What is the observable universe?

How far can we see into the universe beyond the night sky?

The “observable universe” is the region from which information, such as light, could have reached us between the beginning of the universe and now. Its limit is not determined by the capabilities of our telescopes alone.

The universe is thought to be about 13.8 billion years old. Yet the radius of the observable region, measured in present-day distance, is about 46.5 billion light-years. Its diameter is about 93 billion light-years. Totani's paper: The observable region

Why is it larger than 13.8 billion light-years?

Because space itself has continued to expand while light traveled toward us from far away. The distance when the light set out is different from the distance now. “How many years did the light take to reach us?” and “How many light-years away is that place now?” do not give the same number. NASA: The size and expansion of the universe

You may have heard that “the universe is expanding faster than light.” That needs a little more explanation, too. There is no single speed of expansion for the entire universe. Between places far enough apart, the rate at which the distance grows because of the expansion of space can exceed the speed of light.

This does not mean a spacecraft can overtake light in its immediate surroundings. Nor does it mean that every place receding faster than light is invisible to us. A paper explaining cosmic expansion and the limits of observation

There is one more important point.

The edge of what we can observe is not necessarily the edge of the universe itself. Space is thought to continue beyond it, but we do not know how large the whole universe is, or whether it is finite or infinite. NASA: The size of the whole universe

We draw a sphere centered on us not because Earth is a special center of the universe, but because we are considering the region from which information can reach us. An observer somewhere else has an observable region centered on that location.

The universe we can see is not the same as the whole universe.

This distinction matters when we think about the number of stars, or how rare life might be.