Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).

Redshift and blue shift

In physics and astronomy, redshift occurs when the electromagnetic radiation, usually visible light, that is emitted from or reflected off an object is shifted towards the (less energetic) red end of the electromagnetic spectrum. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared with the wavelength emitted by the source. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. In Physics, emission is the process by which the Energy of a Photon is released by another entity for example by an Atom whose Electrons This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. Frequency is a measure of the number of occurrences of a repeating event per unit Time. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Conversely, a decrease in wavelength is called blue shift. Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that

Any increase in wavelength is called "redshift", even if it occurs in electromagnetic radiation of non-optical wavelengths, such as gamma rays, x-rays and ultraviolet. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays This nomenclature might be confusing since, at wavelengths longer than red (e. g. , infrared, microwaves, and radio waves), redshifts shift the radiation away from the red wavelengths. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum.

An observed redshift due to the Doppler effect occurs whenever a light source moves away from the observer, corresponding to the Doppler shift that changes the perceived frequency of sound waves. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies Although observing such redshifts, or complementary blue shifts, has several terrestrial applications (e. g. , Doppler radar and radar guns),^[1] spectroscopic astrophysics uses Doppler redshifts to determine the movement of distant astronomical objects. A doppler radar is a Radar using the Doppler effect of the returned echoes from targets to measure their Radial velocity. A radar gun or speed gun is a small Doppler radar used to detect the speed of objects Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy. ^[2] This phenomenon was first predicted and observed in the 19th century as scientists began to consider the dynamical implications of the wave-nature of light. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700

Another cause of redshift is the expansion of the universe, which explains the observation that the redshifts of distant galaxies, quasars, and intergalactic gas clouds increase in proportion to their distance from the earth. The metric expansion of space is the averaged increase of metric (i A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. Intergalactic space is the physical space between galaxies. Generally free of dust and debris intergalactic space is very close to a total Vacuum. This article is about proportionality the mathematical relation This mechanism is a key feature of the Big Bang model of physical cosmology. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its ^[3]

Gravitational redshift is observed if the receiver is located at higher gravitational potential than the source. In Physics, Light or other forms of Electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational Potential energy can be thought of as Energy stored within a physical system The cause of gravitational redshift is the time dilation that occurs near massive objects, according to general relativity^[4]

All three of these phenomena, whose wide range of instantiations are the focus of this article, can be understood under the umbrella of frame transformation laws, as described below. This article discusses a concept in physics For the concept in sociology see Time displacement. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 There exist numerous other mechanisms with different physical and mathematical descriptions that can lead to a shift in the frequency of electromagnetic radiation and whose action is generally not referred to as a "redshift", including scattering and optical effects (for more see section on physical optics and radiative transfer). Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from In Physics, physical optics, or wave optics, is the branch of Optics which studies Interference, Diffraction, Polarization

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. In Physics, especially Quantum mechanics, the Schrödinger equation is an equation that describes how the Quantum state of a Physical system The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for The effect is named after Christian Andreas Doppler, who offered the first known physical explanation for the phenomenon in 1842. Christian Andreas Doppler ( November 29, 1803 &ndash March 17, 1853) was an Austrian Mathematician and Physicist ^[5] The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies The Netherlands ( Dutch:, ˈnedərlɑnt is the European part of the Kingdom of the Netherlands, which consists of the Netherlands the Netherlands ^[6] 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. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth ^[7] While this attribution turned out to be incorrect (stellar colors are indicators of a star's temperature, not motion), Doppler would later be vindicated by verified redshift observations. Color temperature is a characteristic of Visible light that has important applications in lighting photography videography publishing and other fields

The first Doppler redshift was described in 1848 by French physicist Armand-Hippolyte-Louis Fizeau, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. Armand Hippolyte Louis Fizeau (1819-1896 French physicist, was born in Paris. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared 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. Sir William Huggins, OM, FRS ( February 7 1824 &ndash May 12 1910) was an English Astronomer ^[8]

In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0. In Physics and Optics, the Fraunhofer lines are a set of Spectral lines named for the German physicist Joseph von Fraunhofer ( 1787 1 Å in the red. ^[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, where he mentions "Two methods of investigating that nature of the nebular red-shift". Walter Sydney Adams ( December 20 1876 &ndash May 11 1956) was an American Astronomer. ^[11] The word doesn't appear unhyphenated, perhaps indicating a more common usage of its German equivalent, Rotverschiebung, until about 1934 by Willem de Sitter. Willem de Sitter ( May 6 1872 &ndash November 20 1934) was a Dutch mathematician physicist and astronomer ^[12]

