What is the relationship between the observable universe and the entire universe?
Because light travels at a finite speed and we observe distant objects by means of the light they emit, we can only see objects whose emitted light had time to travel to us since the beginning of the universe. Imagine a postal service that just started up on January 1 that delivers letters over a distance of 100 miles each day. If your friends around the world starting mailing you news as soon as the post office opened, by January 10 your worldview would be everything within 1000 miles. The whole world, however, is much larger. It is the same thing with light in the universe. We can only see out to about 10 billion lightyears distant, beyond this might be a whole part of the universe from which we can get no news. As time goes on we will be able to see from a larger and larger part of the universe, though of course the news will be over 10 billion years old, just like the letters you receive do not tell you how your friends are now, only how they were when they wrote the letters. If th
