cosmic horizon

Stand on a ship in the middle of the ocean and you can see only so far — the horizon, the line beyond which the curve of the Earth hides everything. The universe has a horizon too, but for a different reason: light travels at a finite speed, and the universe has existed for only a finite time, so light from beyond a certain distance simply has not had time to reach us yet. The cosmic horizon is that boundary — the edge of the part of the universe we can possibly observe.

Naively you might guess the horizon is 13.8 billion light-years away, since that is how long light has been travelling. But there is a twist: while that ancient light was in transit, space itself kept expanding, carrying the source regions much farther away. So the most distant matter we can see — the matter that emitted the cosmic microwave background — is now about 46 billion light-years from us, making the observable universe roughly 93 billion light-years across. This horizon is not a physical wall or an edge of the universe; it is simply the limit of our view, and the universe almost certainly continues far beyond it, perhaps infinitely. Every observer, anywhere, has their own horizon centred on themselves.

The cosmic horizon matters because it bounds what cosmology can ever know directly. We cannot see, even in principle, anything beyond it, which raises deep puzzles — for instance, why distant regions on opposite sides of the sky, which have never been in contact, nonetheless have the same temperature (the horizon problem that motivated inflation). A subtle and important point: because of the accelerating expansion driven by dark energy, galaxies beyond a certain distance are receding so fast that their future light will never reach us — so the observable universe is, in a sense, slowly shrinking in what new things it can show us, even as the galaxies we already see drift redder and fainter.