Beginning with observations in 1912, Vesto Slipher discovered that most spiral nebulae had considerable redshifts. Vesto Melvin Slipher ( November 11, 1875 &ndash November 8, 1969) was an American Astronomer. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the ^[13] Subsequently, Edwin Hubble discovered an approximate relationship between the redshift of such "nebulae" (now known to be galaxies in their own right) and the distance to them with the formulation of his eponymous Hubble's law. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Distance is a numerical description of how far apart objects are Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance ^[14] These observations corroborated Alexander Friedman's 1922 work, in which he derived the famous Friedmann equations. Alexander Alexandrovich Friedman or Friedmann (Александр Александрович Фридман ( June 16 1888, Saint Petersburg, Imperial The Friedmann equations are a set of Equations in cosmology that govern the expansion of space in homogeneous and isotropic models ^[15] They are today considered strong evidence for an expanding universe and the Big Bang theory. The metric expansion of space is the averaged increase of metric (i The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. ^[16]

Measurement, characterization, and interpretation

The spectrum of light that comes from a single source (see idealized spectrum illustration top-right) can be measured. To determine the redshift, features in the spectrum such as absorption lines, emission lines, or other variations in light intensity, are searched for. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared If found, these features can be compared with known features in the spectrum of various chemical compounds found in experiments where that compound is located on earth. A very common atomic element in space is hydrogen. A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 The spectrum of originally featureless light shined through hydrogen will show a signature spectrum specific to hydrogen that has features at regular intervals. A hydrogen atom is an atom of the chemical element Hydrogen. The electrically neutral If restricted to absorption lines it would look similar to the illustration (top right). If the same pattern of intervals is seen in an observed spectrum from a distant source but occurring at shifted wavelengths, it can be identified as hydrogen too. If the same spectral line is identified in both spectra but at different wavelengths then the redshift can be calculated using the table below. Determining the redshift of an object in this way requires a frequency- or wavelength-range. In order to calculate the redshift one has to know the wavelength of the emitted light in the rest frame of the source, in other words, the wavelength that would be measured by an observer located adjacent to and comoving with the source. Since in astronomical applications this measurement cannot be done directly, because that would require travelling to the distant star of interest, the method using spectral lines described here is used instead. Redshifts cannot be calculated by looking at unidentified features whose rest-frame frequency is unknown, or with a spectrum that is featureless or white noise (random fluctuations in a spectrum). White noise is a random signal (or process with a flat Power spectral density. ^[17]

Redshift (and blue shift) may be characterized by the relative difference between the observed and emitted wavelengths (or frequency) of an object. In astronomy, it is customary to refer to this change using a dimensionless quantity called z. In Dimensional analysis, a dimensionless quantity (or more precisely a quantity with the dimensions of 1) is a Quantity without any Physical units If λ represents wavelength and f represents frequency (note, λf = c where c is the speed of light), then z is defined by the equations:

Calculation of redshift, z

Based on wavelength	Based on frequency

After z is measured, the distinction between redshift and blue shift is simply a matter of whether z is positive or negative. See the mechanisms section below for some basic interpretations that follow when either a redshift or blue shift is observed. For example, Doppler effect blue shifts (z < 0) are associated with objects approaching (moving closer to) the observer with the light shifting to greater energies. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Conversely, Doppler effect redshifts (z > 0) are associated with objects receding (moving away) from the observer with the light shifting to lower energies. Likewise, gravitational blue shifts are associated with light emitted from a source residing within a weaker gravitational field observed within a stronger gravitational field, while gravitational redshifting implies the opposite conditions. A gravitational field is a model used within Physics to explain how gravity exists in the universe A gravitational field is a model used within Physics to explain how gravity exists in the universe

Mechanisms

A single photon propagated through a vacuum can redshift in several distinct ways. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. Each of these mechanisms produces a Doppler-like redshift, meaning that z is independent of wavelength. These mechanisms are described with Galilean, Lorentz, or general relativistic transformations between one frame of reference and another. The Galilean transformation is used to transform between the coordinates of two Reference frames which differ only by constant relative motion within the constructs of Newtonian In Physics, the Lorentz transformation converts between two different observers' measurements of space and time where one observer is in constant motion with respect to General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 See also Inertial frame A frame of reference in Physics, may refer to a Coordinate system or set of axes within which to ^[2]

Doppler effect, yellow ball appears greenish (blueshift) approaching observer, turns orange (redshift) as it passes, and returns to yellow when motion stops.

