What determines the size of the observable Universe?

There is a boundary around us not drawn by human limitation but by physics itself — a horizon born from the marriage of time, light, and the restless expansion of space. Since the Big Bang some 13.8 billion years ago, light has been racing toward us from every direction, yet the universe it crosses has never stopped stretching, carrying distant galaxies beyond any messenger's reach. The observable universe — a sphere of roughly 46.5 billion light-years in radius — is the sum of that ancient race between light and expansion, a boundary that defines not what we have failed to see, but what reali