universe under certain assumptions has a zero net energy. If a closed universe were to tunnel into existence in a way suggested by Vilenkin in order for it to have the properties we see it would have to inflate. In the standard inflationary scenario inflation makes the universe appear flat to us no matter whether it started off as flat or not. So the flatness of the universe is taken as a sign that it underwent inflation. As small universe that nucleated from "nothing" would have had to have inflated and flat universe is what we should expect to see in simple inflationary models.
    My problem with this is a universe that came from something i.e a non singular bounce may also have undergone inflation and in fact may predict inflation.
    See here :
    This has been discussed on the forum here:
    So observing the universe to be flat doesn't tell us anything about whether the universe came from something or nothing.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook