An ambulance siren drops in pitch as it speeds away; light from galaxies rushing away stretches too, shifting redder. That redshift is how Hubble discovered the universe is expanding — the farther a galaxy, the faster it flees.
Everything is rushing away
Light from distant galaxies arrives stretched toward longer, redder wavelengths — redshifted. In the 1920s Edwin Hubble found that the farther a galaxy is, the faster it recedes: its recession velocity is proportional to distance, v = H₀d. This simple law is the fingerprint of a universe that is expanding.
Galaxies are not flying through space like shrapnel from a bomb at a center. Rather, the space between them is expanding — like raisins in rising dough, every raisin sees every other move away, with none at the center. Cosmological redshift is light stretched by that expanding space in transit.
Run the expansion backwards and everything was once compressed, hot, and dense — the origin of the Big Bang model. The Hubble constant H₀ sets the expansion rate and, roughly inverted, the age of the universe.
- Draw dots on a partly inflated balloon to represent galaxies; pick one as home.
- Inflate the balloon further and measure how far each dot moved from home.
- Notice that dots twice as far away moved roughly twice as much — that is Hubble's law.
- Explain why home is not special: repeat from any other dot and you would see the same pattern.
What you should see: You reproduced Hubble's linear velocity–distance relation and saw firsthand why expansion has no center.