Equations · equation
Cosmological Redshift
Cosmology · 1927
- Equation
- 1 + z = λ_obs/λ_emit = a(t_obs)/a(t_emit)
- Field
- Cosmology
- Named After
- Vesto Slipher · Georges Lemaître
- Year
- 1927
Symbols
z redshift · λ_emit wavelength when the light left · λ_obs wavelength when it arrives · a(t) scale factor of the universe at each moment · z = 0 is here and now
Significance
The single most-used number in observational cosmology. Light is not merely Doppler-shifted by galaxies rushing away — the wavelength is stretched by the expansion of space itself while the photon is in flight, so redshift measures how much smaller the universe was when the light set out. The microwave background left at z ≈ 1100, when everything was a thousand times more compact and 3000 K hot; the most distant galaxies JWST sees sit near z ≈ 14. Redshift is the closest thing cosmology has to a clock.
Referenced by
constant
equation
event
instrument
theory
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