Overview
In physics, a redshift denotes an increase in the wavelength (or a corresponding decrease in frequency) of electromagnetic radiation. When light from a source is observed at longer wavelengths than those emitted, the spectrum appears shifted toward the red part of the visible range, though the effect applies to all wavelengths, from radio waves to gamma rays. The converse—shortening of wavelength and rise in frequency—is called a blueshift. Redshift is not a change in the intrinsic color of the source; rather, it is a relativistic imprint left on the light as it travels through space, encoding information about the source’s velocity, the geometry of spacetime, and the large‑scale dynamics of the universe.Three principal mechanisms produce redshift. Doppler redshift arises from the relative motion of source and observer: if the source recedes, the emitted waves are stretched. Gravitational redshift occurs when photons climb out of a strong gravitational field, losing energy and lengthening their wavelength as predicted by General Relativity. Finally, cosmological redshift is a consequence of the expansion of space itself; as the universe stretches, so does the wavelength of photons traveling across intergalactic distances. Distinguishing among these causes is essential for interpreting astronomical observations, from the motion of nearby stars to the accelerating expansion of the cosmos.
Redshift is quantified by the dimensionless parameter z, defined as z = (λ_observed – λ_emitted) / λ_emitted. A redshift of z = 0.5 means the observed wavelength is 1.5 times the emitted wavelength. For small velocities (v << c), the Doppler approximation z ≈ v/c holds, linking redshift directly to recession speed. At higher z values, relativistic formulas and the cosmological scale factor become necessary, linking redshift to the age and size of the universe at the time the light was emitted.
History/Background
The concept of redshift emerged in the early 20th century alongside the development of spectroscopy and relativity. In 1901, Vesto Slipher measured the spectral lines of the Andromeda Nebula (now M31) and found them shifted toward the red, hinting at large‑scale motions beyond the Solar System. The term “redshift” itself entered the literature after Edwin Hubble’s 1929 paper, where he plotted galaxy recession velocities against distance, revealing a linear relationship now known as Hubble’s Law. This discovery cemented redshift as the primary tool for probing the expanding universe.Parallel theoretical work by Albert Einstein (1916) on gravitational redshift and Arthur Eddington (1920) on the relativistic Doppler effect provided the physical framework. The first direct measurement of gravitational redshift came in 1960 from the Pound–Rebka experiment, confirming General Relativity’s prediction. In the late 1990s, observations of distant Type Ia supernovae showed unexpectedly high redshifts, leading to the discovery of dark energy and the accelerated expansion of the universe—a paradigm shift still under active investigation.
