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| {{Cosmology}} | |||
| '''Doppler redshift''' (or ''redshift'' for short), is a phenomenon caused by the motion of an object away from the observer due to the ], that increases the ] of ] such as ], as received by a detector and compared to its original source. It is so-named because in the ], orange and yellow light, after being subjected to a ], appears to redden as the wavelength increases (and hence the frequency decreases). The opposite shift in wavelength to shorter wavelength is called ]. | |||
| The Doppler redshift is readily observed in astronomy, where the light from celestial objects moving away from the Earth, appear to be redshifted. The degree of redshifting allows the velocity of such objects to be calculated. Other causes of ] are also known. | |||
| ==History in astronomy== | |||
| The Doppler effect as applied to all kinds of waves, is named after ] who proposed the effect in ]. The hypothesis was tested and confirmed for ] by the ] scientist ] in ]. | |||
| The first Doppler redshift was independently discovered by French physicist ] in ]. The effect is sometimes called the "Doppler-Fizeau effect". | |||
| In ], British astronomer ] was the first to determine the velocity of a star moving away from the Earth by this method {{ref|Huggins1868}}. | |||
| In ] American astronomer ] of the Lowell Observatory discovered that the redshift of spectra of spiral and some elliptical ] (galaxies) {{ref|Slipher1912}} indicated that they were moving at very high radial velocities {{ref|Slipher1912b}}, questioning their membership in the ]. He was the first person to measure galactic redshifts. | |||
| In ] ] {{ref|Hubble1929}} discovered that the redshift of light from distant galaxies is proportional to their distance, from which was formulated the ] of galaxies, nowadays known as ]. | |||
| ==Characteristics of Doppler redshift== | |||
| ] of ]s in the ] of a supercluster of distant galaxies (right), as compared to that of the Sun (left).]] | |||
| If a source of the light is moving away from an observer, then redshift (''z'' > 0) occurs; if the source moves towards the observer, then ] (''z'' < 0) occurs. This is true for all waves and is explained by the ]. Consequently, this type of redshift is also called the ''Doppler redshift''. If the source moves away from the observer with ] ''v'' and this velocity is much smaller than the ] ''c'', then the redshift is approximately given by :''z'' ≈ ''v''/''c'' However, it is important to note that this expression is only approximate, and needs modification for speeds close to the ]. (For an exact equation for the frequency shift, see the article on the ]). | |||
| Additionally, there is a special form of Doppler redshift due to ] in ] where a redshift is seen even when the source is moving at right angles to the detector. The ''transverse redshift'' was first observed in the a 1938 experiment performed by Herbert E. Ives and G.R. Stilwell, called the Ives-Stilwell experiment {{ref|Ives1938}}. | |||
| Three defining characteristics of the Doppler redshifts, are that it is | |||
| #Full-spectrum, that is, applies to all wavelengths | |||
| #Distortion free, that is, does not produce blurring or splitting of spectral lines. Some broadening of spectral lines is due to thermal effects (see ]) | |||
| #Frequency independent, that is, affects all wavelengths in a similar manner. | |||
| In this respect, the Doppler redshift shares the same characteristics as: | |||
| *the ] (also called the Hubble redshift) | |||
| *the ] (also called the Einstein Shift) | |||
| Together, these types of redshift are sometimes called Doppler-like redshifts. | |||
| ==Mathematical treatment== | |||
| Redshift (and ]) is represented by the letter ''z'', and is quantified by the equation: | |||
| <center><math>z = \frac{f_{\mathrm{emitted}} - f_{\mathrm{observed}}}{f_{\mathrm{observed}}} = \frac{\lambda_{\mathrm{observed}} - \lambda_{\mathrm{emitted}}}{\lambda_{\mathrm{emitted}}}</math></center> | |||
| where ''f'' is frequency and λ is wavelength. This quantity is ]. ''f''<sub>emitted</sub> is taken to be that as measured in the laboratory with a ] (for example, the frequency of a specific ]), and ''f''<sub>observered</sub> is the frequency of the same spectral line as observed from the target object (eg. a celestial object, or another laboratory source). | |||
| ==See also== | |||
| *] | |||
| *] as a general phenomenon | |||
| *] (also called the Hubble redshift) | |||
| *] (also called the Einstein Shift) | |||
| ==References== | |||
| {{note|doppler1842}} Christian Andreas Doppler, "Über das farbige Licht der Doppelsterne und einige andere Gestirne des Himmels" (On the colored light of the binary star and other stars) (1842) Monograph | |||
| {{note|Hubble1929}} Edwin Hubble, "" (1929) ''Proceedings of the National Academy of Sciences of the United States of America'', Volume 15, Issue 3, pp. 168-173 | |||
| {{note|Huggins1868}} William Huggins, ", Also Observations on the Spectra of the Sun and of Comet II." (1868) ''Philosophical Transactions of the Royal Society of London'', Volume 158, pp. 529-564 | |||
| {{note|Slipher1912}} Slipher, V. M., "" (1912) ''Lowell Observatory Bulletin'', vol. 1, pp.2.26-2.27 | |||
| {{note|Slipher1912b}} Slipher, V. M., "" (1913) ''Lowell Observatory Bulletin'', vol. 1, pp.2.56-2.57 | |||
| {{note|Ives1938}} Ives, Herbert E.; Stilwell, G. R., "" (1938) ''Journal of the Optical Society of America'', vol. 28, issue 7, p.215 | |||
| ] | |||
| ] | |||
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