If the Universe is 13.8 billion years old, and the speed of light is truly our cosmic speed limit, how far away should we be able to see? The answer seems obvious: 13.8 billion light-years, since a light-year is the distance light can travel in a year, and nothing can go faster than that.
Unfortunately, like a great many answers that seem obvious when you apply your logical common sense to them, that’s not how things actually work. In reality, if you were to look at the most distant thing of all you can possibly see, and ask “how far away is it,” the answer is much farther than that: 46 billion light-years. That might sound impossible, but it’s not. You just have to expand your way of thinking.
Traditionally, the way you most often think of a distance is by taking two points and drawing a line between them. It’s something we learn to do as kids, and keep with us into adulthood. For most applications, there’s no problem in doing this, whether we use a ruler, an odometer, or a light clock: by measuring the amount of time it takes a light signal to take either a one-way or round-trip journey.
But this assumption isn’t strictly valid when it comes to the Universe. Distance isn’t necessarily defined by a straight line, nor do those distances remain the same over time. The reason for this is something we don’t think about in our day-to-day experience: space isn’t flat, and it’s also inextricably linked to time, in the form of spacetime.
The “space isn’t flat” part is perhaps easier to understand. When you think about the Earth revolving around the Sun, you probably think about it the same way that Newton did: in terms of an invisible, attractive force acting from one object (the Sun) on another (the Earth).