   #copyright

Redshift

2007 Schools Wikipedia Selection. Related subjects: General Physics; Space
(Astronomy)

   Redshift of spectral lines in the optical spectrum of a supercluster of
   distant galaxies (right), as compared with that of the Sun (left).
   Wavelength increases up towards the red and beyond, (frequency
   decreases)
   Enlarge
   Redshift of spectral lines in the optical spectrum of a supercluster of
   distant galaxies (right), as compared with that of the Sun (left).
   Wavelength increases up towards the red and beyond, (frequency
   decreases)

   In physics and astronomy, redshift occurs when the visible light from
   an object is shifted towards the red end of the spectrum. 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. This increase in wavelength
   corresponds to a decrease in the frequency of the electromagnetic
   radiation. Conversely, a decrease in wavelength is called blueshift.

   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. 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.

   A redshift can occur when a light source moves away from an observer,
   corresponding to the Doppler shift that changes the frequency of sound
   waves. Although observing such redshifts has several terrestrial
   applications (e.g. Doppler radar and radar guns), spectroscopic
   astrophysics uses Doppler redshifts to determine the movement of
   distant astronomical objects. This Doppler redshift phenomenon was
   first predicted and observed in the nineteenth century as scientists
   began to consider the dynamical implications of the wave-nature of
   light.

   Another redshift mechanism accounts for the famous observation that the
   spectral redshifts of distant galaxies, quasars, and intergalactic gas
   clouds are observed to increase proportionally with their distance to
   the observer. This relation is accounted for by models that predict the
   universe is expanding, seen in, for example, the Big Bang model. Yet a
   third type of redshift, the gravitational redshift also known as the
   Einstein effect, results from the time dilation that occurs in general
   relativity near massive objects.

History

   Hippolyte Fizeau who first described the Doppler redshift
   Enlarge
   Hippolyte Fizeau who first described the Doppler redshift

   The history of the subject begins with the development in the
   nineteenth century of wave mechanics and the exploration of phenomena
   associated with the Doppler effect. The effect is named after Christian
   Andreas Doppler who offered the first known physical explanation for
   the phenomenon in 1842. The hypothesis was tested and confirmed for
   sound waves by the Dutch scientist Christoph Hendrik Diederik Buys
   Ballot in 1845. 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.
   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.

   The first Doppler redshift was described by French physicist
   Armand-Hippolyte-Louis 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.

   In 1871, optical redshift is confirmed when the phenomenon is observed
   in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In
   1901 Aristarkh Belopolsky verified optical redshift in the laboratory
   using a system of rotating mirrors.

   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". The word doesn't appear unhyphenated, perhaps
   indicating a more common usage of its German equivalent,
   Rotverschiebung, until about 1934 by Willem de Sitter.

   Beginning with observations in 1912, Vesto Slipher discovered that most
   spiral nebulae had considerable redshifts. 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.
   These observations are today considered strong evidence for an
   expanding universe and the Big Bang theory.

Measurement, characterization, and interpretation

   A redshift can be measured by looking at the spectrum of light that
   comes from a single source (see idealized spectrum illustration
   top-right). If there are features in this spectrum such as absorption
   lines, emission lines, or other variations in light intensity, then a
   redshift can in principle be calculated. This requires comparing the
   observed spectrum to a known spectrum with similar features. For
   example, the atomic element hydrogen, when exposed to light, has a
   definite signature spectrum that shows features at regular intervals.
   If the same pattern of intervals is seen in an observed spectrum
   occurring at shifted wavelengths, then a redshift can be measured for
   the object. Determining the redshift of an object therefore requires a
   frequency- or wavelength-range. Redshifts cannot be calculated by
   looking at isolated features or with a spectrum that is featureless or
   white noise (random fluctuations in a spectrum).

   Redshift (and blueshift) 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. 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:

                     CAPTION: Measurement of redshift, z

                   Based on wavelength Based on frequency
                   z = \frac{\lambda_{\mathrm{observed}} -
         \lambda_{\mathrm{emitted}}}{\lambda_{\mathrm{emitted}}} z =
                        \frac{f_{\mathrm{emitted}} -
                f_{\mathrm{observed}}}{f_{\mathrm{observed}}}
    1+z = \frac{\lambda_{\mathrm{observed}}}{\lambda_{\mathrm{emitted}}}
          1+z = \frac{f_{\mathrm{emitted}}}{f_{\mathrm{observed}}}

   After z is measured, the distinction between redshift and blueshift is
   simply a matter of whether z is positive or negative. According to the
   mechanisms section below, there are some basic interpretations that
   follow when either a redshift or blueshift is observed. For example,
   Doppler effect blueshifts (z < 0) are associated with objects
   approaching (moving closer) to the observer with the light shifting to
   greater energies. Conversely, Doppler effect redshifts (z > 0) are
   associated with objects receding (moving away) from the observer with
   the light shifting to lower energies. Likewise, Einstein effect
   blueshifts are associated with light entering a strong gravitational
   field while Einstein effect redshifts imply light is leaving the field.

