Assuming that the Universe is and has been expanding ever since its creation (Big Bang theory), the galaxies that are now at the 'edge' (not visible theoretically) must have been (at some point in time) at a place around where the Earth is at now.

If it is assumed to be true, then the photons emitted by these galaxies from this point onwards should be around for us to see these galaxies (and even the first light emerged from the Big Bang must be visible).

Then why is the observable so small? It should depend upon the direction in which we look. That is, if we look in the direction of the center of the expanding sphere, it should be a small observable universe. If we train our telesopes in 'exactly' opposite direction of the center, it should be 'fully' observable.

Does it work this way? Or are my basics wrong?

  • $\begingroup$ Your basics are wrong because it is not an expanding sphere with a center. It has no center an it is expanding in every direction. $\endgroup$ Feb 3, 2014 at 13:20
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    $\begingroup$ Thanks for your comment. Being non professional, I had doubt that it could be novice question. Nevertheless, when we say 'something is expanding' means its path can be traced back to its earlire position. Can it be a possibility that our milky way has formed in such 'expantion path' of some distant galaxy/star? If yes, then that object must be visible if it is still streaming photons no matter how far it has reached it can not be non observable. $\endgroup$
    – Vivek
    Feb 3, 2014 at 14:28
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    $\begingroup$ The problem lies in the fact that space itself is expanding. You're used to the idea of matter expanding an so it can be traced to it's point of origin; with spacetime it's different, our universe has no boundaries and no center, not an easy thing to imagine. $\endgroup$ Feb 3, 2014 at 14:55
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    $\begingroup$ @EduardoSerra As for the no-center aspect, I've found the "problem" of locating the center of the surface of the Earth to be a good analogy. Back when people still thought the Earth was a flat circle, people would draw maps as circular area of explored territory with their capital city at the center of the circle. As more and more cities started producing maps with their city at the center, people began to wonder which city is actually the World Center. Then the Greeks realized that Earth is spherical and so it has no center; only maps have centers, and map-centers are arbitrary anyway. $\endgroup$
    – David H
    Feb 3, 2014 at 19:15

2 Answers 2


Quick answer: Because they didn't entered our event horizon. Some never will. And some will move out of our event horizon - their last photons that'll be received here being sent right now.

Let's do some fact checks first:

[...]the galaxies that are now at the 'edge' (not visible theoretically) must have been (at some point in time) at place around where the Earth is at now[...]

According to current calculations, the first galaxies may have formed around 200 million years after the Big Bang - older estimates went with the 400-500 MY range. For a long, long while, there were no stars to be seen. So if you go back in time you won't be seeing the same structures we see today.

Second, and that may be an awkward mental experiment, nothing else was occupying our place other than ourselves. I'll ask you to excuse the cliché, but the old balloon example is very apt to explain this:

enter image description here

As the universe expands, the distance between celestial bodies increases. Now, here's a way to put it: space is being generated in between the objects.

And not only that - while there's a limit on how fast you can move on the balloon's surface, it doesn't apply to the amount of space being generated.

As a direct consequence there's a bubble around us that basically works in the very same way as a black hole's event horizon does:

  • In a black hole, gravity is so strong that its pull on photons exceeds C (the speed of light, or 299,792,458 m/s);
  • On a fringe object, the amount of space generated by the expansion, per second, may exceed C; its photons will never reach Earth.

And it seems that we live in a accelerated universe - that is, the spatial acceleration ratio is actually going up. If that's correct, some fringe objects will slip away form our event horizon, disappearing (from our point of view) into a sample of the heat death of the universe.

Scary, huh?

  • $\begingroup$ Fantastic explanation!!! Thanks a lot for elaboration. $\endgroup$
    – Vivek
    Feb 3, 2014 at 20:35

The space we observe behaves roughly like the surface of an expanding sphere, and we can only look along its surface. The interior of the sphere has existed in the past, the outside will exist in the future.

The surface of a sphere is two-dimensional. Our space has one more dimension; it may be thought of being embedded in a 4-dimensional space (hyperspace), although this embedding hyperspace doesn't need to exist to describe our universe.

Like the surface of an expanding sphere has finite area and no boundary, our three-dimensional space can be finite for a fixed time of expansion, with no boundary.

The universe (the radius of the sphere, if you want) expands faster than the speed of light, therefore we cannot look completely around the universe to see our own past, but just to the point it started its expansion. The starting point (big bang) seems to be opaque.

We see the afterglow of the big bang in any direction as cosmic microwave background.


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