What do you think that astronomers mean when they use the term observable universe? (hint: think of the time it takes for light from very distant objects to reach earth.)

What does "observable universe" mean?

Light does not travel instantaneously between points in space. It has a finite speed "c", measured experimentally to be about 3 x 108 meters/second (or about 1.1 x 109 kilometers/hour. Flying at this rate you could get from NYC to Tokyo in about 1/30th of a second.)

Since light takes time to travel, we never actually see the current moment. Looking down at your hand, you do not see it as it is right now, but rather as it was a miniscule fraction of a moment earlier. Now, this interval is so small, given the short distance between your retina and your hand, that the difference is utterly negligible. In fact, bound by Earth's meager scope, the phenomenon isn't really worth mentioning.

The discrepancy becomes significant, however, when exploring much larger distances. Light years, for example. A light year is the distance light travels in one year. If you look at a star that's 50 light years away, you are seeing it as it was 50 years ago. Thus the deeper you peer into space, the farther you are seeing back in time. If this star had exploded 49 years ago, in a spectacular event called a Supernova, we would not know it until 1 more year from now.

Likewise, any event that happened beyond a certain point in the past is unknowable to us if the signal from it hasn't had time to reach us. It is not that our telescopes are too weak, or our instrumentation insensitive. We simply do not yet have access to the information. (No matter how prolific a reader you may be, you'd be hard pressed to read a friend's email if it has yet to arrive in your inbox.)

As a consequence of this limitation, astronomers often refer to the observable universe, a term referring to the volume of space that we are physically able to detect. The question of what lies outside this observable region is a tempting one to ponder. Yet inspiring though it may be, there is a certain futility in such a pursuit.