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Size of the Universe? I'm confused.

  1. Jul 5, 2012 #1
    I have to preface my questions by saying that I have only a little better than a high school understanding of physics; as such I humbly ask that any answers be termed in a way that I'll be able to understand them.

    I've read that the universe has a radius of ~ 46 billion light years. And we know that it's about 13.7 billion years old.

    This hurts my head. How could we possibly see something that far away? Light simply wouldn't have had time to get to us from that far? How is it that we can see beyond the 13.7 billion year mark? Wouldn't the largest area we can view be roughly double that, 27.4 billion light years? And if we can't see it, how can we know?

    Furthermore, how can the universe even have a measurable size? Are we even certain it's finite?

    I have seen Hubble photographs of the earliest galaxies, which formed when the universe was about a billion years old. Wouldn't they be so far from us by now that there's no way the light could have reached us? Did we see them with some sort of gravitational lensing, allowing us to see beyond the 13.7 billion year mark?

    I've also read that the universe, for a time, was pure energy - no matter. And after that it was still a while before there were even photons. Have we been able to see back to this time? Physicists seem fairly certain what happened right up to only a few Planck times after the Big Bang. How can they know? Do we have any evidence of what happened prior to the existence of photons, other than math?

    Forgive me if these seem simplistic, but my desire to understand is strong enough to risk the embarrassment.
  2. jcsd
  3. Jul 5, 2012 #2
    Light does not move 1 billion miles per year..
  4. Jul 5, 2012 #3
    This is the size of the "observable universe" which is different from the size of the total universe.

    We can't. 13 billion years ago some gas emitted some light. We calculate that that gas is 46 billion light years right now. But the light that gets emitted right now, won't reach us for another 46 billion years.

    We can't.

    No. As something gets further away, it's going to move away faster.

    Educated guess.

    The figure that's quoted is "how much of the universe we can see". It could be infinite.

    Correct. It's a "reasonable guess" that those galaxies would have evolved in the same way that nearby galaxies evolve.

    No. Once the universe gets to be about 3000K, it no longer transmits light so that's as far back as we can directly see. We can make guesses about what happened before. For example, based on the amount of helium in the universe, we can make reasonable guesses as to how the universe behaved when it was 3 minutes old.

    Also, at some point we may be able to "see" gravity waves or neutrinos.

    Educated guesses. It's like trying to figure out what's in the center of the earth. You can't see it directly, but you can guess based on how sound gets transmitted through the earth.

    In the case of the big bang, you have what are basically sound waves create fluctuations in the cosmic background, and that provides a *lot* of information. You can also figure stuff out by nuclear reaction rates.

    Yup. The interesting thing is that we know a lot about the big bang because we can see the ripples in the cosmic background. The big mystery right now is the "dark ages". Once the universe can transmit light, things "go dark" until the time that the first stars form which gives you about a billion years of mystery. How galaxies formed is something of a bigger mystery than the big bang.

    Also photons did exist. The problem is that once you heat gas hot enough, it starts absorbing and reabsorbing photons.

    But people are figuring this out.
  5. Jul 5, 2012 #4
    Thank you very much Two-Fish.

    One follow up question if you don't mind.

    With earth at the centre, how can the observable universe be greater in size than the time allowed for light to travel to us? Since, as you said, it's rate of expansion increases proportionally with distance from us. Wouldn't the observable size be some number less than 27 billion light years across?
  6. Jul 5, 2012 #5
    The expanding universe is analogous to an expanding soap bubble, but with one more dimension.

    Currently the bubble has a radius estimated to be about 46 billion light years. But the center of that bubble is a point lying outside of the surface of the bubble itself. We live in the surface of the bubble and we can see only 13.7 billion light years in all directions from our point on the surface. The rest of the surface is there, but light from that part has not yet had time to reach us.

    This is all theoretical, however. The universe may have a more complex 'shape' than this analogy would suggest.
  7. Jul 5, 2012 #6


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    Actually, isn't it correct that if it emits light right "now", it will NEVER get to us since the point of emission is receding from us "now" at about 3c ?
  8. Jul 5, 2012 #7


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    Phinds, you are making an important point about the light NEVER reaching us. It's hard to explain why in just 10 words or less and what you said is not the full explanation (I'm sure you realize it was simplified.) The full story involves *acceleration*, not just the fact that the material emitting the light is receding at 3c.

    The threshold of Never-See---the distance to current events we will never see no matter how long we wait, called the "cosmic event horizon", is about 16 billion LY. If a galaxy is now, say 17 billion LY away from us and has a supernova explode today we would NEVER see that explosion. Not in 46 billion years or 50 or 60 or ever, no matter how long we waited.

    It's not JUST that the distance to it is expanding superluminally, which it certainly is. It's that (according to standard cosmology) the Hubble constant is not slated to decline to below around 60 km/s per Mpc which means the Hubble radius (within which distances grow slower than light) is not slated to extend beyond about 16 billion LY, ever. And that's because of the positive Lambda--the acceleration.

    Does the FAQ have a good simple discussion of the cosmic event horizon? The usual abbreviation is CEH. Lineweaver has some clear discussion in his "Inflation and the CMB" article from back around 2003. If our FAQ doesn't have a simple clear account then maybe somebody (you? twofish?) should contribute one.
    (Whoever is in charge might or might not accept your essay but it would be good to submit something and see, if they don't have anything on CEH)
    Last edited: Jul 5, 2012
  9. Jul 10, 2012 #8
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