Q & A: visible universe

Q:
I have often come across terms like "the observable universe" and "the un-observable universe" distinguishing between the parts of the universe that is close enough and too far away for us to observe their light. If all matter in the universe was concentrated in the so called big bang singularity (Stephen Hawking) before expanding to what we see today, how can there be parts of the universe that are too far away to be observed (so far away that the light from them hasn’t had time to reach us yet)? In another phrasing: How can there be matter that has traveled so far from us that the time it took for it to get there isn’t time enough for its light to travel back to us? Thank you!
- Tobias Johansson
Chalmers University of Technology, Sweden, Göteborg
A:
Great question! There are several levels of complication in giving the answer, so please bear with me.

The picture you've assumed is one with a finite, initially miniscule, universe blowing apart at some constant rate. If that were the case light from any part would have time to reach us on a direct path. The parts that were moving away from us quickly would have had to send their light early after the big bang, but they'd still be visible at some point in their history. One could consider that events which have happened already but the light hasn't had time to reach us yet are in a currently unobservable portion of the universe which may become observable at some later time. You may do a quick calcuation and wonder why even for things running away at nearly the speed of light from us the time of visibility wouldn't be almost half the the age of the universe- not all that young. However, that would be age in our reference frame, and the important age is that in the object's own frame, which is much less.

Now there are several things missing from this simple picture. The first problem is the assumption that the universe is finite. It may have always been infinite, even though all the parts we can see were once arbitrarily close to each other. In that case of course there would be parts that have always been too distant to see.

The second problem is more complicated, but actually more definitely known to be relevant. The expansion of space is governed by the laws of General Relativity, not Special Relativity (which is a good approximation for small patches). If for some reason a background energy density is present in space itself, the rules say that the expansion of space will accelerate. That process is called inflation. The speed limit that says that nothing moves faster than the speed of light applies only to objects close to each other, in one of those patches where Special Relativity works. On a large scale, inflation can make spaceexpand so that two objects reach outside each other's horizons. The evidence that there was a period of very rapid inflation early in the universe is now extremely compelling. In fact, there's compelling evidence that a much weaker inflation is going on right now. The background energy densities driving either of these inflations are not currently understood- that's the so-called 'dark energy' problem.

Portions of the universe inside the event horizons of black holes are also unobservable parts of the universe (although, eventually, the energy in a black hole may eventually be radiated away via a process called Hawking radiation, and this radiation may contain all the information about what fell inside and so perhaps even these portions of the universe are "observable", although you have to wait a loooong time for a black hole to evaporate and then disentangle the radiation that comes out).

I know this answer wasn't very explanatory, but perhaps it will help a bit. For further reading, I suggest Scientific American articles on cosmology, but only from the last few years.

Mike W. (with a little bit from Tom).

(published on 10/22/2007)