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My observable universe

I've been stretching the ol' grey matter recently tackling- how the universe is expanding in all directions, that wherever you are in the universe it will appear to be expanding in all directions etc etc
I understand that in whichever direction we look the universe appears to be homogeneous, and that my perceivable universe is limited by the amount of it that I can actually observe - that the further I look the longer I look back in time and the farthest viewable points away are those whose light has only just reached me here on Earth.
Does that mean then that if I were somehow able to look beyond the furthest images I can see that I would see a black sky where lights would blink on over time as the light from those objects reached me?
Also, if the universe if expanding faster than the speed of light, meaning I will never see the farthest objects as they are travelling away from me quicker than the light can make it back to me to observe, will I eventually reach a point where I can see so far that I can see no more? Are there any guess-timates to this threshold (if indeed it exists)?

  • Apr 6 2013: You can only look back so far in time - not back to the big bang because until the universe cooled below about 3000 K it was a plasma that is opaque to photons. That wall is the cosmic microwave background.
    If you are really patient and are watching an area that has a lot of gas and dust collecting, you would be able to see a star "switch on". Reeeeaaalllllyyyy patient.
    Its significantly easier to find one about to switch off with a nova or supernova in comparison.
    If current theories are correct and space is expanding at an ever increasing rate then at some point objects will be moving away from us at greater than the speed of light and will be forever invisible to us.
    Likely long before that the light from those receding objects will be red shifted so much that it will be impossible to see anyways. Eventually so far that it cannot penetrate the gas around our own galaxy and so will become invisible at that point.
    Eventually all we will be able to see is our own milky way galaxy and maybe the Andromeda galaxy if it hasn't already whammed into us.
    • Apr 7 2013: Hi Gordon
      Thanks for your comment. I didn't think we would be able to see back to the big bang - although presumably if we look away/back far enough you would see the early universe, and in every direction you looked!
      I cant seem to get my head around the fact that the farther away we look, the further back in time we see
      The universe is bigger now than it was, and expanding all the time
      So when we look away/back we are looking at an earlier smaller version of the universe?!
      Perhaps you would need to see the universe from outside of it to gain this perspective?
  • mary T

    • +1
    Apr 6 2013: That's a cool idea -- stars blinking on as we watch their formation backwards in time. I myself wonder whether anything exists that we can not see, until we see it. Oh, now my head is hurting from thinking about your questions. I hope others respond who have studied this field.
    • Apr 7 2013: Hi Mary
      Thanks for your comment. I wasn't thinking about stars switching on, but rather seeing objects for the first time since their light has only just reached Earth. Although thinking about it some more that would mean receiving the first light they sent (unless they had somehow moved into a position nearer to us as an adult star) and so of course they would all be infant stars - you were ahead of me there ;)
      I think it is fair to say that things do exist before we see them for the first time, althouhg I have no way of proving the point, just an intuitive sense, but I wonder whether physics will eventually determine that our reality is in some way bound to our consciousness. Of course we can only experience reality via our perception but what I'm thinking of when I say this is the famous double-slit experiment, where that tricksy photon behaves differently when it is being observed than it does when it isn't - I mean it can't know its being observed, right? so the act of our observing in some way alters why?.....
      • mary T

        • +1
        Apr 10 2013: I just don't understand time at all. I don't see why it's all one way, and I don't see why it has to be that way. It seems to me that everything is now, all time is now, and we just can't see it somehow. I really wonder whether our brains are just not developed enough yet. Maybe we are the Neanderthals to some future version of us that will be able to see all time at once. That makes sense to me. Also, it makes my head spin.

        Plus, if I could see all time all of the time, I'd check out a few things that I'm personally convinced about nut can't prove.
  • Apr 7 2013: That would certainly be a good trick.
    You are always looking back in time the further you look away. That is because the speed of light is a fixed value.
    So, it takes 8 minutes and 20 seconds for light from the sun to get to the earth. Therefore the sun you are looking at is 8 minutes and 20 seconds back in time vis a vis its actual condition right now. I could have blown up 4 minutest and 10 seconds ago and you will not know about it for another 4 minutes and 10 seconds.
    Extrapolate that to the outer planets. There you are look back hours.
    The nearest star, over 4 years ago.
    You have probably heard the theory that alien races if they exist may just be getting our first radio and tv transmissions and watching I love Lucy without even getting the DejaVu channel.
    From their point of view, they are looking at the earth back in time.
  • Apr 7 2013: This aspect of cosmology has always confused me.

    As I understand it:

    Physicists refer to "the observable universe" because our ability to observe the universe is inherently limited.
    Our understanding of the big bang and other aspects of cosmology is based on our limited observations.

    So, how do our theories account for the part of the universe that we cannot observe? How can a physicist calculate whether the universe will continue expanding or not, if we do not know how much matter exists which we cannot possibly observe? How can we conclude that the universe is homogeneous if we can see only part of it?

    In summary, if we know that we can observe only part of the universe, how do we know that observations of the remaining universe would not invalidate all of our theories? How can we have confidence in our theories?