Mission 06
How Old Is the Universe?
Every galaxy outside our own neighbourhood is moving away from us, and the further one is, the faster it goes. That single fact, measured properly, gives you the age of everything. You are going to measure it from twelve real galaxies, using the same two numbers professional astronomers use.
Read the shift
Calcium leaves a dark line at a known wavelength. In a receding galaxy it arrives stretched. The fraction it stretched by is the fraction of light speed.
Plot the twelve
Distance across, velocity up. One galaxy is a dot. Four is a hint. Twelve is a law.
Read the age
The slope says how fast the universe expands. Turn it upside down and it says how long it has been expanding.
Measure 4 more, then the slope starts to mean something.
The scatter is real. Galaxies have their own motion on top of the expansion, typically a few hundred km/s, which is why no single galaxy can give you this number and twelve of them can.