Redshift and the expanding universe

What redshift is

Elements emit and absorb light at fixed wavelengths, producing lines in a spectrum. In light from distant galaxies the same pattern appears shifted toward the red, spacing preserved, so the shift is measurable with precision. The redshift z puts a number on it: at z = 1 the light is stretched to twice the emitted wavelength. The most distant galaxies known sit above z = 14.

The discovery

Vesto Slipher measured redshifts of spiral nebulae in the 1910s. Edwin Hubble measured their distances using Cepheid variables calibrated by Henrietta Swan Leavitt, see the Harvard computers, and found that further galaxies recede faster. Georges Lemaître had predicted the relation from general relativity, so it is increasingly called the Hubble–Lemaître law. It is the foundation of the Big Bang model.

It is not a Doppler shift, and I would stop calling it one even in beginner explanations.

Why it is not a Doppler shift

The ambulance-siren explanation is an approximation. In the standard picture, distant galaxies are not moving through space away from us. Space between us is expanding, and light is stretched along with it. That is why recession velocities computed naively from high redshifts exceed the speed of light without breaking anything: expansion is not motion through space. Nearby, real Doppler shifts do occur, so Andromeda is blueshifted.

The Hubble tension

The expansion rate can be measured locally, with Cepheids and Type Ia supernovae, or from the cosmic microwave background projected forward. The two disagree by more than their stated uncertainties, and the gap has persisted as measurements improved. It may be a systematic error or something missing from the model. Nobody knows, and the field says so openly.

What is redshift?

The shift of light toward longer wavelengths. In distant galaxies the pattern of spectral lines appears stretched toward the red, in proportion to distance.

Is cosmological redshift a Doppler shift?

Not really. The space between us and distant galaxies is expanding, stretching the light as it travels. Naive Doppler calculations at high redshift give velocities above light speed.

What is the Hubble tension?

Local measurements of the expansion rate disagree with those from the cosmic microwave background by more than their stated uncertainties, and the gap has not closed.