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Specific question about infinite size and age of the universe

  1. Apr 22, 2013 #1
    Before I say the exact question, this is a inquiry about a hypothetical situation that says the Universe is infinite in both size and age. If possible, I would like a full explanation to what happens to the Universe going beyond the specific question, but I have the question here basically as a reference. I believe that the answer would be A. there would be no concepts of space and time in our understanding of nature, but I am not certain. Additionally, I looked to post this in the questions forum, but there was no forum for astronomy based questions. Thank you in advance.

    If the Universe were infinite in size and infinite in time (age),
    A.there would be no concepts of space and time in our understanding of nature.
    B.one plus one would equal to three.
    C.energy would not be conserved.
    D.there would be no night (dark sky) on Earth.
    E.there would be no day (bright sky) on Earth.
  2. jcsd
  3. Apr 22, 2013 #2
    I would have to agree that the answer is in fact A.

    I believe A is true because the concepts of space and time of our understanding of nature comes from the fact that they only exist if they they are finite quantities. These finite quantities are what gives special relativity its backing, in an infinite spacetime relativity can't exist.
  4. Apr 22, 2013 #3
    if the universe is infinite and time and space then A would be closest however that answer is worded in an ambiquous manner. If the sentence read "We would not know the overall size of space nor how old the universe is" that would be accurate. However A implies that we would not understand time or space. Which would not be true. C) In C we cannot say for certain if energy is conserved or not as we cannot account for all the energy losses in the universe today.

    B D and E make no sense at all from the viewpoint of the original question.

    I should now note that the age of the universe is definetely finite. Its still not known however if its finite or infinite.
  5. Apr 22, 2013 #4
    OK how do you explain how Einstein understood spacetime when he believed in a static eternal model of the universe?

    by the way welcome to PF to both the OP and Leomclaughlin.

    In C: in an infinite universe energy would not be conserved as the laws of energy conservation apply to isolated systems. One could argue that an infinite volume can never be considered isolated and probably be correct. However it fails to mention that we cannot account for all the energy loss in our current universe which could be finite or infinite
    Last edited: Apr 22, 2013
  6. Apr 22, 2013 #5


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    There is nothing about an infinite universe that would preclude the concepts of space and time, so A is not logical. I assume B is a humor check. Energy is not conserved in our current universe. It is not even well defined under GR, so C is technically true irrespective of the size of the universe. D is better known as Olber's paradox. It is false because the observable universe is not infinitely ancient. As long as the sun is around, we will continue to experience daylight, so E is false - again, irrespective of the size of the universe.
  7. Apr 22, 2013 #6
    Thank you for the answers thus far, obviously it is between A and C depending on what is taken into consideration when it comes to the choices. Still unsure on which, maybe some other users will post as well.

    Also, I have another topic that I posted in the General Astronomy that is just a broad question on Eddington Luminosity. Here is the link in case I put it in the wrong place and someone is able to help: https://www.physicsforums.com/showthread.php?t=687329 . Thanks for everything thus far.
  8. Apr 22, 2013 #7
    As far as scientists know the Universe could theoretically expand forever (although probably not) so the Universe to answer is most likely not infinite in size, although it is very big and expanding. The other thing you mentioned is the age of the Universe. We know that the Universe started around 13.798 ± 0.037 billion years ago. So during that "time" before that (I can't think of a better word so please don't assume there was time before the Universe unless of course you apply different theories) so the Universe is not infinite in age but rather finite. So I think A is wrong and you might need a better question or a better answer for your multiple guess question.

    Hope that helped.
  9. Apr 23, 2013 #8
    I disagree, on what basis do you say the universe probably wont expand forever? If what is termed "dark energy" is a constant then it will expand forever. Current observations indicate it is constant. Now I accept it might not be and it could be dynmical and there's nothing wrong with entertaining the idea as a possibility, but we have no reason to say its probable that it will change.
    Moreover the size of the universe is independent of this question. It could have been infinite at the big bang, in which case it is infinite now. So yes the universe could be infinite in size.
    The unvierse could also be infite in age. We know the expansion started 13.8 bio years ago. But one can only belive thats the beginning of time if one trusts GR in the deep Planck regime. Anybody here think we should do that?
    There are many attempts to deal with this problem, they include string theory , loop quanutm gravity, Horava gravity. There are other ideas that do not invoke quanutm gravity but some other mechanism such as eternal inflation or conformal cyclic cosmology that also imply our big bang is not a unique event. All of these approaches have led to us conlcude that the big bang is not the beginning. They could all be wrong but that doesnt make GR in the planck regime correct.

    So to say the universe is 13.8 bio years old I think is just wrong. Its better to say the expansion of the observable universe is 13.8 bio years old. That maybe more of a mouthful but it has the benefit of being correct.
  10. Apr 23, 2013 #9


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    None of these follow.

    A) Doesn't work, because having a concept of space and time merely requires some local entropy increase. An infinite universe doesn't disallow local changes in entropy.
    B) Can never be true in any universe, because of the definitions of "one", "plus", "equals", and "three".
    C) Energy isn't conserved globally, but this doesn't follow from the premise.
    D) Again, the global nature of space-time has no impact on local behavior. So no.
    E) This could follow if we were to add the extra stipulation that the universe is static. That is, neither expanding nor contracting. In a static universe, every direction you could possibly look would eventually terminate on the surface of a star, meaning that the sky would be the average temperature of all the stars in the universe.

    But since this extra condition wasn't supplied, we're back to the same statement as D: global properties like whether or not the universe is infinite do not have an impact on local physics.
  11. Apr 23, 2013 #10

    I think your answer labelled [E] applies to [D]...??


    anyway, if the universe were infinite as described AND STATIC, [D] is the answer..the sky would be BRIGHT as you describe.

    Olbers Paradox explains why this would be the best answer.

    "there was no forum for astronomy based questions."

    There IS an astronomy forum...see above the title of your post...but the 'cosmology' category is fine...
    Last edited: Apr 23, 2013
  12. Apr 23, 2013 #11


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    Ahh, right. I misread the answers. But the question doesn't specify a static universe, so I'm not sure it applies.
  13. Apr 24, 2013 #12
    Well it turned out the correct answer, according to the person who devised the question, was D. From the explanations that everyone provided, as well as my own breakdown of the question, I honestly don't see how D can be the correct answer. I will be appealing this soon...
  14. Apr 24, 2013 #13


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    D is correct with the added stipulation of a static, homogeneous universe. So yes, I think it should be appealed, because the question isn't sufficient to narrow it down that far.
  15. Apr 24, 2013 #14


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    seeker7, you are posing this like a multiple choice question, yet, none of those statements can be derived from an infinite universe without some more assumptions. Maybe I don't understand what it is that you are asking.
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