21 - Horizons in the universe

Summary

I am a part of all that I have met;

Yet all experience is an arch wherethro'

Gleams that untravelled world, whose margin fades

For ever and for ever when I move.

Tennyson (1809–92), Ulysses

WHAT ARE COSMOLOGICAL HORIZONS?

Horizons

We look out in space and back in time and do not see the galaxies stretching away endlessly to an infinite distance in an infinite past. Instead, we look out a finite distance and see only things within the “observable universe.” Like the sea-watching folk in Robert Frost's poem, we “cannot look out far” and “cannot look in deep.”

The observable universe is normally only a portion of the whole universe. We are at the center of our observable universe; its distant boundary acts as a cosmic horizon beyond which lie things that cannot be observed. Observers in other galaxies are located at the centers of their observable universes that are also bounded by horizons. A person on a ship far from land, who sees the sea stretching away to a horizon, is at the center of an “observable sea.” People on other ships are at the centers of their own observable seas that are bounded by horizons. Despite this analogy the horizons of the universe are not as simple as the horizons of the sea.