Redshift Summary

Redshift type	Frame transformation law	Example of a metric[18]	Definition[19]
Doppler redshift	Galilean transformation	Euclidean metric
Relativistic Doppler	Lorentz transformation	Minkowski metric
Cosmological redshift	General relativistic tr.	FRW metric
Gravitational redshift	General relativistic tr.	Schwarzschild metric

Doppler effect

Main article: Doppler effect

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 blue shift (z < 0) occurs. The Galilean transformation is used to transform between the coordinates of two Reference frames which differ only by constant relative motion within the constructs of Newtonian In Mathematics, the Euclidean distance or Euclidean metric is the "ordinary" Distance between two points that one would measure with a ruler In Physics, the Lorentz transformation converts between two different observers' measurements of space and time where one observer is in constant motion with respect to In Physics and Mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einstein's theory of Special relativity General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In Einstein's theory of General relativity, the Schwarzschild solution (or the Schwarzschild vacuum) describes the Gravitational field outside The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that This is true for all electromagnetic waves and is explained by the Doppler effect. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Consequently, this type of redshift is called the Doppler redshift. If the source moves away from the observer with velocity v, then, ignoring relativistic effects, the redshift is given by

    (Since , see below)

where c is the speed of light. In Physics, velocity is defined as the rate of change of Position. In the classical Doppler effect, the frequency of the source is not modified, but the recessional motion causes the illusion of a lower frequency.

Relativistic Doppler effect

Main article: Relativistic Doppler effect

A more complete treatment of the Doppler redshift requires considering relativistic effects associated with motion of sources close to the speed of light. The relativistic Doppler effect is the change in Frequency (and Wavelength) of Light, caused by the relative motion of the source and the observer (as in A complete derivation of the effect can be found in the article on the relativistic Doppler effect. The relativistic Doppler effect is the change in Frequency (and Wavelength) of Light, caused by the relative motion of the source and the observer (as in In brief, objects moving close to the speed of light will experience deviations from the above formula due to the time dilation of special relativity which can be corrected for by introducing the Lorentz factor γ into the classical Doppler formula as follows:

This phenomenon was first observed in a 1938 experiment performed by Herbert E. This article discusses a concept in physics For the concept in sociology see Time displacement. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial The Lorentz factor or Lorentz term appears in several equations in Special relativity, including Time dilation, Length contraction, and the Ives and G. R. Stilwell, called the Ives-Stilwell experiment. The Ives–Stilwell experiment exploits the Transverse Doppler effect (TDE described by Albert Einstein in his 1905 paper ^[20]

Since the Lorentz factor is dependent only on the magnitude of the velocity, this causes the redshift associated with the relativistic correction to be independent of the orientation of the source movement. The magnitude of a mathematical object is its size a property by which it can be larger or smaller than other objects of the same kind in technical terms an Ordering In contrast, the classical part of the formula is dependent on the projection of the movement of the source into the line of sight which yields different results for different orientations. The scalar resolute, also known as the scalar projection or scalar component, of a vector \mathbf{b} in the direction of a vector \mathbf{a} Consequently, for an object moving at an angle θ to the observer (zero angle is directly away from the observer), the full form for the relativistic Doppler effect becomes:

and for motion solely in the line of sight (θ = 0°), this equation reduces to:

For the special case that the source is moving at right angles (θ = 90°) to the detector, the relativistic redshift is known as the transverse redshift, and a redshift:

is measured, even though the object is not moving away from the observer. In Geometry and Trigonometry, a right angle is an angle of 90 degrees corresponding to a quarter turn (that is a quarter of a full circle In Special relativity, the transverse Doppler effect is the nominal Redshift component associated with Transverse (i Even if the source is moving towards the observer, if there is a transverse component to the motion then there is some speed at which the dilation just cancels the expected blue shift and at higher speed the approaching source will be redshifted. ^[21]

Expansion of space

Main article: Metric expansion of space

In the early part of the twentieth century, Slipher, Hubble and others made the first measurements of the redshifts and blue shifts of galaxies beyond the Milky Way. The metric expansion of space is the averaged increase of metric (i The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply They initially interpreted these redshifts and blue shifts as due solely to the Doppler effect, but later Hubble discovered a rough correlation between the increasing redshifts and the increasing distance of galaxies. Theorists almost immediately realized that these observations could be explained by a different mechanism for producing redshifts. Hubble's law of the correlation between redshifts and distances is required by models of cosmology derived from general relativity that have a metric expansion of space. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance The metric expansion of space is the averaged increase of metric (i ^[16] As a result, photons propagating through the expanding space are stretched, creating the cosmological redshift. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance This differs from the Doppler effect redshifts described above because the velocity boost (i. e. the Lorentz transformation) between the source and observer is not due to classical momentum and energy transfer, but instead the photons increase in wavelength and redshift as the space through which they are traveling expands. In Physics, the Lorentz transformation converts between two different observers' measurements of space and time where one observer is in constant motion with respect to In Classical mechanics, momentum ( pl momenta SI unit kg · m/s, or equivalently N · s) is the product In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός ^[22] This effect is prescribed by the current cosmological model as an observable manifestation of the time-dependent cosmic scale factor (a) in the following way:

This type of redshift is called the cosmological redshift or Hubble redshift. The scale factor, parameter of Friedmann-Lemaître-Robertson-Walker model is a function of time which represents the relative expansion of the Universe Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance If the universe were contracting instead of expanding, we would see distant galaxies blue shifted by an amount proportional to their distance instead of redshifted. ^[23]

