Galaxy RedshiftEdit
Galaxy redshift is the change in the wavelength of light from distant galaxies toward longer wavelengths, a signal that astronomers interpret as the galaxies receding from us as the universe expands. The measurement of redshift relies on spectroscopy, where spectral lines from atoms and ions are shifted in a way that can be quantified and compared across objects. The culmination of decades of observation is a coherent cosmological picture in which the expansion of space itself stretches light as it travels through the cosmos, a cornerstone of the standard model of cosmology.
Redshift, denoted by z, is determined from the observed and emitted wavelengths through the relation 1 + z = λ_observed / λ_emitted. For nearby galaxies, the redshift translates roughly into a recession velocity via v ≈ cz, where c is the speed of light, and this linear relation is known as Hubble's Law. At greater distances, the connection between redshift and distance depends on the cosmic expansion history, described by the framework of cosmology and the parameters of the prevailing model, such as the energy content of the universe and the geometry of space.
Core concepts
What redshift measures
Redshift provides a practical handle on how far light has traveled and, by extension, how the universe has changed during that travel time. It is a key observable that ties together the motion of galaxies, the expansion of space, and the large-scale structure of the cosmos. The concept sits at the intersection of spectroscopy and cosmology, enabling astronomers to build maps of galaxy distributions and to test theories about the origin and fate of the universe.
Mechanisms of redshift
Redshift can arise from several physical effects, and disentangling them is essential for interpretation. - Cosmological redshift or expansion redshift: light loses energy as space itself stretches during travel, a phenomenon central to modern cosmology and a major pillar of the mainstream view that the universe has evolved through expansion. This is often described as the cosmological redshift and is encoded in the mathematics of the evolving ΛCDM model. - Doppler redshift: motion of a galaxy through space toward or away from us changes the observed wavelength of its light, analogous to the familiar Doppler effect for sound. In practice, both the cosmological component and any peculiar velocity contribute to the measured redshift. - Gravitational redshift: light climbing out of a gravitational well loses energy and shifts toward longer wavelengths. This effect is usually small for distant galaxies but is part of the broader set of relativistic phenomena that can affect spectral lines in extreme environments.
Observational history and methods
The first broad evidence for extragalactic redshift came from spectral measurements in the early 20th century, led by pioneers such as Vesto Slipher and later synthesized by Edwin Hubble and colleagues. Since then, advances in spectroscopy and large surveys have produced redshift measurements for hundreds of thousands of galaxies, enabling three-dimensional mappings of the universe. Observational programs such as the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey have been instrumental in charting the cosmic web and refining estimates of cosmological parameters.
Implications for cosmology
Redshift is more than a distance indicator; it is a probe of cosmic history. The observed redshift-distance relationship supports a model in which the universe began in a hot, dense state and has since expanded, with components such as dark energy driving acceleration in the recent epoch. This interpretation rests on a network of independent lines of evidence, including the Cosmic Microwave Background, the distribution of galaxies, and supernova observations. Ongoing work seeks to refine the precise expansion history of the universe, test the consistency of different measurement methods, and explore the implications for the content and fate of the cosmos. Tensions in current measurements, such as the so-called Hubble tension, remain active areas of research that motivate careful examination of assumptions and systematics in both local and early-universe observations.
Observational programs and surveys
Redshift surveys collect spectra for many galaxies to produce three-dimensional maps of the sky. Notable projects include the Sloan Digital Sky Survey, the 2dF Galaxy Redshift Survey, and the 6dF Galaxy Survey. These surveys provide statistically robust vistas of how galaxies cluster on large scales, how peculiar velocities influence redshift measurements, and how the growth of structure proceeds over cosmic time.
Controversies and debates
While the cosmological interpretation of redshift is widely supported, there have historically been alternative ideas about redshift mechanisms, such as non-expansion explanations for the observed shifts. The consensus today favors expansion as the dominant driver for cosmological redshift, with other effects (Doppler and gravitational contributions) playing subsidiary roles within specific contexts. Historically, other hypotheses like tired-light scenarios have largely fallen out of favor because they struggle to account for a suite of observations, including the detailed way light curves and spectral features behave over time and the structure observed in the CMB. Ongoing debates in cosmology center on questions such as the precise value of the Hubble constant and how best to reconcile measurements from the local distance ladder with those inferred from the early universe. These discussions emphasize methodological rigor, cross-checks across independent methods, and openness to new physics should data favor it.