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Size of university and Dark Matter

  1. Mar 5, 2014 #1
    I have a Physic 201 understanding of physic.

    I understand that the most distant star (galaxy) that we can see from the earth is 4.4 billion light years (the time of the big bang) away. I think this is called the viewing horizon.

    Assume a person on a planet 4.4 billion light years from earth (let us call it B1) looks back at earth. I expect he will see the earth.

    What does he see if he looks in the opposite direction from earth? I expect he will see B2 – 4.4 billion light years away.

    How far is B2 from the earth???? – 8.8 Billion light years ???

    This analogy could go on for B3, B4, B5 …..

    Could this account for dark matter????
  2. jcsd
  3. Mar 5, 2014 #2


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    (1) It is more like 14.4 than 4.4
    (2) this has nothing to do with dark matter. I would suggest you read up a bit on dark matter.
    (3) Yes, someone at the edge of our observable universe will have us at one edge of their observable universe and will be able to see stuff at the other side of their observable universe that is, and always will be, causally disconnected from the Earth.
  4. Mar 5, 2014 #3


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    Actually, the most remote galaxies currently known were less than a billion years older than the BB when the photons we now observe were emitted. That translates into a little over 13 billion light years. An observer currently 4.4 billion light years distant would observe the earth as it appeared 4.4 billion years ago. If that observer looks in the opposite direction, the most remote galaxies would appear to be as they were about 9 billion years ago. Cosmological distances are difficult to picture. CMB photons we currently observe, for instance, originated a mere 42 million light years from earth, but, took nearly 14 billion years to reach earth.
  5. Mar 5, 2014 #4


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    Yes but what does any of this have to do with the question asked - about the size of the University? Is this something to do with distance learning?
  6. Mar 5, 2014 #5


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    Could what account for dark matter?
  7. Mar 6, 2014 #6


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    Yes. What you say makes sense. Actually the most distant MATTER we can see from Earth is estimated to NOW be a little over 45 billion LY from us*. We see that matter when it was still hot gas and had not yet formed stars.
    But we expect that matter has in the course of many years formed galaxies and stars and planets just as our matter has.
    If there are observers there and they look towards Earth they will see our matter, 45 billion LY from them, as it was in early times, a glowing hot gas. That glow is what has become the "microwave background" that we see and they see.

    And if they look in the opposite direction (as you imagined and said in your post) they will see matter that is 45 billion LY from them and 90 billion LY from us.

    So what you said about 2X distance was right. We do not know how big the universe is. We know that it is very very big, much more than 90 billion LY. Maybe infinite, maybe finite, but in either case extremely large. And these distances are increasing.

    You also asked about "Dark Matter". That is a separate topic. It is not related to this SIZE question--the first question you asked.
    You can make a separate post to ask about dark matter. Also you should ask about dark energy.

    Dark matter and dark energy are two very different ideas. Dark matter exists as a real material and it SLOWS DOWN expansion by its gravity. Dark energy may be a name for something that is not exactly an energy. We don't know the cause of the "dark energy effect" which is ACCELERATING the expansion. So DM and DE are different things and DM slows expansion while DE speeds it up. NEITHER of them directly causes what you said about distances to stars and planets beyond the visible horizon.

    Welcome to Physicsforum. Asking questions is the best thing for us. Thanks for starting off with a good question about distances :smile:

    *ask questions to find out how distances are defined. that distance is 45 billion LY distance now, but a long time ago (when the light started traveling to us) it was much smaller. Distances change size over time, even while the light is traveling.
    Last edited: Mar 6, 2014
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