How can the visible universe be 46 billion light years in radius when the universe is only 13.8 billion years old?

The expansion of the universe explains how it can be much larger than 13.8 billion light years across.

How can the visible universe be 46 billion light years in radius when the universe is only 13.8 billion years old? And how can we detect light 46 billion light years away when the universe has existed for a fraction of that time?

Joe Murchison
Placerville, California

Astronomers widely agree that the universe formed in the Big Bang approximately 13.8 billion years ago. It has since expanded. This expansion explains how a 13.8 billion year old universe can be much larger than 13.8 billion light years across.

First of all, we must explain that the light speed limit that relativity places on objects within the universe does not apply to the universe itself. In fact, we cannot refer to an absolute rate of expansion of the universe because we cannot measure it relative to anything external. We can only estimate the “velocity” of distant galaxies moving away from us relative to our position. Except for those that are gravitationally bound (such as the Milky Way and Andromeda galaxies), all galaxies appear to be moving away from each other as the space-time in which they are embedded expands. The more distant the galaxy, the faster its recession rate, as observed by Hubble’s Law. As a result of this expansion, a galaxy’s location changes significantly during the time it takes its light to travel to us.

Let’s put this in human terms: You are throwing a ball to a friend from a certain distance, say 50 feet (15 meters). Provided you both stay still (and aim properly), the 50-foot path of the ball will always equal the distance between you and your friend. But now let’s assume that instead of standing still, you are pulled back 10 feet (3 m) every time you throw the ball. In that case, by the time the ball reaches your friend, its travel distance will be less than the distance that now separates the two of you.

In the same way, the light we are seeing from an object 13 billion light years away took 13 billion years to travel from it to us, but during that time, the amount of space separating us from that object has increased significantly. Calculations show that this expansion would make the current radius of the universe about 46 billion light years.

Edward Herrick-Gleason
Planetarium Director, Southworth Planetarium,
University of Southern Maine, Portland, Maine