Mechanisms

   A single photon propagated through a vacuum can redshift in several
   distinct ways. 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.

                          CAPTION: Redshift Summary

      Redshift type Transformation frame Example of a metric Definition
        Doppler redshift Galilean transformation Euclidean metric z =
                                 \frac{v}{c}
      Relativistic Doppler Lorentz transformation Minkowski metric z =
                   \left(1 + \frac{v}{c}\right) \gamma - 1
        Cosmological redshift General relativistic tr. FRW metric z =
               \frac{a_{\mathrm{now}}}{a_{\mathrm{then}}} - 1
    Gravitational redshift General relativistic tr. Schwarzschild metric
            z=\frac{1}{\sqrt{1-\left(\frac{2GM}{rc^2}\right)}}-1

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
   blueshift (z < 0) occurs. This is true for all electromagnetic waves
   and is explained by the Doppler effect. 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

          z \approx \frac{v}{c}     (Since \gamma \approx 1 , see below)

   where c is the speed of light. 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

   A more complete treatment of the Doppler redshift requires considering
   relativistic effects associated with motion of sources close to the
   speed of light. A complete derivation of the effect can be found in the
   article on the relativistic Doppler effect. 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:

          1 + z = \left(1 + \frac{v}{c}\right) \gamma

   This phenomenon was first observed in a 1938 experiment performed by
   Herbert E. Ives and G.R. Stilwell, called the Ives-Stilwell experiment.

   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.
   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. 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:

          1+ z = \frac{1 + v \cos (\theta)/c}{\sqrt{1-v^2/c^2}}

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

          1 + z = \sqrt{\frac{1 + \frac{v}{c}}{1 - \frac{v}{c}}}

   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. 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 blueshift and at higher speed the approaching source will be
   redshifted.

Expansion of space

   In the early part of the twentieth century, Slipher, Hubble and others
   made the first measurements of the redshifts and blueshifts of galaxies
   beyond the Milky Way. They initially interpreted these redshifts and
   blueshifts 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. As a result, photons
   propagating through the expanding space are stretched, creating the
   cosmological redshift. 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. 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:

          1+z = \frac{a_{\mathrm{now}}}{a_{\mathrm{then}}}.

   This type of redshift is called the cosmological redshift or Hubble
   redshift. If the universe were contracting instead of expanding, we
   would see distant galaxies blueshifted by an amount proportional to
   their distance instead of redshifted.

   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. 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
   blueshifts. The difference between physical velocity and space
   expansion is clearly illustrated by the Expanding Rubber Sheet
   Universe, a common cosmological analogy used to describe the expansion
   of space. 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 if the distance
   between two objects is far enough.)

   In spite of the distinction between redshifts caused by the velocity of
   objects and the redshifts associated with the expanding universe,
   astronomers (especially professional ones) 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. 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. 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.g., a quasar from the Earth) can expand faster than the
   speed of light. 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.
   Expressing this precisely requires working with the mathematics of the
   Robertson-Walker metric.

Gravitational redshift

   A graphical representation of the gravitational redshift due to a
   neutron star.
   Enlarge
   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. This is known as the gravitational redshift or
   Einstein Shift. 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:

          1+z=\frac{1}{\sqrt{1-\left(\frac{2GM}{rc^2}\right)}} ,

   where
     * G is the gravitational constant,
     * M is the mass of the object creating the gravitational field,
     * r is the radial coordinate of the observer (which is analogous to
       the classical distance from the centre of the object, but is
       actually a Schwarzschild coordinate), and
     * c is the speed of light.

   This gravitational redshift results can be derived from the assumptions
   of special relativity and the equivalence principle; the full theory of
   general relativity is not required.

   The effect is very small but measurable on Earth using the Mossbauer
   effect and was first observed in the Pound-Rebka experiment. However,
   it is significant near a black hole, and as an object approaches the
   event horizon the red shift becomes infinite. It is also the dominant
   cause of large angular-scale temperature fluctuations in the cosmic
   microwave background radiation (see Sachs-Wolfe effect).

