How can the universe be about 13.8 billion years old if the observable universe is roughly 93 billion light-years across? The number looks impossible only if a light-year is treated as a travel-time stopwatch. It is a unit of distance, while the universe has been changing during the entire journey of the light we receive.
When astronomers say that a galaxy is billions of light-years away, they are describing a relationship between distance, time and the expansion of space. The light may have been travelling for more than 13 billion years, but the galaxy that released it has not remained at the same distance while that light crossed the cosmos.
What is a light-year?
A light-year is the distance light travels in one year: about 9.46 trillion kilometres, or 5.88 trillion miles. It is not a measure of time. Saying that a galaxy is 10 light-years away means that its light takes 10 years to reach us, and that we see the galaxy as it looked 10 years ago.
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That simple rule works well for nearby stars. At cosmological distances, the space between galaxies is expanding while the light is on its way. For a closer look at what the universe expands into, see our companion explainer. The distance at the moment the light was emitted, the distance the light has travelled, and the distance to the same galaxy today are therefore different quantities.
Why the observable universe is larger than 13.8 billion light-years
The universe has been expanding for roughly 13.8 billion years. Light from the oldest visible afterglow has been travelling toward us for almost that entire history. During that journey, the space through which the light moved expanded. The source of the light is now much farther away than the simple travel-time figure of 13.8 billion light-years.
NASA explains the scale in straightforward terms: the observable universe is about 92 billion light-years across, while ESA gives a similar estimate of at least 93 billion light-years for the visible universe. The exact figure depends on the cosmological parameters used, but the basic reason is the same. The distance between the source and us grew while the light was in transit.
We are seeing ancient light, not a live view
Telescopes do not show distant galaxies as they look today. They show the light that has just arrived. A galaxy whose light has travelled for 13 billion years is being seen in a very young phase of its history. The galaxy may now be far more developed, or it may have merged with another galaxy, but information about that later history has not had time to reach us.
That is why the observable universe is also a record of cosmic time. Looking farther away means looking further into the past. The most distant light we can detect comes from an era before stars and galaxies had formed, when the universe was a hot, opaque plasma. We observe that early glow as the cosmic microwave background, not as a photograph of the Big Bang itself.
What is expanding?
On the largest scales, it is space itself that expands. Galaxies are not simply flying through a fixed empty room from one central explosion. The distances between unbound galaxies increase as the geometry of the universe changes. NASA’s James Webb Space Telescope Q&A puts it plainly: the universe has no known centre of expansion, and the Big Bang happened everywhere rather than at one location in space.
The usual raisin-bread analogy captures one part of the idea. Our guide to the Laniakea Supercluster places the Milky Way in that much larger cosmic web. As the dough rises, raisins move farther apart because the dough between them expands. No raisin is the centre of the surface. The analogy has limits—the universe is not a loaf with an outside crust—but it helps explain why every large galaxy group can see other distant groups receding.
Does the universe have an edge?
The observable universe has a horizon, but that is not the same as a wall or a physical edge. The horizon marks the greatest distance from which light has had time to reach us since the early universe. Beyond it may be more galaxies and more space; we simply cannot receive their light yet, and some regions may never become observable.
The entire universe could be much larger than the part we can see. It might be infinite, or it might be finite without having an edge in the ordinary sense, like the two-dimensional surface of a sphere has finite area but no boundary. Observations tell us a great deal about the observable region, but they do not reveal the total size of everything that exists.
Why the number is sometimes 92 and sometimes 93 billion light-years
You will see estimates of 92, 93, or about 94 billion light-years across. These are not competing claims that require one of them to be thrown away. They reflect updated measurements and slightly different choices of cosmological parameters, including the expansion rate and the contents of the universe.
Popular articles also round the number for readability. The radius of the observable universe is usually placed at roughly 46 billion light-years, which gives a diameter of about 92 to 93 billion light-years. The uncertainty is not a sign that astronomers do not know how to measure distance; it reflects the fact that the calculation combines several measurements across immense scales.
Is anything moving faster than light?
Very distant galaxies can have their distances from us increase faster than the speed of light because space between us is expanding. That does not mean a local spacecraft or signal has broken Einstein’s speed limit. Special relativity limits motion through local space; cosmic expansion changes the distance between far-separated regions of space.
There is another important distinction. Galaxies inside a gravitationally bound group, such as the Milky Way and its closest companions, do not simply expand away from one another. Gravity holds local structures together. Expansion becomes the dominant effect only across much larger, weakly bound regions. The Great Attractor is another example of how astronomers describe large-scale gravitational flows.
What does “observable” really mean?
“Observable” does not mean “the whole universe.” It means the portion from which information has been able to reach us. Every observer has a similar horizon centred on their own location, not because they occupy a privileged place, but because light has a finite speed and the universe has a finite age.
That horizon will not remain fixed. As the universe expands, some new light may eventually arrive from regions currently beyond our view. At the same time, the accelerating expansion associated with dark energy means that many galaxies we can see today will eventually become unreachable and invisible to future observers.
The short answer
The universe can be 13.8 billion years old and the observable universe about 93 billion light-years across because space expanded while ancient light was travelling toward us. A light-year measures distance, not the current separation of a galaxy. The 93-billion-light-year figure describes the visible region of the cosmos, not a hard edge and not necessarily the size of the whole universe.
The apparent contradiction disappears once distance is treated as part of an evolving universe. We are not looking at a static map. We are receiving old light from a cosmos whose scale has been changing since that light began its journey.
