Redshift

Absorption lines in the visible spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the visible spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).

In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light.

Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles. A redshift occurs whenever a light source moves away from an observer. A special instance of this is the cosmological redshift, which is due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase in their distance from Earth. Finally, gravitational redshift is a relativistic effect observed in electromagnetic radiation moving out of gravitational fields. Conversely, a decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift.

Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns.[1] Redshifts are also seen in the spectroscopic observations of astronomical objects.[2] Its value is represented by the letter z.

A special relativistic redshift formula (and its classical approximation) can be used to calculate the redshift of a nearby object when spacetime is flat. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity.[3] Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from true redshift and are not generally referred to as such (see section on physical optics and radiative transfer).

Redshift and blueshift

History

The history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842.[4] The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845.[5] Doppler correctly predicted that the phenomenon should apply to all waves, and in particular suggested that the varying colors of stars could be attributed to their motion with respect to the Earth.[6] Before this was verified, however, it was found that stellar colors were primarily due to a star's temperature, not motion. Only later was Doppler vindicated by verified redshift observations.

The first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is sometimes called the "Doppler–Fizeau effect". In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method.[7] In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red.[8] In 1887, Vogel and Scheiner discovered the annual Doppler effect, the yearly change in the Doppler shift of stars located near the ecliptic due to the orbital velocity of the Earth.[9] In 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors.[10]

The earliest occurrence of the term red-shift in print (in this hyphenated form) appears to be by American astronomer Walter S. Adams in 1908, in which he mentions "Two methods of investigating that nature of the nebular red-shift".[11] The word does not appear unhyphenated until about 1934 by Willem de Sitter, perhaps indicating that up to that point its German equivalent, Rotverschiebung, was more commonly used.[12]

Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies, then mostly thought to be spiral nebulae, had considerable redshifts. Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin.[13] Three years later, he wrote a review in the journal Popular Astronomy.[14] In it he states that "the early discovery that the great Andromeda spiral had the quite exceptional velocity of –300 km(/s) showed the means then available, capable of investigating not only the spectra of the spirals but their velocities as well."[15] Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable "positive" (that is recessional) velocities. Subsequently, Edwin Hubble discovered an approximate relationship between the redshifts of such "nebulae" and the distances to them with the formulation of his eponymous Hubble's law.[16] These observations corroborated Alexander Friedmann's 1922 work, in which he derived the Friedmann-Lemaître equations.[17] They are today considered strong evidence for an expanding universe and the Big Bang theory.[18]

Other Languages
Afrikaans: Rooiverskuiwing
العربية: انزياح أحمر
বাংলা: লোহিত সরণ
беларуская: Чырвонае зрушэнне
bosanski: Crveni pomak
čeština: Rudý posuv
Cymraeg: Rhuddiad
eesti: Punanihe
Esperanto: Ruĝenŝoviĝo
Gaeilge: Deargaistriú
한국어: 적색편이
hrvatski: Crveni pomak
Ido: Redesko
Bahasa Indonesia: Pergeseran merah
íslenska: Rauðvik
къарачай-малкъар: Къызыл тебиуню ёлчеми
ქართული: წითელი ძვრა
latviešu: Sarkanā nobīde
Lëtzebuergesch: Routverrécklung
Bahasa Melayu: Anjakan merah
မြန်မာဘာသာ: အနီရောင်အရွေ့
Nederlands: Roodverschuiving
नेपाली: रेडसिफ्ट
日本語: 赤方偏移
norsk nynorsk: Raudforskuving
Novial: Redesko
Scots: Reidshift
shqip: Redshift
Simple English: Red shift
slovenčina: Červený posun
slovenščina: Rdeči premik
српски / srpski: Црвени помак
srpskohrvatski / српскохрватски: Crveni pomak
татарча/tatarça: Qızıl taypılma
українська: Червоний зсув
Tiếng Việt: Dịch chuyển đỏ
粵語: 紅移
中文: 紅移