Here on Earth, in the Milky Way galaxy, there is a limit to how far out into the universe humans can observe, regardless of what technology is used. Imagine, for example, being on a ship in the middle of the ocean. If you look in all directions, all you see is water, out to a certain distance. But Earth’s surface extends far beyond that horizon limit. The farthest limit to our viewing is called the cosmic horizon. Everything within that cosmic horizon is called the observable universe. In many cases, for the sake of brevity, astronomers refer to the “observable universe” as merely the “universe.”

The size of the observable universe is determined by a combination of the age and the expansion rate of the universe. Taking both these effects into account, the current distance to the cosmic horizon can be computed at any point in time. However, since the cosmic horizon is always getting farther away, astronomers usually prefer to describe cosmic sizes using the distance that light must travel from one point to another in our expanding universe. This distance is based on the age of the universe and the speed of light. Thus, since the universe is 13.7 billion years old, we usually say that the cosmic horizon is 13.7 billion light-years away, or about eighty billion trillion miles, in every direction.