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Size of our universe

  1. Sep 22, 2015 #1
    my wife asked me after watching firefly, "how big is the actual universe?". I actually had no idea at the time and I was too lazy to get up and research it but I did have a calculator on my desk and a pad of paper so I wrote this. The actual size of the universe itself? I'm not sure of, but the actual size of the universe that could possibly contain anything besides light and trace energy tid bits is what I was going for. If you find something wrong with it please tell me!

    imperial measurement system
    (5.87e+12 x 1.38e+10)2 x 0.999999 = 1.619958e+23 miles across
    metric measurement system
    (9.454255e+12 x 1.38e+10)2 x 0.999999 = 2.6091134E+23 km across

    (cy x au)2 x fs

    cy = distance light travels in one year
    au = estimated age of our universe
    fs = closet percentage to the speed of light we've seen standard elements travel
    Last edited: Sep 22, 2015
  2. jcsd
  3. Sep 22, 2015 #2
    I don't get the meaning of that formula but "speed of light in a year" does not make physical sense.
  4. Sep 22, 2015 #3
    the distance light travels in one year's time. editing post to clear that up it wasn't written well. Basically I'm trying to state the size of the universe that actually contains elements exuding photons and neutrino etc etc.
  5. Sep 22, 2015 #4
    Your workings assume that all the matter in the universe originated at a single point in 3D space and then moved away from that point at a speed that is no greater than light speed. There is overwhelming evidence that that is not what happened. It is possible that the universe is infinite in size: there is no evidence that it is not.
  6. Sep 22, 2015 #5
    And why the square?
    You should get some quantity in km^2 if you use it like this.
  7. Sep 22, 2015 #6
    (cy x au) explains how far a photon could have traveled since the "big bang" in on direction, the square is there to include the opposite direction.
  8. Sep 22, 2015 #7
    absolutely agree with you. I'm not trying to explain the actual size of space-time because that seems to be unknowable at this time. what I'm chasing is the actual size of the universe that could possibly contain actual matter. The limits of the universe are more than likely infinite but there is only a certain amount of space that could actually contain matter.
  9. Sep 22, 2015 #8
    Then you should multiply it by two, not square it. Assuming that this thing with the "two directions" works the way you think.
  10. Sep 22, 2015 #9
    You are mistaken. Search for "big bang misconceptions" on this site or (almost) anywhere else.
  11. Sep 22, 2015 #10
    In a pistol duel the opponents start back to back and walk away from each other at 1 ms-1. After 10 s how far apart are they:
    1. 10 m
    2. 20 m
    3. 1 hectare (10,000 m2)
    However the origin of the universe cannot be compared to a pistol duel.
  12. Sep 22, 2015 #11
    Wouldn't a finite universe break translation invariance?
  13. Sep 22, 2015 #12
    We can only observe isotropy within the observable universe; that does not tell us anything about the universe beyond the observable portion.
  14. Sep 22, 2015 #13


    Staff: Mentor

    I don't understand what this means.

    Where are you getting this formula from? It doesn't look like anything derived from valid physics or cosmology.

    The universe itself may be infinite in spatial size; we don't know for sure (but our best current model suggests that it is). The observable universe at the present epoch is about 46 billion light-years in radius; see here:

  15. Sep 22, 2015 #14


    Staff: Mentor

    As MrAnchovy pointed out, this is not correct. On cosmological scales, the density of matter/energy in the universe is the same everywhere; if it's infinite in size, then there is an infinite expanse of matter/energy that's the same density everywhere.
  16. Sep 22, 2015 #15


    Staff: Mentor

    Not if it has a compact spatial topology, like a 3-sphere. That's how closed universe models work.
  17. Sep 22, 2015 #16


    Staff: Mentor

    Not directly, no, if there were significant anisotropy outside the observable universe, we would expect it to have detectable indirect effects on what we can see.
  18. Sep 22, 2015 #17


    User Avatar

    Yes. Sure: the top of my head is 2.5 ft above my navel and my feet are 3.5 ft below my navel, therefore I am 2.5x3.5 = 8.75 sq ft tall. Right.
    As already answered, there are several reasons to believe that the Universe might, right now, be of infinite size. The "theoretical" size of what Cosmologist call the "Observable Universe" depends on the model (the system of equations and assumptions) we use to calculate it. Often it is estimated to be about 46 billion lightyears in radius, meaning it has a diameter of 92-93 billion light years. But what is meant by "observable universe"? It means, crudely, that all of the stuff that was close enough to the stuff we are made out of for it to interact with our stuff. You may know that the Universe is expanding. This means we are slowly "losing touch" with some of the matter (the stuff, which includes light). RIGHT NOW (and we'll ignore what "now" means in Relativity Theory, just assume it has its common meaning) Stuff which is more than about 15 billion light years away from us is moving away from us at ever increasing speed. This stuff can NOT ever be seen by us; meaning there is no way it can influence us in any way. - its essentially not there as far as we are concerned. But at one time the Universe was much smaller. Some estimate it might have been a few meters in diameter, others think it started out smaller than an atom, but our Physics fails at the start of the Big Bang, so that remains an open question. Anyway, most of us believe that some of the stuff which was close enough to interact (at the speed of light) then has expanded out to about 46 billion light years. And yes, if you calcuate its "average expansion speed" you get 46 รท 14 = 3.3X the speed of light. But this expansion is not the stuff moving through space, but the actual expansion of space, so there is no limit on how fast this can be.
    Two aspects to point out. 1. The Observable Universe isn't all observable. Things which are 46 billion light years away will never again be close enough to us for them to have any meaning for us. The OU contains all the stuff that we would have seen at one time, if we were present at the beginning. 2. A lot of Cosmology assumes that the Universe is isotropic and homogeneous. These assumptions mean that there is (on average) no special place in the Universe: it looks the same (on average) from anywhere. (There are good reasons for these assumptions, and there is some evidence that they are approximately true). What this means is for an alien living on a planet 14 billion light years (bly) from us, that it would look up at night and see a sky which was pretty similar to ours. (Different constellations, different galaxies, but the average 'structure' of the Universe would look the same.) This means the alien's OU would look about the same but include things that are NOT in our OU. So, we'd need to add that "extra" volume to the size of the ENTIRE universe. But there's no reason that another alien couldn't be on a planet 14 bly from the first (and 28 bly from us), and we'd add that volume too. And another could be at the edge of Alien 2's OU. Since homogenous means none of these aliens would be at a special point (the edge of a cliff or wall IS a special place) that we should continue to assume the universe goes on forever, even though the stuff that was ever close enough to us has only been able to "travel" to no more than ~46 bly from us, the stuff that we couldn't interact with at the start (or ever since) could be billions, trillions, and more, light years distant "now". But talking about stuff that we never did and never will interact with is like talking about the angels dancing on the head of a pin; you can have a multitude of different opinions about them, and we'll never be able to determine which, if any of them, is correct.
  19. Sep 23, 2015 #18

    I believe that MrAnchovy was specifically referring to the paper _Misconceptions About the Big Bang_ authored by Charles Lineweaver and Tamara Davis. This was published in _Scientific American_ in March of 2005. It addresses your question as well as many others and is one of the best introductions or perhaps re-introductions to the unfortunately named Big Bang Theory as well as exploring elements of the Universe's 'architecture'.

    Here is a link to the article hosted on Dr. Lineweaver's own website:


    Highly recommended.

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