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The size of the universe, and what we can see.

  1. Oct 25, 2011 #1
    Hi. Just joined the forums. I've not taken a physics class since high school, but it is an interest of mine, so forgive me if I'm way off on some of the science.

    Please tell me if I have any of these facts wrong:
    - The universe is about 13.7 billion years old.
    - Our light horizon is roughly 27(?) billion light years across.
    - We estimate the diameter of the (known?) universe to be around 46 billion light years.
    - The oldest/earliest galaxy we've ever seen is from around .9 billion years (of the age of the universe).
    - The very earliest galaxies are estimated to be from about .3 billion years

    - We have estimated the size of the universe by looking at the earliest/most distant galaxies and then calculating how much the universe has expanded since then.

    If my numbers aren't far off, is it just serendipity that we have been able to see almost to the beginning?
    If humans had developed to the point we're at now about 1 billion years ago, would we be able to see almost to the big bang? I've read the first few hundred thousand years had no photons.

    I hope this makes sense to someone. :smile:
  2. jcsd
  3. Oct 25, 2011 #2


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    The diameter of the observable universe is about 95 billion light years (I think you have the radius, thinking it's the diameter).

    I don't know about the light horizon.

    All else seems good.

    We can see back to the surface of last scattering, which is about 400,000 thousand years after the singularity. I believe I have read hypotheses that we should be able to see somewhat further back using neutrinos, which were not obscured as photons were prior to 400,000 years.

    At not time would anything have been "visible" behind the surface of last scattering. In the past, the CMB would have been warmer than the 3 degrees or so that it is now.
  4. Oct 25, 2011 #3
    Thanks for the response.

    What is the 'surface of last scattering'? By your other comments, does this mean we can see the very first stars? My understanding is they formed around the 200 million year mark. This would obviously predate all galaxies.

    If humans had started looking a billion years later, would we have never been able to see this early stuff? With the accelerating expansion of the universe, it just seems awfully lucky that we started to look out when we did.
  5. Oct 25, 2011 #4


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    Up until about 400,000 years, the universe was opaque to light due to the massive density of particles/energy boiling all around. At that point, things clear up (you would likely find it interesting to read the details ... just google it) and that point in time is called the surface of last scattering and is what we see referred to as the Cosmic Microwave Background. From that point forward, it has never been possible to see past this "surface". Stars started forming some time after that so theoretically at least we could see them if we had good enough equipment.
  6. Nov 2, 2011 #5


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    The co-moving distance tends to confuse more than clarify matters of distance. It is not intuitive, even though perfectly reasonable in an Einstein deSitter universe model. The light travel distance [or light travel time] is easier to grasp, but, does not actually tell you anything about distance because the universe has constantly expanded since the light was emitted. The distance was obviously much smaller when the photons were emitted, and, equally obvious, must be much larger now. The easiest [IMO] way to picture distance is based on the formula 1 + z = a(to)/a(te) where z = redshift, a(to) = size of the universe now, and a(te) = size of the universe when the photons were emitted. If you assume a(to) = 1 [size now], a(te) becomes the relative size when the photons were emitted. For further discussion see http://arxiv.org/abs/astro-ph/9905116
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