Red ShiftCosmological Redshift

Red Shift rides away from everything, and the light she leaves behind gets longer.

The first formula on this page is the whole idea. Light leaves a distant galaxy with one wavelength and arrives with a longer one. The ratio, 1 + z, equals how much the universe has expanded while the light was travelling: the ratio of the cosmic scale factor now to its value then. If a galaxy's light arrives with z = 1, the universe has doubled in linear size since that light set out. For distant galaxies this is not best pictured as a Doppler shift from motion through space; the wavelength is stretched in transit, along with the space it crosses.

The most stretched light anyone sees is the cosmic microwave background, released when the universe first became transparent. It has a redshift of about 1,090. It left as the glow of gas at roughly 3,000 kelvin and now arrives as microwaves at 2.7 kelvin.

Vesto Slipher measured the first galaxy redshifts in the 1910s. In 1929 Edwin Hubble showed that they grow with distance, the second formula: for nearby galaxies, recession speed is the Hubble constant times distance.

That constant is where the argument starts. The Planck satellite's fit to the microwave background gives 67.4 ± 0.5 kilometres per second per megaparsec. The SH0ES team's distance ladder of Cepheid stars and supernovae gave 73.04 ± 1.04 in 2022. The two disagree by more than their error bars allow. This is the Hubble tension, and it is open: either one method has a hidden error, or the standard model of cosmology is missing something.

So this page prints no single value for H₀. Red Shift does not wait for the answer. The longer she rides, the redder the trail.