Hubble's Law

In 1929 Edwin Hubble noticed that the farther a galaxy is, the faster it appears to move away — and the two are simply proportional:

where $d$ is distance, $v$ is recession speed, and $H_0$ (the Hubble constant) is today's expansion rate. This isn't galaxies flying through space but space itself stretching, so every observer sees the same outward flow. Measuring $v$ from redshift and $d$ from a standard candle gives $H_0$ — and its inverse sets the rough timescale of the expansion.

Each dot below is a galaxy. Slide $H_0$ until the line threads the data — the slope that fits is the expansion rate, and its inverse is the naive age: