Working definition
Redshift is a measured ratio between observed and reference wavelengths. Its physical interpretation can include relative motion, gravity, and cosmic expansion. Converting redshift to a distance or lookback time requires a regime-appropriate model and, at cosmological scales, declared cosmological parameters.
What is measured
- Spectral features provide observed wavelengths and redshift.
- Parallax, standard candles, and standard rulers provide overlapping distance constraints.
- Galaxy surveys map redshift distributions and angular positions.
What is inferred
- Nearby distance indicators calibrate more distant ones in a ladder.
- Cosmological models relate redshift to several distinct distance measures.
- Expansion history is constrained jointly from multiple probes.
Fact-layer dependency
The explanatory layer cannot rewrite these fields.
time.utcInstantsubject.identifiersreference.framecoordinates.valuescoordinates.uncertaintyprovenance.providerWhat is observed
Spectral features provide observed wavelengths and redshift. Parallax, standard candles, and standard rulers provide overlapping distance constraints. Galaxy surveys map redshift distributions and angular positions.
Cosmological redshift is observed as a shift of spectral features toward longer wavelengths for receding cosmic sources.
Boundary: Redshift is not synonymous with recessional speed in every regime.
How inference enters
Nearby distance indicators calibrate more distant ones in a ladder. Cosmological models relate redshift to several distinct distance measures. Expansion history is constrained jointly from multiple probes.
Limits and unresolved questions
Redshift is not synonymous with recessional speed in every regime. Distance definitions differ in an expanding spacetime. The ladder carries correlated calibration systematics.
Calibration, peculiar motion, population effects, and model choice can limit the inferred expansion rate.
Boundary: The ladder carries correlated calibration systematics.
Sources
Each source states both what it establishes and where its authority ends. Access dates record the last public verification.
- [1]Tour of the Electromagnetic Spectrum · NASA Science · accessed 2026-08-15
Establishes: The wavelength and frequency domains of electromagnetic radiation and why different observing bands require different detectors and reveal different processes.
Boundary: A wavelength band does not uniquely identify a physical source; interpretation also depends on calibration, spectral shape, environment, and competing mechanisms.
- [2]What Is the Universe? · NASA Science · accessed 2026-08-15
Establishes: A broad synthesis of cosmic contents, scale, history, expansion, and the observationally inferred roles of dark matter and dark energy.
Boundary: Percentages and ages summarize model fits and observations; the physical nature of dark matter and dark energy remains unresolved.
- [3]Planck and the Cosmic Microwave Background · European Space Agency · accessed 2026-08-15
Establishes: The CMB as relic radiation, its near-blackbody spectrum, anisotropies, foregrounds, and use in constraining cosmological models.
Boundary: Cosmological parameters are conditional on a model, likelihood, foreground treatment, calibration, and combination of data sets.