Cosmology
Redshift
Stretching of light wavelengths — measures distance and motion in an expanding universe
Redshift is the increase in wavelength of light from distant galaxies. Three causes: (1) Doppler shift — recession motion. (2) Cosmological — expansion of space. (3) Gravitational — light leaving strong gravity. Most cosmological observations: redshift z = (λ_observed - λ_rest)/λ_rest. For Hubble's law: cz = H₀ d (low z); relativistic for high z. Highest z observed: ~10-13 (early universe, ~13 Gyr ago). Quasar at z = 7.5: ~13 billion light-years away.
- Definitionz = (λ_obs - λ_rest)/λ_rest
- CausesDoppler, cosmological, gravitational
- Sun's redshift~10⁻⁶ (gravitational, very small)
- Highest observed~10-13 (JWST, Lyman break)
- Hubble's lawcz = H₀ d (low z); H₀ ~70 km/s/Mpc
- First measuredVesto Slipher (1912); Hubble (1929)
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Why redshift matters
- Distance measurement. Hubble's law connects z to d.
- Universe expansion. Direct evidence.
- Lookback time. See past events at high z.
- Galaxy evolution. Compare different epochs.
- Cosmological constraints. Test cosmological models.
- BBN. Trace primordial nucleosynthesis at high z.
- Quasar surveys. Probe early universe.
Common misconceptions
- Redshift means object moving away. Cosmological z is space expanding, not motion.
- Higher z means farther. Generally true; complications at very high z.
- Redshift is distance. Indirect; need cosmological model to get distance.
- Doppler redshift only. Cosmological important too.
- Light loses energy traveling. Energy decreases via stretching; not absorbed.
- Redshift is straightforward. Multiple effects can contribute.
Frequently asked questions
What's the difference between Doppler and cosmological redshift?
Doppler — wavelength shift from object's motion through space. Cosmological — wavelength shift from space itself expanding. Locally: indistinguishable. Cosmologically: different — cosmological z applies to galaxies that are not moving through space, just being carried by expansion. Total: combination.
How is redshift measured?
Compare absorption or emission lines from distant galaxy to laboratory wavelengths. Same lines — Hα, Mg, Fe, Lyman series, etc. Shift gives z. Resolution depends on spectrograph; modern: z accuracy 10⁻⁵. Multiple lines confirm z.
What's the Hubble's law?
v = H₀ d (for low z). Hubble's discovery (1929): galaxies recede with speed proportional to distance. H₀: Hubble constant ~70 km/s/Mpc. At larger z: not exact — full general relativity. cz = H₀ d only valid for v << c. Higher z: relativistic correction.
How does redshift relate to time?
Light from distant objects took time to reach us. Light from z = 1: ~7 Gyr ago. z = 10: ~13.4 Gyr ago. Combined with expansion, redshift is both distance and time. Looking at high-z objects = looking back in time. Universe's "lookback time" mapped from z.
What's the highest measured redshift?
~10-13 (JWST observations 2022+). Galaxies forming ~300-500 Myr after BB. Some quasars at z > 7. Earliest detected light: CMB at z ~1100 (effectively, surface of last scattering). Beyond that: opaque universe (electromagnetic invisible). Cosmic neutrino background at z ~10⁹.
What's gravitational redshift?
Light leaving strong gravity loses energy → wavelength stretches. Tested at solar surface, Pound-Rebka experiment, GPS satellites, neutron star observations. Quantitatively predicted by general relativity. Important near black holes (Sgr A* near horizon). Different cause from cosmological/Doppler redshift.
Are there blueshifts?
Yes. Galaxies approaching us → blueshift. Local Group: Milky Way and M31 approaching → M31 has slight blueshift. Most galaxies redshifted (universal expansion). Local motions can override expansion. Sun's motion through Milky Way: stars in some directions blueshifted.