These galaxies are not receding simply by means of a physical velocity in the direction away from the observer; instead, the intervening space is stretching, which accounts for the large-scale isotropy of the effect demanded by the cosmological principle. The cosmological principle is an assumption invoked in Cosmology that when applied severely restricts the large variety of possible cosmological theories ^[24] For cosmological redshifts of z < 0. 1 the effects of spacetime expansion are minimal and observed redshifts dominated by the peculiar motions of the galaxies relative to one another that cause additional Doppler redshifts and blue shifts. SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS ^[25] The difference between physical velocity and space expansion can be illustrated by the Expanding Rubber Sheet Universe, a common cosmological analogy used to describe the expansion of space. The metric expansion of space is the averaged increase of metric (i If two objects are represented by ball bearings and spacetime by a stretching rubber sheet, the Doppler effect is caused by rolling the balls across the sheet to create peculiar motion. The cosmological redshift occurs when the ball bearings are stuck to the sheet and the sheet is stretched. (Obviously, there are dimensional problems with the model, as the ball bearings should be in the sheet, and cosmological redshift produces higher velocities than Doppler does if the distance between two objects is large enough. )

In spite of the distinction between redshifts caused by the velocity of objects and the redshifts associated with the expanding universe, astronomers sometimes refer to "recession velocity" in the context of the redshifting of distant galaxies from the expansion of the Universe, even though it is only an apparent recession. ^[26] As a consequence, popular literature often uses the expression "Doppler redshift" instead of "cosmological redshift" to describe the motion of galaxies dominated by the expansion of spacetime, despite the fact that a "cosmological recessional speed" when calculated will not equal the velocity in the relativistic Doppler equation. ^[27] In particular, Doppler redshift is bound by special relativity; thus v > c is impossible while, in contrast, v > c is possible for cosmological redshift because the space which separates the objects (e. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial g. , a quasar from the Earth) can expand faster than the speed of light. ^[28] More mathematically, the viewpoint that "distant galaxies are receding" and the viewpoint that "the space between galaxies is expanding" are related by changing coordinate systems. In Mathematics and its applications a coordinate system is a system for assigning an n - Tuple of Numbers or scalars to each point Expressing this precisely requires working with the mathematics of the Friedmann-Robertson-Walker metric. ^[29]

Gravitational redshift

Main article: Gravitational redshift

A graphical representation of the gravitational redshift due to a neutron star

In the theory of general relativity, there is time dilation within a gravitational well. In Physics, Light or other forms of Electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational In Physics, Light or other forms of Electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 This is known as the gravitational redshift or Einstein Shift. In Physics, Light or other forms of Electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational ^[30] The theoretical derivation of this effect follows from the Schwarzschild solution of the Einstein equations which yields the following formula for redshift associated with a photon traveling in the gravitational field of an uncharged, nonrotating, spherically symmetric mass:

where

• is the gravitational constant,
• is the mass of the object creating the gravitational field,
• is the radial coordinate of the observer (which is analogous to the classical distance from the center of the object, but is actually a Schwarzschild coordinate), and
• is the speed of light. In Einstein's theory of General relativity, the Schwarzschild solution (or the Schwarzschild vacuum) describes the Gravitational field outside The Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the A gravitational field is a model used within Physics to explain how gravity exists in the universe Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation This article is about rotations in three-dimensional Euclidean space The gravitational constant, denoted G, is a Physical constant involved in the calculation of the gravitational attraction between objects with mass Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In the theory of Lorentzian manifolds Spherically symmetric spacetimes admit a family of nested round spheres.

This gravitational redshift result can be derived from the assumptions of special relativity and the equivalence principle; the full theory of general relativity is not required. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial The equivalence principle ^[31]

The effect is very small but measurable on Earth using the Mössbauer effect and was first observed in the Pound-Rebka experiment. The Mössbauer effect (Mößbauer) a physical phenomenon discovered by Rudolf Mößbauer in 1957, refers to the resonant and recoil-free emission and absorption The Pound-Rebka experiment is a well known experiment to test Albert Einstein 's theory of General relativity. ^[32] However, it is significant near a black hole, and as an object approaches the event horizon the red shift becomes infinite. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot It is also the dominant cause of large angular-scale temperature fluctuations in the cosmic microwave background radiation (see Sachs-Wolfe effect). The Sachs-Wolfe effect, named after Rainer Kurt Sachs and Arthur Michael Wolfe, is a property of the Cosmic microwave background radiation (CMB in which ^[33]

