Ask a Physicist Answers
Prof. Brian Cox recently stated that the universe is 100 billion light years in diameter. Since the universe is no more than 14 billion years old and nothing exceeds the speed of light, shouldn't it be about 28 billion light years in diameter, not 100?
Cosmic Background Radiation — the leftover radiation from the Big Bang. Image Credit: NASA/WMAP
That's a great question! The answer is both simple and tricky: while nothing in space can travel faster than the speed of light, space itself can expand at any speed.
Usually, when scientists talk about the expanding universe, they use the example of blowing up a balloon. Imagine drawing a bunch of little galaxies on the surface of the balloon, then blowing up the balloon to expand the space between them. For our balloon example, we'll pretend that the speed of light is 1 inch per minute.
The "light" has to travel on the surface of the balloon from one galaxy to the next at that speed, no faster. But we can blow up the balloon faster than that: we can get the balloon's surface to stretch by more than one inch per minute. The same is true for space. While everything inside of space must obey the speed limit, space's expansion — just like the balloon — can go faster.
Because of the finite speed of light, we can only see a limited part of the universe. If the universe is 14 billion years old, then in a way you are correct - we can only "see" a universe of 14 billion lightyears in any direction (28 billion lightyears across). So as strange as it seems, the universe can be bigger than the portion of it we can observe!
Kelly Chipps (AKA nuclear.kelly)
Department of Physics
Colorado School of Mines
Jack from Clearwater, Fl