How big is the universe?

Nobody can see the whole universe. Light travels fast, but not infinitely fast, and the universe is 13.8 billion years old. So the only light that has reached us is light that has had time to. Round the Earth there is a great ball of space from which light has arrived. Anything beyond it is out of sight, however good the telescope. That ball is called the observable universe.

This is its wall. In every direction, the furthest thing that can be seen is the glow left from the time when the young universe was a hot, shining fog. This real map shows that glow over the whole sky. Nothing behind it can be seen at all, because before that time light could not travel freely.

How far away is the wall? Not 13.8 billion light years. While the light was on its way, space itself was stretching, and the place the light set out from was carried further off. When the glow began its journey, its source was only about 42 million light years from here. Today that same matter is about 46 billion light years away.

So the part we can see is a ball 93 billion light years across, with the Earth at the centre. It holds hundreds of billions of galaxies, and by some counts as many as two trillion. This real photograph covers a patch of sky only a tenth as wide as the full Moon, and it holds nearly 10,000 of them.

The Earth is at the centre only because this is where we are looking from. Someone in a far galaxy has a ball of their own, the same size, centred on them. Part of theirs lies outside ours. They can see places we never shall, and we can see places hidden from them. The edge is like the horizon at sea. It is where your view stops, and the sea goes on beyond it.

How far it goes on is not known. Measurements show that space is flat, to within less than half of one per cent, and the simplest flat space has no end at all. If instead it curves gently back on itself, the curve is too slight to detect, and the whole must be very much larger than the part we see. Either way there is no sign of a middle and no sign of an edge.