Observations in astronomy

The redshift observed in astronomy can be measured because the emission and absorption spectra for atoms are distinctive and well known, calibrated from spectroscopic experiments in laboratories on Earth. An element's 'emission spectrum' is the relative intensity of Electromagnetic radiation of each Frequency it emits when it is Heated (or more generally when A material's absorption spectrum shows the fraction of incident Electromagnetic radiation absorbed by the material over a range of Frequencies. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ A laboratory (informally lab) is a facility that provides controlled conditions in which scientific Research, Experiments and When the redshift of various absorption and emission lines from a single astronomical object is measured, z is found to be remarkably constant. Although distant objects may be slightly blurred and lines broadened, it is by no more than can be explained by thermal or mechanical motion of the source. Thermal motion is the random motion of Molecules or other small objects that results from their being in thermal equilibrium at a particular Temperature. In Physics, motion means a constant change in the location of a body For these reasons and others, the consensus among astronomers is that the redshifts they observe are due to some combination of the three established forms of Doppler-like redshifts. Alternative hypotheses are not generally considered plausible. ^[34]

Spectroscopy, as a measurement, is considerably more difficult than simple photometry, which measures the brightness of astronomical objects through certain filters. Photometry is a technique of Astronomy concerned with measuring the Flux, or intensity of an Astronomical object 's Electromagnetic Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light. An optical filter is a device which selectively transmits light having certain properties (often a particular range of Wavelengths that is range of Colours ^[35] When photometric data is all that is available (for example, the Hubble Deep Field and the Hubble Ultra Deep Field), astronomers rely on a technique for measuring photometric redshifts. The Hubble Deep Field (HDF is an image of a small region in the Constellation Ursa Major, based on the results of a series of observations by the Hubble The Hubble Ultra Deep Field, or HUDF, is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope A photometric redshift is an estimate for the distance of an astronomical object such as a Galaxy or Quasar. ^[36] Due to the filter being sensitive to a range of wavelengths and the technique relying on making many assumptions about the nature of the spectrum at the light-source, errors for these sorts of measurements can range up to δz = 0. Observational error is the difference between a measured value of quantity and its true value 5, and are much less reliable than spectroscopic determinations. ^[37] However, photometry does allow at least for a qualitative characterization of a redshift. For example, if a sun-like spectrum had a redshift of z = 1, it would be brightest in the infrared rather than at the yellow-green color associated with the peak of its blackbody spectrum, and the light intensity will be reduced in the filter by a factor of two (1+z) (see K correction for more details on the photometric consequences of redshift). Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of In Physics, a black body is an object that absorbs all light that falls on it K correction is a correction to an astronomical object's magnitude (or equivalently its Flux) that allows a measurement of a quantity of light from an ^[38]

Local observations

A picture of the solar corona taken with the LASCO C1 coronagraph. The Solar and Heliospheric Observatory ( SOHO) is a Spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December The picture is a color coded image of the doppler shift of the FeXIV 5308 Å line, caused by the coronal plasma velocity towards or away from the satellite.

In nearby objects (within our Milky Way galaxy) observed redshifts are almost always related to the line of sight velocities associated with the objects being observed. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Observations of such redshifts and blue shifts have enabled astronomers to measure velocities and parametrize the masses of the orbiting stars in spectroscopic binaries, a method first employed in 1868 by British astronomer William Huggins. In Physics, velocity is defined as the rate of change of Position. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Sir William Huggins, OM, FRS ( February 7 1824 &ndash May 12 1910) was an English Astronomer ^[8] Similarly, small redshifts and blue shifts detected in the spectroscopic measurements of individual stars are one way astronomers have been able to diagnose and measure the presence and characteristics of planetary systems around other stars. Any Planet is an extremely faint light source compared to its parent Star. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 ^[39] Measurements of redshifts to fine detail are used in helioseismology to determine the precise movements of the photosphere of the Sun. Helioseismology is the study of the propagation of Pressure waves in the Sun. The Sun (Sol is the Star at the center of the Solar System. ^[40] Redshifts have also been used to make the first measurements of the rotation rates of planets,^[41] velocities of interstellar clouds,^[42] the rotation of galaxies,^[2] and the dynamics of accretion onto neutron stars and black holes which exhibit both Doppler and gravitational redshifts. A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Interstellar cloud is the generic name given to an accumulation of gas plasma and dust in our and other galaxies. The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy In physics the term dynamics customarily refers to the time evolution of physical processes An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e ^[43] Additionally, the temperatures of various emitting and absorbing objects can be obtained by measuring Doppler broadening — effectively redshifts and blue shifts over a single emission or absorption line. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Atomic physics, Doppler broadening is the broadening of Spectral lines due to the Doppler effect in which the thermal movement of ^[44] By measuring the broadening and shifts of the 21-centimeter hydrogen line in different directions, astronomers have been able to measure the recessional velocities of interstellar gas, which in turn reveals the rotation curve of our Milky Way. The hydrogen line, 21 centimeter line or HI line refers to the Spectral line created by changes in the energy state of neutral Hydrogen and Recessional Velocity is a term used to describe the rate at which an object is moving away typically from Earth. The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy ^[2] Similar measurements have been performed on other galaxies, such as Andromeda. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda ^[2] As a diagnostic tool, redshift measurements are one of the most important spectroscopic measurements made in astronomy. Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy.

