In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method. The effect is sometimes called the "Doppler–Fizeau effect". 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. Only later was Doppler vindicated by verified redshift observations. Before this was verified, however, it was found that stellar colors were primarily due to a star's temperature, not motion. 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. The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842. The history of the subject began with the development in the 19th century of classical wave mechanics and the exploration of phenomena associated with the Doppler effect. The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is >1 for redshifts, <1 for blueshifts). Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer). Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns. Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.Įxamples of strong redshifting are a gamma ray perceived as an X-ray, or initially visible light perceived as radio waves. Relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. The observation that all sufficiently distant light sources show redshift for a velocity proportionate to their distance from Earth is known as Hubble's law. The radiation travels through expanding space ( cosmological redshift).towards an object in less strongly curved (flatter) spacetime ( gravitational redshift) The radiation travels towards an object in a weaker gravitational potential, i.e.The radiation travels between objects which are moving apart (" relativistic" redshift, an example of the relativistic Doppler effect).In astronomy and cosmology, the three main causes of electromagnetic redshift are The terms derive from the colours red and blue which form the extremes of the visible light spectrum. The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light).
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