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Looking back 10 billion years

  1. Mar 24, 2010 #1
    We continue reaching further and further back in time by observing galaxies that were born during the early part of the universe's existence. Some of the furthest galaxies we can see are 10 billion light years away, that is, their light has taken that long to reach us.

    In light of the Big Bang, if these galaxies are at the edge of the visible universe wouldn't they all appear on the same side of the sky?

    That is, the light from galaxies of the same age on the "other" side of the universe has not reached us yet?

    Actually, wouldn't all the galaxies at the edge of the universe that we can see all be concentrated in one general area of the sky?
  2. jcsd
  3. Mar 24, 2010 #2


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    correct, the earliest are around 300M years after big bang.

    Nope - we live IN the universe remember.
    The universe was created at a point, it then spread out into nothingness (not empty space - but really nothingness!)

    We aren't at any special central point in the universe, in fact for some slightly difficult to picture reasons there isn't a centre of the universe.
    Whichever direction we look we see further back in time, until ultimately at the microwave background we see the energy that filled the universe before any matter was created.
  4. Mar 24, 2010 #3
    For sure this is what I'm not seeing. I think of the Milkyway as halfway from the peach pit to the skin, give or take a few million light-years. I guess when dealing with such massive distances and space-time fluctuations one needs to abandon geometrical thinking.

    Thanks for the reply.
  5. Mar 24, 2010 #4


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    On the scale of the galaxy stars aren't randomly distributed - they are clumped in the
    centre of the galaxy and in a thin disk, that's easy to see (at least with dark skies)

    But the milky way is really tiny backyard stuff.
    It's only about 100,000lyr across compared to a universe 80Bn lyr in size and is only one of 100Billion galaxies in the universe.
    The few galaxies near us are clumped together because they feel each other's gravity - but as you look further out into space the galaxies are randomly spaced.
  6. Mar 24, 2010 #5


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    Wait. What?

    You think the universe is a sphere like a peach and we are somewhere halfway from the centre?

    No. We are at the centre. As is every other point in the universe. Everywhere is the centre.
  7. Mar 25, 2010 #6
    Well, no, I don't actually believe there is a peach pit in the "center" of the universe. But it is rather difficult (for me) to really understand the concept of "everywhere is the center". I do understand the notion that everything in the sky is moving away from everything else in the sky, which may or may not lead one to hypothesize everywhere is the center. Of course, if everything is moving away from me then I must be in the center.

    I'm sure I'm not visualizing the time-space dialation/expansion correctly, but I assume that the universe was once a singularity, so the big bang began at a point and the universe is expanding from that point in every direction. That assumption, however wrong, leads me to the peach-pit analogy.
  8. Mar 25, 2010 #7


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    The important (and tricky) bit to 'get' is that the universe wasn't empty space and everything then exploded out of one point at the centre.

    It was the universe, ie. empty space and time itself, that was created.
    Now the tricky bit - the universe is 4dimensional so in 3d has no edges and no centre.
    The normal example is to imagine a 2d world on a balloon - as the balloon inflates every point on it seems to be moving away from any point on the surface - there is no centre point on the surface and no edge.
    The unverse is like that - except we are a 3d planet on the surface of a 4d balloon!
  9. Mar 25, 2010 #8
    Thanks for the thoughtful response. I'll need to chew on that one for a while.
  10. Mar 25, 2010 #9
    The often-used analogy is that of the skin of an idealized balloon which is being inflated. Initially, all points on its surface are close together. As the balloon expands, every point on its surface recedes from every other point. There is a centre of expansion - the centre of the balloon - but this centre is not part of the skin. The same holds for our universe: the centre of expansion is something other than a point in space.

    The reason the analogy uses a two-dimensional surface is that it can then use the third spatial dimension to represent concepts like the centre and the curvature of the skin. The equivalent for a three- (four-) -dimensional space (-time) is naturally much harder for our three-dimensional spatial imagination to process.

    ETA: Double-post...
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