Extragalactic observations

Physical cosmology

Universe · Big Bang Age of the universe Timeline of the Big Bang Ultimate fate of the universe

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The most distant objects exhibit larger redshifts corresponding to the Hubble flow of the universe. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. The age of the Universe is the time elapsed between the theory of the Big Bang and the present day This timeline of the Big Bang describes the events according to the Scientific theory of the Big Bang, using the cosmological time parameter of Comoving coordinates The ultimate fate of the universe is a topic in Physical cosmology. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance The largest observed redshift, corresponding to the greatest distance and furthest back in time, is that of the cosmic microwave background radiation; the numerical value of its redshift is about z = 1089 (z = 0 corresponds to present time), and it shows the state of the Universe about 13. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy 7 billion years ago, and 379,000 years after the initial moments of the Big Bang. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. ^[45]

The luminous point-like cores of quasars were the first "high-redshift" (z > 0. A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. 1) objects discovered before the improvement of telescopes allowed for the discovery of other high-redshift galaxies.

For galaxies more distant than the Local Group and the nearby Virgo Cluster, but within a thousand megaparsecs or so, the redshift is approximately proportional to the galaxy's distance. The Local Group is the group of galaxies that includes our galaxy the Milky Way. The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18 History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 This correlation was first observed by Edwin Hubble and has come to be known as Hubble's law. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance Vesto Slipher was the first to discover galactic redshifts, in about the year 1912, while Hubble correlated Slipher's measurements with distances he measured by other means to formulate his Law. Vesto Melvin Slipher ( November 11, 1875 &ndash November 8, 1969) was an American Astronomer. The cosmic distance ladder (also known as the Extragalactic Distance Scale) is the succession of methods by which astronomers determine the Distances to celestial In the widely accepted cosmological model based on general relativity, redshift is mainly a result of the expansion of space: this means that the farther away a galaxy is from us, the more the space has expanded in the time since the light left that galaxy, so the more the light has been stretched, the more redshifted the light is, and so the faster it appears to be moving away from us. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Hubble's law follows in part from the Copernican principle. Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance In Cosmology, the Copernican principle, named after Nicolaus Copernicus, states the Earth is not in a central specially favoured position ^[46] Because it is usually not known how luminous objects are, measuring the redshift is easier than more direct distance measurements, so redshift is sometimes in practice converted to a crude distance measurement using Hubble's law. Luminosity has different meanings in several different fields of science

Gravitational interactions of galaxies with each other and clusters cause a significant scatter in the normal plot of the Hubble diagram. Gravitation is a natural Phenomenon by which objects with Mass attract one another In Probability theory and Statistics, the variance of a Random variable, Probability distribution, or sample is one measure of The peculiar velocities associated with galaxies superimpose a rough trace of the mass of virialized objects in the universe. In Physical cosmology, the term peculiar velocity refers to the components of a receding Galaxy 's velocity that cannot be explained by Hubble's law. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Mechanics, the virial theorem provides a general equation relating the average total Kinetic energy, \left\langle T \right\rangle of a stable system This effect leads to such phenomena as nearby galaxies (such as the Andromeda Galaxy) exhibiting blue shifts as we fall towards a common barycenter, and redshift maps of clusters showing a Finger of God effect due to the scatter of peculiar velocities in a roughly spherical distribution. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda ^[47] This added component gives cosmologists a chance to measure the masses of objects independent of the mass to light ratio (the ratio of a galaxy's mass in solar masses to its brightness in solar luminosities), an important tool for measuring dark matter. In Physical cosmology the mass to light ratio is the quotient between the total Mass of a Galaxy or a cluster and its Luminosity. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from ^[48]

The Hubble law's linear relationship between distance and redshift assumes that the rate of expansion of the universe is constant. However, when the universe was much younger, the expansion rate, and thus the Hubble "constant", was larger than it is today. For more distant galaxies, then, whose light has been travelling to us for much longer times, the approximation of constant expansion rate fails, and the Hubble law becomes a non-linear integral relationship and dependent on the history of the expansion rate since the emission of the light from the galaxy in question. Observations of the redshift-distance relationship can be used, then, to determine the expansion history of the universe and thus the matter and energy content.

