One of the things that I have always wondered was how the universe could be so big. I mean, it is about 13.8-year-olds, so why is it that we can observe the universe at about 93 billion light years across? I mean, shouldn’t we only be able to observe about 27.6 billion light years in total?
When you think about it, nothing can travel faster than light, and light travels at a fixed speed. With the universe existing for about 13.8 billion years, the furthest light we should be able to see is around 13.8 billion years in one direction. When you double that for both directions, you get 27.6 billion light-years across. While that seems logical, why is that not what we observe?
To answer this question, we must take a look at cosmic expansion. This is where things discontinue acting like the way we expect. Not only are galaxies moving through space, but space itself is expanding.
Let’s say a galaxy is very far away, and about 13 billion years ago, that galaxy gave off light. The light started to travel toward us. Here is the part most people miss: while that light was traveling, the universe itself was expanding. Imagine that light is moving toward us, but the space between us and the galaxy is expanding. What this means is that the distance the light has traveled is getting longer while it’s traveling.
So the galaxy that emitted the light is no longer 13 billion light years away; it might now be 45 or even more billion light years away. That particularly explains how galaxies can be further away than 13 billion light-years. However, now you have the problem of things moving faster than the speed of light. Isn’t it impossible for something to move faster than light?
While it is true that nothing in space can move faster than light, it’s also true that space itself can expand at any speed. Those galaxies you see are not going through space faster than light. Rather, they’re being carried away because space itself is stretching and some of them are receding from us faster than the speed of light.
That means that there are parts of the universe that we can see that are moving away from us faster than light. The only reason we can see them is that the universe has been expanding the entire time their light was traveling.
It’s interesting to think that there are galaxies out there currently whose light will never reach us. This is not because it hasn’t had enough time, but because space is expanding too fast. This means that not only is the universe big, but it’s growing in a way that breaks our everyday intuition.
So, space itself can expand at a rate faster than light. The universe that we currently see isn’t the universe as it is now. It’s a snapshot of the past; stretched, expanded, and still moving away from you.
Further Reading (Affiliate Links)
The First Three Minutes by Steven Weinberg
The Fabric of the Cosmos by Brian Greene
Welcome to the Universe by Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott
Cosmos by Carl Sagan
A Brief History of Time by Stephen Hawking
Something Deeply Hidden by Sean Carroll
Introduction to Cosmology – Barbara Ryden