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

Light travel just after the big bang

  1. Apr 16, 2015 #1
    If the universe had stayed the size of a baseball, would it have taken light 13.5 billion years to travel across it?
     
  2. jcsd
  3. Apr 16, 2015 #2
    What is your thought about it?
     
  4. Apr 16, 2015 #3

    phinds

    User Avatar
    Gold Member
    2016 Award

    The universe has never been "the size of a baseball" so I'm not clear why you are asking what would have happened if it has "stayed" that size.
     
  5. Apr 16, 2015 #4
    I don't know, maybe something about how the density(?) of spacetime effects light.
     
  6. Apr 16, 2015 #5

    phinds

    User Avatar
    Gold Member
    2016 Award

    This posits an incorrect assumption that spacetime is an "ether". It is not.
     
  7. Apr 16, 2015 #6

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    The universe, as best we can tell, is spatially infinite, so, as phinds said, it has never been "the size of a baseball".

    Spacetime doesn't have a density. The matter and energy present in spacetime does, but spacetime itself doesn't.
     
  8. Apr 20, 2015 #7

    Maybe the question s/h/b stated as: If the currently visible universe had stopped expanding at an earlier time, would the greater energy density cause light to take the same 13.5 b.l.y. to traverse the radius?
     
  9. Apr 20, 2015 #8

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    The only way this could have happened would be if the universe were closed and that earlier time was the moment of maximum expansion. So the universe would currently be contracting.

    First of all, it's years, not light-years, since you're talking about time.

    The time it takes light to cover a given distance in the universe is not affected by the energy density, except in so far as the energy density determines how the expansion proceeds. All you need to know to figure out light paths is the expansion profile.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook