The short answer
The observable universe is a sphere centered on us, with a radius of roughly 46 billion light years and so a diameter of about 93 billion light years. These figures are rounded and depend on measured values for the universe’s age and expansion, so treat them as good estimates, not exact numbers.
That may seem puzzling, because the universe is about 13.8 billion years old. Light that has traveled for 13.8 billion years should have come from at most 13.8 billion light years away. The resolution is that space itself has been expanding the whole time.
What observable means
We see an object only if light from it has had time to reach us. The universe has a finite age, so light from very distant regions has not arrived yet. The observable universe is simply the region from which light, or other signals, could have reached us since the beginning.
Two points are easy to miss:
- It is not an edge or wall. There is nothing special at the boundary, and an observer in a distant galaxy would have their own observable universe, centered on them and partly different from ours.
- It is not the same as the whole universe. It is the part we can in principle detect.
Why it is bigger than 13.8 billion light years
Imagine ants on a stretching rubber band. An ant moving along the band covers some distance, but the band under it is also growing, so the starting point ends up much farther away than the distance the ant walked. Something similar happens to light crossing expanding space.
When we see the most distant light today, the matter that emitted it was much closer to us at that time. Over the billions of years the light traveled, that matter was carried away by expansion. Its present-day distance is now about 46 billion light years. This is a calculated “current” distance, not something we measure directly, which is why published numbers rely on a cosmological model.
The oldest light we can see
The farthest signal we detect is not from a galaxy but from the cosmic microwave background, a faint glow that dates from when the universe was young, roughly 380,000 years old. Before that, the universe was too hot and dense for light to travel freely. This background radiation is one of the key observations supporting the Big Bang picture.
Beyond that, no light can reach us from earlier times, although other types of signal might someday, such as gravitational waves. The farthest individual galaxies detected are seen as they were when the universe was only a few hundred million years old, and observations from space telescopes continue to push this limit.
How we get the distances
Distances at this scale are not measured with a ruler. Astronomers build a ladder of methods, from nearby parallax to using the brightness of stars and exploding stars as “standard candles”, and finally using the redshift of light, the stretching of its wavelengths as space expands. This ladder of cosmic distances carries some uncertainty on every rung. The final numbers also depend on a model of the universe that includes dark matter and a still poorly understood expansion-driving component.
Can the observable region grow?
As time goes on, light from farther regions keeps arriving, so the observable universe grows. However, measurements indicate that the expansion is accelerating. If that continues, some galaxies will eventually recede so fast that their light never reaches us, and they will drop out of view. This outcome depends on assumptions about the future, so it is a projection and not a certainty.
Faster-than-light separation of distant regions does not contradict the speed of light being a limit. That limit applies to motion through space; the expansion of space itself is a different matter.
What lies beyond
Nobody knows how large the whole universe is. Observations of its geometry are consistent with space being very nearly flat on large scales. A flat universe could be infinite, but a finite universe that is simply much larger than our observable portion would also fit. Measurements can only place limits: if the universe curves back on itself, the curvature is too small to detect so far.
Most cosmologists expect that the region beyond our horizon looks broadly similar to what we see, with galaxies and stars throughout. This is a reasonable expectation based on how uniform the observable part is, but it cannot be directly checked.
Why the number is worth knowing
The size matters less as a record than as a reminder of how our knowledge is limited by light travel time. A single galaxy such as the Milky Way is about 100,000 light years across, and the observable universe is hundreds of thousands of times larger than that. Within it, estimates of the number of galaxies run to at least hundreds of billions, with some estimates higher. Each figure is a rounded estimate that may change as telescopes improve.