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I How can the universe be infinitely big?

  1. Apr 22, 2016 #1
    I was just watching a video in which the famous physicist Brian Greene was interviewed about his book The Hidden Reality. During this interview, Greene said that it is a matter of significant debate among cosmologists about whether the size of the universe in finite or infinite. In Stephen Hawking's book A Brief History of Time, Hawking says that the universe expanded at a finite rate after the Big Bang. If the universe expanded at a finite rate after the Big Bang, how can the universe now be infinitely big? Before I saw this interview of Brian Greene, I thought that no cosmologist who believes in the Big Bang Theory would think that the universe was infinitely big.

    My sources on this are the interview of Brian Greene that I watched on youtube and Stephen Hawking's A Brief History of Time.
  2. jcsd
  3. Apr 22, 2016 #2


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    There is no contradiction at all between the universe being infinite (then and now) and expanding between then and now. Google The Hilbert Hotel.

    The consensus these days seems to be that it most likely IS infinite but it's not clear that we'll ever have any actual evidence/proof of that.
  4. Apr 22, 2016 #3
    Phinds, I agree with the person who wrote the Wikipedia article about the Hilbert Hotel paradox that it's counterintuitive, and it's interesting. I now agree with you that there is no contradiction between the universe being infinite and expanding between then and now.
  5. Apr 22, 2016 #4


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    Yeah, counter-intuitive and a bit weird when you first encounter it.
  6. Apr 22, 2016 #5
    I was thinking about the implications of an infinitely large universe, and at first, for about one second, I thought to myself: The universe could not be infinitely big because that would require the Mass/energy in one spot at the singularity to be infinitely big mass/energy. But then one second later, I remember that whenever I read about the Big Bang, my sources say that the Mass/Energy at the moment of the Big Bang was infinitely big, so there is no contradiction. Indeed, the contradiction would be if the universe was not infinitely big.
  7. Apr 22, 2016 #6


    Staff: Mentor

    Neither of which are valid sources here on PF. Please take some time to understand the actual theories involved from sources other than pop science books or videos. In the meantime, phinds' post is correct and answers the question as best it can be. Thread closed.
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