While it was long believed that the expansion rate has been continuously decreasing since the Big Bang, recent observations of the redshift-distance relationship using Type Ia supernovae have suggested that in comparatively recent times the expansion rate of the universe has begun to accelerate. A Type Ia supernova is a sub-category of cataclysmic Variable The accelerating universe is the observation that the universe appears to be expanding at an accelerated rate

Highest redshifts

Currently, the objects with the highest known redshifts are galaxies. The most reliable redshifts are from spectroscopic data, and the highest confirmed spectroscopic redshift of a galaxy is that of IOK-1^[49], at a redshift z = 6. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ IOK-1, probably one of the oldest and most distant Galaxy yet found - seen as it was 12 96. Slightly less reliable are Lyman-break redshifts, the highest of which is the lensed galaxy A1689-zD1 at a redshift z = 7. In Astronomical spectroscopy, the Lyman alpha forest is the sum of Absorption lines arising from the Lyman alpha transition of the neutral Hydrogen 6 ^[50] and the next highest being z = 7. 0^[51] while as-yet unconfirmed reports from a gravitational lens observed in a distant galaxy cluster may indicate a galaxy with a redshift of z = 10. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation ^[52]

The highest measured quasar redshift is z = 6. 4 ^[53]. The highest known redshift radio galaxy (TN J0924-2201) is at a redshift z = 5. 2 ^[54] and the highest known redshift molecular material is the detection of emission from the CO molecule from the quasar SDSS J1148+5251 at z = 6. 42 ^[55]

Redshift surveys

Main article: Redshift survey

Rendering of the 2dFGRS data

With the advent of automated telescopes and improvements in spectroscopes, a number of collaborations have been made to map the universe in redshift space. In Astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the Redshift of astronomical objects A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the large-scale structure of the universe. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light The Great Wall, a vast supercluster of galaxies over 500 million light-years wide, provides a dramatic example of a large-scale structure that redshift surveys can detect. The Great Wall (also called Coma Wall) sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the Cosmos. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by ^[56]

The first redshift survey was the CfA Redshift Survey, started in 1977 with the initial data collection completed in 1982. The Center for Astrophysics (CfA Redshift Survey was the first attempt to map the Large-scale structure of the universe ^[57] More recently, the 2dF Galaxy Redshift Survey determined the large-scale structure of one section of the Universe, measuring z-values for over 220,000 galaxies; data collection was completed in 2002, and the final data set was released 30 June 2003. In Astronomy, the 2dF Galaxy Redshift Survey (Two-degree-Field Galaxy Redshift Survey 2dF or 2dFGRS is a Redshift survey conducted by the A data set (or dataset) is a collection of Data, usually presented in tabular form Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. ^[58] (In addition to mapping large-scale patterns of galaxies, 2dF established an upper limit on neutrino mass. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost ) Another notable investigation, the Sloan Digital Sky Survey (SDSS), is ongoing as of 2005 and aims to obtain measurements on around 100 million objects. The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic Redshift survey using a dedicated 2 ^[59] SDSS has recorded redshifts for galaxies as high as 0. 4, and has been involved in the detection of quasars beyond z = 6. A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. The DEEP2 Redshift Survey uses the Keck telescopes with the new "DEIMOS" spectrograph; a follow-up to the pilot program DEEP1, DEEP2 is designed to measure faint galaxies with redshifts 0. The DEEP2 Survey or DEEP2 is a Redshift survey of the Redshift ~1 universe The W M Keck Observatory is a two-telescope Astronomical observatory at the 4145 meter (13600 ft summit of Mauna Kea in Hawai'i. A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used 7 and above, and it is therefore planned to provide a complement to SDSS and 2dF. ^[60]

Effects due to physical optics or radiative transfer

The interactions and phenomena summarized in the subjects of radiative transfer and physical optics can result in shifts in the wavelength and frequency of electromagnetic radiation. The term radiative transfer refers to the physical phenomena of energy transfer in the form of electromagnetic radiation In Physics, physical optics, or wave optics, is the branch of Optics which studies Interference, Diffraction, Polarization In such cases the shifts correspond to a physical energy transfer to matter or other photons rather than being due to a transformation between reference frames. These shifts can be due to such physical phenomena as coherence effects or the scattering of electromagnetic radiation whether from charged elementary particles, from particulates, or from fluctuations of the index of refraction in a dielectric medium as occurs in the radio phenomenon of radio whistlers. Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium A dielectric is a nonconducting substance ie an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. A Whistler is a Very low frequency electromagnetic (radio wave generated by Lightning. ^[2] While such phenomena are sometimes referred to as "redshifts" and "blue shifts", the physical interactions of the electromagnetic radiation field with itself or intervening matter distinguishes these phenomena from the reference-frame effects. In astrophysics, light-matter interactions that result in energy shifts in the radiation field are generally referred to as "reddening" rather than "redshifting" which, as a term, is normally reserved for the effects discussed above. ^[2]

