What causes cosmological redshift?

The expansion of space stretches photon wavelengths in transit. It is not a classical Doppler effect.

When does the classical formula break down?

For z > 0.3 use the relativistic Doppler formula; for z > 1 use full cosmological models.

The cosmic microwave background has z ≈ 1100. The most distant galaxies observed have z > 10.

Estimate a galaxy's recession velocity from its cosmological redshift using the classical approximation v = z * c.

Redshift (z)

Recession Velocity

2,997.92 km/s

Hubble Distance (approx)42.83 Mpc

Recession Velocity vs Redshift (z)

For low redshifts (z << 1):

v = z × c where c = 299 792.458 km/s

Combined with Hubble's law v = H0 d gives approximate distance d = v / H0.

For z > 0.3, relativistic corrections are important.

A galaxy has redshift z = 0.01.

Estimate distance from Hubble's law.

Find how far back in time you are seeing.

Relate magnitude to distance.