The Cosmic Horizon

The observable universe has a boundary, but it is not a wall — it is a horizon of time. Light travels at a finite speed, so the farther you look, the older the light. The most distant light we can detect is the Cosmic Microwave Background, emitted when the universe was about 380,000 years old.

The observable universe has a radius of about 46.5 billion light-years — even though the universe is only 13.8 billion years old. This gap exists because space itself has been expanding the whole time, carrying ancient light sources ever farther away.