Spacetime and the cosmos
1922–1929
The expanding universe
Alexander Friedmann, Georges Lemaître, Edwin Hubble
Is the universe getting bigger?
Friedmann and Lemaître found that Einstein’s equations allow a universe that grows. Using Leavitt’s pulsing stars to measure distances, Hubble showed that distant galaxies are moving away, faster the further they are.
First, make a guess
Standing on a different galaxy, would you see everything moving away from you?
- From any galaxy
- speed = Hubble rate × distance
- Today’s measured rate
- about 70 km/s per megaparsec
Space itself stretches, carrying galaxies apart. From any galaxy you pick, every other galaxy moves away, and the further it is, the faster: Hubble’s law. There is no centre. (Real galaxies also have their own small motions, and nearby ones held by gravity do not spread out.)
How they figured it out
In 1917 Einstein added an extra term to his equations to keep the universe still, as everyone assumed it was. Alexander Friedmann, a Russian mathematician and meteorologist, showed in 1922 that the equations also allow a growing universe. Einstein first thought Friedmann had made a mistake, then admitted he hadn’t.
Georges Lemaître, a Belgian priest and physicist, predicted the expansion in 1927 in a journal few people read. In 1929 Edwin Hubble, using Leavitt’s pulsing stars for distances and earlier speed measurements by Vesto Slipher, showed that the further a galaxy is, the faster it recedes.
The equation
A galaxy twice as far away is moving away twice as fast.
See it in real life
Raisin bread
As raisin bread rises in the oven, every raisin moves away from every other, and raisins twice as far apart separate twice as fast. No raisin is at the centre. Police speed radar uses the same Doppler shift astronomers used to measure galaxies.
Where engineers use it
Doppler measurement
Redshift, the same effect as police speed radar, gives galaxies’ speeds.
Worked example: A galaxy 100 megaparsecs away
- km/s.
Where it came from
- General relativity November 1915
Led to
Friedmann and Lemaître found that Einstein’s equations allow a universe that grows.
- A ruler for the universe 1912
Built on
Leavitt’s pulsing stars gave Hubble his distances.
What it led to
- The afterglow of the Big Bang 1965
Led to
If the universe expanded, it was once hot and dense, and that heat should still glow.
- The expansion speeds up 1998
Led to
Measuring how the expansion changes over time revealed it is speeding up.