The observable universe is the part of the entire universe that we can, in principle, receive signals from, because light (or any information) has had enough time to reach us since the Big Bang.

Core idea

  • The universe has a finite age (about 13–14 billion years), and light travels at a finite speed, so there is a maximum distance from which light can have reached Earth by now.
  • All galaxies and radiation inside that distance form a kind of cosmic bubble around us, with Earth at the center, called the observable universe.

Not the whole universe

  • The entire universe may be much larger than, or even infinite compared to, the observable universe; most of it is forever beyond what we can see or measure.
  • “Observable” does not mean “what our telescopes are currently powerful enough to detect”; it means “what is physically able to send us light or signals within the age of the universe.”

How big is it?

  • Because space itself has been expanding while light travels, the radius of the observable universe is about 46 billion light‑years in every direction, making its diameter about 93 billion light‑years.
  • This observable region slowly grows with time as light from more distant parts of the universe finally arrives at Earth.

A simple way to picture it

  • Imagine turning on a flashlight in a dark room; over time, the light sphere grows, lighting up more space. The observable universe is like the reverse: it is the sphere of distant places whose light has had time to reach us.
  • Every observer, anywhere in the cosmos, has their own observable universe bubble centered on them, which may overlap with ours but is not identical.

Information gathered from public forums or data available on the internet and portrayed here.