In many circumstances scattering causes radiation to redden because entropy results in the predominance of many low-energy photons over few high-energy ones (while conserving total energy). In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός In Physics, the law of conservation of energy states that the total amount of Energy in an isolated system remains constant and cannot be created although it may ^[2] Except possibly under carefully controlled conditions, scattering does not produce the same relative change in wavelength across the whole spectrum; that is, any calculated z is generally a function of wavelength. The Mathematical concept of a function expresses dependence between two quantities one of which is given (the independent variable, argument of the function Furthermore, scattering from random media generally occurs at many angles, and z is a function of the scattering angle. Randomness is a lack of order Purpose, cause, or predictability Matter is commonly defined as being anything that has mass and that takes up space. In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called If multiple scattering occurs, or the scattering particles have relative motion, then there is generally distortion of spectral lines as well. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared ^[2]

In interstellar astronomy, visible spectra can appear redder due to scattering processes in a phenomenon referred to as interstellar reddening^[2] — similarly Rayleigh scattering causes the atmospheric reddening of the Sun seen in the sunrise or sunset and causes the rest of the sky to have a blue color. Red is any of a number of similar Colors evoked by light consisting predominantly of the longest wavelengths of Light discernible by the human eye in the wavelength In Astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of Electromagnetic radiation Rayleigh scattering (named after Lord Rayleigh) is the elastic Scattering of Light or other electromagnetic radiation by particles much smaller Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five The Sun (Sol is the Star at the center of the Solar System. Sunrise is the instant at which the upper edge of the Sun appears above the Horizon in the East. Sunset, also called sundown in some American English Dialects is the instant when the trailing edge of the Sun 's disk disappears below The sky is the part of the Atmosphere or of Outer space visible from the surface of any Astronomical object. Blue is a Colour, the Perception of which is evoked by This phenomenon is distinct from redshifting because the spectroscopic lines are not shifted to other wavelengths in reddened objects and there is an additional dimming and distortion associated with the phenomenon due to photons being scattered in and out of the line of sight. In Physics, atomic Spectral lines are of two types An emission line is formed when an electron makes a transition from a particular discrete Extinction is a term used in Astronomy to describe the absorption and Scattering of Electromagnetic radiation emitted by Astronomical objects

For a list of scattering processes, see Scattering. Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from

References

Notes

Articles

• Odenwald, S. & Fienberg, RT. 1993; "Galaxy Redshifts Reconsidered" in Sky & Telescope Feb. 2003; pp31–35 (This article is useful further reading in distinguishing between the 3 types of redshift and their causes. )
• Lineweaver, Charles H. and Tamara M. Davis, "Misconceptions about the Big Bang", Scientific American, March 2005. Scientific American is a Popular science magazine, published (first weekly and later monthly since August 28, 1845, making it (This article is useful for explaining the cosmological redshift mechanism as well as clearing up misconceptions regarding the physics of the expansion of space. )

Book references

• Binney, James; and Michael Merrifeld (1998). Galactic Astronomy. Princeton University Press. ISBN 0-691-02565-7.  
• Carroll, Bradley W. and Dale A. Ostlie (1996). An Introduction to Modern Astrophysics. Addison-Wesley Publishing Company, Inc. . ISBN 0-201-54730-9.  
• Feynman, Richard; Leighton, Robert; Sands, Matthew (1989). Feynman Lectures on Physics. The Feynman Lectures on Physics by Richard Feynman, Robert Leighton, and Matthew Sands is perhaps Feynman's most accessible technical work Vol. 1. Addison-Wesley. ISBN 0-201-51003-0.  
• Grøn, Øyvind; Hervik, Sigbjørn (2007). Einstein's General Theory of Relativity. New York: Springer. ISBN 978-0-387-69199-2.  
• Kutner, Marc (2003). Astronomy: A Physical Perspective. Cambridge University Press. ISBN 0-521-52927-1.  
• Misner, Charles; Thorne, Kip S. and Wheeler, John Archibald (1973). Gravitation. San Francisco: W. H. Freeman. ISBN 0-7167-0344-0.  
• Peebles, P. J. E. (1993). Principles of Physical Cosmology. Princeton University Press. ISBN 0-691-01933-9.  
• Taylor, Edwin F. ; Wheeler, John Archibald (1992). John Archibald Wheeler ( July 9, 1911 &ndash April 13, 2008) was an eminent American Theoretical physicist. Spacetime Physics: Introduction to Special Relativity (2nd ed. ). W. H. Freeman. ISBN 0-7167-2327-1.  
• Weinberg, Steven (1971). Gravitation and Cosmology. John Wiley. ISBN 0-471-92567-5.  
• See also physical cosmology textbooks for applications of the cosmological and gravitational redshifts. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its

External links

• Ned Wright's Cosmology tutorial
• Article on redshift from SPACE.com
• Cosmic reference guide entry on redshift
• Mike Luciuk's Astronomical Redshift tutorial