Redshift

Redshift
Name	Redshift
Field	Astronomy; Astrophysics; Cosmology
Introduced	19th century
Discovered by	Vesto Slipher; William Huggins
Units	Dimensionless (z); kilometers per second

Redshift

Redshift is the increase in wavelength (shift toward longer wavelengths) of electromagnetic radiation from an object, observed as a displacement of spectral features. It is a central observable in astronomy and astrophysics used to infer motion, distance, and expansion, and it underpins modern cosmology models such as the Lambda-CDM model and the interpretation of the Hubble–Lemaître law. Historical measurements by Vesto Slipher, William Huggins, and later surveys like the Sloan Digital Sky Survey established redshift as a fundamental tool across observational programs led by institutions like the Royal Observatory, Mount Wilson Observatory, and Palomar Observatory.

Introduction

Redshift is quantified by the dimensionless parameter z, defined by comparing observed and emitted wavelengths; it appears in spectra of objects ranging from stars in the Milky Way to quasars in the early universe probed by the Hubble Space Telescope and the James Webb Space Telescope. Early spectroscopic studies by Fraunhofer and analyses by Gustav Kirchhoff and Robert Bunsen set the stage for identifying spectral line displacements used by Vesto Slipher to measure galaxy motions. Modern large-scale mapping projects such as the 2dF Galaxy Redshift Survey and DEEP2 rely on redshift catalogs produced by collaborations like the European Southern Observatory and the National Optical Astronomy Observatory.

Types of Redshift

Astronomers classify redshift by mechanism: Doppler redshift describes recession or approach relative to an observer, gravitational redshift arises in strong gravitational potentials as predicted by Albert Einstein's general relativity, and cosmological redshift results from the metric expansion of space in models developed by Alexander Friedmann and Georges Lemaître. Observational categories include stellar radial-velocity redshifts measured in programs such as those at Keck Observatory and European Space Agency missions, galaxy redshifts cataloged by 2MASS and GALEX, and high-z quasar redshifts discovered by teams using Very Large Telescope instrumentation. Distinct phenomena like transverse Doppler effects were predicted by Hendrik Lorentz and tested with experiments at facilities such as CERN.

Causes and Physical Mechanisms

Doppler redshift stems from relative motion described by Christian Doppler's principle and formalized in the context of Albert Einstein's special relativity; it is applied in radial-velocity measurements of exoplanets by groups at Harvard-Smithsonian Center for Astrophysics and Geneva Observatory. Gravitational redshift is derived from general relativity and has been measured in solar spectra from the Mount Wilson Observatory and in timing signals from pulsars in binaries observed by teams using the Arecibo Observatory and the Very Long Baseline Array. Cosmological redshift is a manifestation of the expanding spacetime metric in solutions to the Einstein field equations, with key theoretical contributions by Friedmann and Lemaître and observational confirmation via the Hubble–Lemaître law from measurements by Edwin Hubble and modern instruments such as the Planck (spacecraft). Additional frequency shifts arise from interactions such as the Sunyaev–Zel'dovich effect measured by the Atacama Cosmology Telescope and Wilkinson Microwave Anisotropy Probe teams.

Measurement and Units

Redshift z is defined as (λ_observed − λ_emitted)/λ_emitted and is dimensionless, but velocity equivalents use units of kilometers per second via the non-relativistic v ≈ cz and the relativistic formula derived from special relativity. Spectroscopic methods exploit emission and absorption lines from elements first cataloged by Joseph von Fraunhofer and studied in stellar atlases from Henry Draper Catalogue efforts. Instruments such as echelle spectrographs on Keck Observatory and the Very Large Telescope deliver high-resolution measurements for projects like the HARPS exoplanet survey and the GALAH stellar archaeology program. Photometric redshift estimation uses broad-band filters in surveys like the Pan-STARRS and LSST (now Vera C. Rubin Observatory) to infer z when spectroscopy is unavailable, calibrated against spectroscopic samples from Sloan Digital Sky Survey.

Applications and Observational Evidence

Redshift underlies distance ladders used by teams analyzing Type Ia supernovae from collaborations like the Supernova Cosmology Project and the High-Z Supernova Search Team, which contributed to the discovery of cosmic acceleration and the inference of dark energy. Large-scale structure mapping via redshift surveys such as BOSS and eBOSS reveals baryon acoustic oscillation signatures predicted by James Peebles and measured with instruments including the Subaru Telescope, constraining parameters in the Lambda-CDM model and informing missions like Euclid (spacecraft). High-redshift quasars found by groups using Chandra X-ray Observatory and Spitzer Space Telescope probe reionization epochs studied by teams working with the Atacama Large Millimeter/submillimeter Array. Precision tests of gravitational redshift and time dilation use atomic clocks in experiments involving institutions like NIST and observations of pulsars cataloged by the International Pulsar Timing Array.

Implications for Cosmology

Cosmological redshift is central to interpreting the universe's dynamics: the expansion history inferred from redshift–distance relations informs estimates of the Hubble constant, a current tension between measurements by the Planck Collaboration cosmic microwave background analyses and local-distance ladders led by SH0ES and Carnegie Institution for Science. Redshift surveys map the growth of structure, constraining models of dark matter and alternative theories proposed by researchers at institutions such as Perimeter Institute and Kavli Institute. Observations of high-z objects by James Webb Space Telescope and other observatories continue to refine timelines for galaxy formation, cosmic reionization, and tests of fundamental physics, linking spectral shifts to the deepest questions pursued by contemporary astrophysical and cosmological collaborations.

Category:Astronomical spectroscopy