How far can we see? The observable universe explained
13.8 billion years old, 46 billion light years across. Both are correct.
4 min read · Published 27 August 2026
The short answer
The observable universe extends about 46 billion light years in every direction, despite the universe being 13.8 billion years old. There is no contradiction: space has expanded while light was travelling, so objects whose light has taken 13.8 billion years to reach us are now much further away. The boundary is a limit on what we can see, not a physical edge.
Why 46 and not 13.8
The naive expectation is that in a 13.8-billion-year-old universe we could see 13.8 billion light years. That would hold in static space.
Space is expanding. Light emitted by a distant galaxy 13 billion years ago has been travelling through space that stretched during the journey. The galaxy that emitted it has been carried further away the whole time.
So we receive light from an object that was relatively close when it emitted, and is now roughly 46 billion light years away. The light travelled for 13.8 billion years; the distance covered grew underneath it.
None of this involves anything moving faster than light through space. Space itself expanding is not motion in the relevant sense, and relativity places no limit on it.
What sets the boundary
The limit is the distance light could have travelled since the universe became transparent, roughly 380,000 years after the beginning. Before that the universe was an opaque plasma and light could not travel freely.
So the earliest thing observable in light is the cosmic microwave background — the surface where the universe became transparent. It is the wall we cannot see past, not because of distance but because of opacity.
Neutrinos and gravitational waves could in principle carry information from earlier, since both decoupled sooner. Detecting a primordial background of either would let us see past that wall, and is a long-term goal.
It is a horizon, not an edge
The observable universe is centred on the observer, and every observer has their own. A galaxy 40 billion light years away has its own observable universe centred on itself, overlapping ours and extending well beyond it.
The universe beyond our horizon is not different or absent. It is simply outside the range from which light has had time to reach us.
How much more there is is unknown. The universe may be infinite. Measurements indicate space is flat to within observational precision, which is consistent with infinite extent — but a sufficiently large finite universe would also look flat locally, and the two cannot be distinguished from inside.
The horizon is shrinking
Because expansion is accelerating, distant galaxies are receding ever faster. Beyond a certain distance, the space between us and them expands faster than light can cross it, and their light will never arrive.
Galaxies currently visible will progressively redshift and fade beyond detection. Over very long timescales, everything outside our local gravitationally bound group will disappear from view.
The consequence is that observers in the far future would see only their own galaxy, with no evidence of cosmic expansion, no cosmic microwave background, and no way to infer the Big Bang. The observations that let us reconstruct cosmic history are available during a window, and we are inside it.
That is a genuinely unsettling thought, and it is one of the more striking things modern cosmology has established.
Common questions
- How big is the observable universe?
- About 46 billion light years in radius, or roughly 93 billion light years across. This exceeds the 13.8-billion-year age because space expanded while light was in transit, carrying the emitting objects further away.
- How can we see further than the universe is old?
- Because space itself has expanded during the light's journey. An object whose light took 13.8 billion years to reach us has been carried to a present distance of around 46 billion light years. Nothing moved faster than light through space.
- What is beyond the observable universe?
- Presumably more universe, simply too far for light to have reached us yet. It is a horizon rather than an edge, and every observer has their own. Whether the universe is finite or infinite is not determinable from inside.
- Is the observable universe getting smaller?
- In effect, yes. Accelerating expansion means distant galaxies will eventually recede faster than their light can cross the intervening space, progressively fading from view. Far-future observers would see only their own galaxy.