July 14, 2024
1 Solar System Way, Planet Earth, USA

How can the visible universe have a radius of 46 billion light years 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.

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

Joe Murchison
Placerville, California

Astronomers widely accept that the universe formed in the Big Bang approximately 13.8 billion years ago. Since then it has been expanding. This expansion explains how a 13.8 billion-year-old universe can be much larger than 13.8 billion light years in diameter.

First, we must explain that the speed of light limit that relativity imposes on objects within the universe does not apply to the universe itself. In fact, we cannot refer to an absolute expansion rate of the universe because we cannot measure it in reference to anything external. We can only measure the “speeds” of distant galaxies moving away from us relative to our own position. Apart from those that are gravitationally bound (such as the Milky Way and the Andromeda galaxies), all galaxies appear to move away from each other as the space-time in which they are embedded expands. The more distant the galaxy is, the faster its recession speed, as noted by Hubble's Law. As a consequence of this expansion, the location of a galaxy changes considerably during the period of time that its light requires to travel to us.

Let's put this in human terms: you're throwing a ball to a friend from a certain distance, say 50 feet (15 meters). As long as you both stay still (and aim correctly), the ball's 50-foot path will always be equal to the distance between you and your friend. But now suppose that instead of staying in place, you move back 10 feet (3 m) each time you throw the ball. In that case, when the ball reaches your friend, its distance traveled will be less than the distance that now separates the two of you.

Likewise, the light we are seeing from an object 13 billion light years away required 13 billion years to reach us, but during that period, the amount of space separating us from that object has increased. substantially. 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

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