The galaxies are not sitting still — the space between us and them is stretching, and it stretches their light along the way. The farther a galaxy, the faster it flees, and the redder it burns. That simple line — velocity rises with distance — is how we learned the universe expands.
Hubble's law, exactly: recession velocity v = H₀·d, with H₀ ≈ 70 km/s/Mpc (adjustable), and the redshift z = v/c for the low-z regime shown. Both are computed live and the spectral lines slide redward by Δλ/λ = z — a real, linear distance–velocity relation, the observational bedrock of cosmic expansion. Lookback time is estimated as ≈ d/c. The line is straight because expansion is (to first order) uniform.
'The galaxies flee', 'the universe expands' — the picture is real but the instrument uses the low-redshift linear approximation (z = v/c); at large z the exact relativistic/cosmological treatment departs from a straight line, and H₀ itself is contested (the Hubble tension). Redshift is cosmological stretching of space, not a literal Doppler flight — the toy Doppler framing is the figure.