Bake raisin bread and every raisin drifts away from every other as the dough puffs up — none is the centre. Galaxies do the same as space itself expands, which is how we know the universe is growing outward.
Islands of stars, all rushing apart
A galaxy is a vast gravitationally bound system of stars, gas, and dust — our Milky Way holds hundreds of billions of stars. In the 1920s Edwin Hubble found that distant galaxies' light is redshifted, and the farther away a galaxy is, the faster it recedes: the universe is expanding.
As a galaxy moves away, its light waves stretch toward longer, redder wavelengths — like a siren dropping in pitch as it passes. Greater redshift means greater speed away from us.
Running the expansion backward implies everything began from an extremely hot, dense state ~13.8 billion years ago — the Big Bang.
- A distant galaxy's spectral lines are shifted toward the red.
- Recall what a redshift indicates about its motion.
- State what a larger redshift tells you.
What you should see: Redshift means the galaxy is moving away; a larger redshift means it recedes faster — and, being farther, reflects the universe's expansion.