Reading this wiki page about the observable universe, (and visible universe), implies that we humans, on this rock in the milkyway are seeing a static representation of the universe that is still effectively in the past. They're also saying that the observable universe is about 93 billion light years to observe it, because of Hubble's law, (I think), which implies that other objects far away in the universe are moving away from us faster than the speed of light (SoL). Now if we're moving slower than SoL then it would infer that we would never, from Earth be able to see the entire universe. In fact our sun is expected to consume us long before this would even be conceivably possible.

So, if we are seeing all of the observable universe when it was 13.75 billions years old and there is actually more "universe" to observe beyond this static reference, will we actually see more of it ? Or are we doomed to observe the universe at a specific place in time ?

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    $\begingroup$ Call me crazy, but isn't the observable Universe by definition the part of the Universe that we can see? I think the header should be something more like Will we ever be able to observe all of the Universe? $\endgroup$
    – Warrick
    Commented Apr 12, 2012 at 7:06
  • $\begingroup$ My thoughts exactly, Warrick. The question as it stands is an oxymoron. Or maybe a paradox. $\endgroup$
    – Andrew
    Commented Apr 12, 2012 at 10:53
  • $\begingroup$ If you follow the links (second paragraph in the web page) they go on about visible and observable. From what I can work out, "observable" universe includes signals transmitted since the beginning of time. $\endgroup$
    – giulio
    Commented Apr 12, 2012 at 12:35

1 Answer 1


All of the universe which is observable, we can see :) But you're right---there's lots of additional universe out there that we can't see, and we'll never be able to. In fact, because of expansion, more and more of the visible universe is actually leaving the region which we can see---which is called our 'light-cone'.

Star with a point in space-time---called point 'A'. Now imagine light traveling away from that point in all directions. In the plot below, time is graphed on the vertical axis, and space is in the horizontal plane. Every second that goes by, the light goes 1-light-second (ls) further away. If you trace out the path of those light-rays, it defines two cones:

enter image description here

Everything in the cone behind point A is in its 'past', everything in the cone in front of it is in its 'future'. Everything outside of both cones is 'causally disconnected' (like point 'E'). Points outside of the cones will never interact with point 'A' (because information from them is limited to the speed of light).

If you imagine that you are at point 'A'. Then the visible universe is everything within your past-light-cone. The circle at the bottom would be the border ('event horizon') of your visible universe.

Because space-time is expanding, the shape of the 'cones' is actually bent, kind of like this:

enter image description here

But anyway, there is lots of universe outside of your light-cone (outside of out 'observable universe'), possibly infinite amounts of it (we don't know).


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