REDSHIFTA field of galaxies receding as space stretches — every one dragged toward red, farther meaning faster.

Nothing is flying. Space is growing.

The galaxies are not rushing away through space. Space itself is swelling between them — and the farther one sits, the more stretched-out space lies in the gap.

Picture raisins in rising dough. No raisin moves through the loaf, yet every raisin sees all the others drift away, and the distant ones recede fastest because there is simply more dough expanding between you and them. That is the cosmos: not an explosion into space, but the swelling of space everywhere at once.

So a wave of light crossing that swelling gap gets stretched with it. A stretched light-wave is a redder light-wave. The colour you see is not the galaxy's — it is the distance the light crossed, printed onto it.

Colour became a ruler.

In 1929 Edwin Hubble noticed the rule that governs this whole field: a galaxy's recession speed is simply its distance times a constant. Draw one against the other and the points fall on a straight line.

That line is the diagram in the panel. Its slope is the Hubble constant — about 70 kilometres per second for every megaparsec of distance. Measure how far a wave has been stretched and you read the speed straight off; read the speed and the line hands you the distance. Colour became a ruler that reaches to the edge of the observable universe.

Watch one galaxy long enough and its point climbs the line: distance grows, velocity grows, its spectral line slides right toward red. When it finally crosses the horizon — receding faster than its light can close the gap — the survey lets it go and picks up the next.

Left running, the field never empties: new galaxies drift up from the centre as far ones redden out.