The observable universe: how far can we see?

The universe is about 13.8 billion years old (Planck 2018: 13.787 billion, give or take 0.020 billion). Light can only reach us from places that were close enough for it to arrive in that time. Anything beyond that is out of sight, simply because its light has not got here yet. The sphere of everything we can in principle see is the observable universe. We sit at its centre, because it is defined by what reaches us.

Is it 13.8 billion light-years in radius? Almost, but no. It is about 46.5 billion light-years in radius (so about 93 billion light-years across).

Why 46 and not 13.8? Because space expanded while the light was travelling. Picture a runner on a track that is being stretched: while they run, the finish line moves farther away. The light that has just reached us left its source long ago, when the source was much closer. In the time the light travelled, the source was carried away by the expanding space. Today the source is about 46 billion light-years from us.

Two different distances are involved:

• the light-travel distance (13.8 billion light-years for the oldest light), and
• the distance today, after the stretching (up to 46.5 billion light-years). Astronomers call this the comoving distance; we meet it again in World 2.

The matter that emitted the oldest light we can see was only about 40 million light-years away from our own matter when it did so.

The edge of sight, not the edge of space. The edge of the observable universe is like the horizon at sea. You cannot see past it, but that does not mean the ocean stops there. Someone standing on the horizon sees their own horizon, centred on them. The same holds for the universe: there is no known edge. Whether the whole universe is finite or infinite is not known; we can only say that it is at least as large as the part we see, and possibly much larger.

How many galaxies? The observable universe contains hundreds of billions to roughly two trillion galaxies. The estimates keep changing as telescopes improve, so any single number is a guess with a wide range.

The farthest things we see. There are two records.

• The most distant galaxies seen so far, found by the James Webb Space Telescope (JWST), are seen as they were less than 300 million years after the Big Bang (World 7).
• The oldest light of all is the cosmic microwave background (World 4), released about 380,000 years after the Big Bang. Before that, the universe was an opaque glowing fog, so we cannot see further back in light. That is our true horizon.