QUESTION: 
How far does the universe extend?


ANSWER from Tom Roellig on October 31, 1995: 
We believe that the Big Bang occurred about 10 to 25 billion years 
ago.  As you can see, even this number is very uncertain. Anyway, this 
would indicate that the observable universe would be 20 to 50 
billion light-years in diameter. However, currently our best theories 
of the formation of universe postulate that there was a brief period 
right after the Big Bang, called the inflationary period, that would 
cause our observable universe to be only a very small fraction of the 
true size of the universe. In addition, under certain circumstances, 
Einstein's General Theory of Relativity would result in an observable 
universe that is finite, but also without end. (If you like, think of 
walking on a planet without any oceans. You could keep walking 
forever and ever without changing direction, but you would 
eventually get back to where you started out.) Anyway, that is our 
best answer to your question. I am sorry that it is so complicated and 
so outside our everyday experience.