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.
   When the redshift of various absorption and emission lines from a
   single astronomical object is measured, z is found to be remarkably
   constant. (See Is the fine structure constant really 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. 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.

   Spectroscopy, as a measurement, is considerably more difficult than
   simple photometry which measures the brightness of astronomical objects
   through certain filters. 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.
   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 = 0.5, and are much less reliable than spectroscopic
   determinations. 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 colour 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).

Local observations

   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. Observations of such redshifts and
   blueshifts 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.
   Similarly, small redshifts and blueshifts 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. Measurements of redshifts to fine detail
   are used in helioseismology to determine the precise movements of the
   photosphere of the Sun. Redshifts have also been used to make the first
   measurements of the rotation rates of planets, velocities of
   interstellar clouds, the rotation of galaxies, and the dynamics of
   accretion onto neutron stars and black holes which exhibit both Doppler
   and gravitational redshifts. Additionally, the temperatures of various
   emitting and absorbing objects can be obtained by measuring Doppler
   broadening — effectively redshifts and blueshifts over a single
   emission or absorption line. 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. Similar
   measurements have been performed on other galaxies, such as Andromeda.
   As a diagnostic tool, redshift measurements are one of the most
   important spectroscopic measurements made in astronomy.

Extragalactic observations

          Physical cosmology

     * Age of the universe
     * Big Bang
     * Blueshift
     * Comoving distance
     * Cosmic microwave background
     * Dark energy
     * Dark matter
     * FLRW metric
     * Friedmann equations
     * Galaxy formation
     * Hubble's law
     * Inflation
     * Large-scale structure
     * Lambda-CDM model
     * Metric expansion of space
     * Nucleosynthesis
     * Observable universe
     * Redshift
     * Shape of the universe
     * Structure formation
     * Timeline of the Big Bang
     * Timeline of cosmology
     * Ultimate fate of the universe
     * Universe

            Related topics
     * Astrophysics
     * General relativity
     * Particle physics
     * Quantum gravity


   The most distant objects exhibit larger redshifts corresponding to the
   Hubble flow of the universe. 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 13.7 billion years ago,
   and 379,000 years after the initial moments of the Big Bang.

   The luminous point-like cores of quasars were the first "high-redshift"
   (z > 0.1) objects discovered before the improvement of telescopes
   allowed for the discovery of other high-redshift galaxies. Currently,
   the highest measured quasar redshift is z = 6.4, with the highest
   confirmed galaxy redshift being z = 7.0 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.

   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. This correlation
   was first observed by Edwin Hubble and has come to be known as Hubble's
   law. 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. 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. Hubble's law follows in part from
   the Copernican principle. 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.

   Gravitational interactions of galaxies with each other and clusters
   cause a significant scatter in the normal plot of the Hubble diagram.
   The peculiar velocities associated with galaxies superimpose a rough
   trace of the mass of virialized objects in the universe. This effect
   leads to such phenomena as nearby galaxies (such as the Andromeda
   Galaxy) exhibiting blueshifts 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.
   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.

   For more distant galaxies, the relationship between current distance
   and observed redshift becomes more complex. When one sees a distant
   galaxy, one is seeing the galaxy as it was sometime in the past, when
   the expansion rate of the Universe was different from what it is now.
   At these early times, we expect differences in the expansion rate for
   at least two reasons:
    1. The gravitational attraction between galaxies has been acting to
       slow down the expansion of the Universe since then.
    2. The possible existence of a cosmological constant or quintessence
       may be changing the expansion rate of the Universe

   Recent observations have suggested the expansion of the Universe is not
   slowing down, as expected from the first point, but accelerating. It is
   widely, though not quite universally, believed that this is because
   there is a form of dark energy dominating the evolution of the
   universe. Such a cosmological constant implies that the ultimate fate
   of the Universe is not a Big Crunch, but instead will continue to exist
   foreseeably (though most physical processes within the Universe will
   still come to an eventual end).

   The expanding Universe is a central prediction of the Big Bang theory.
   If extrapolated back in time, the theory predicts a "singularity", a
   point in time when the Universe had infinite density. The theory of
   general relativity, on which the Big Bang theory is based, breaks down
   at this point. It is believed that a yet unknown theory of quantum
   gravity would take over before the density becomes infinite.

Redshift surveys

   Rendering of the 2dFGRS data
   Enlarge
   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. 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. 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 first redshift survey was the CfA Redshift Survey, started in 1977
   with the initial data collection completed in 1982. 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 addition to mapping large-scale patterns of
   galaxies, 2dF established an upper limit on neutrino mass.) Another
   notable investigation, the Sloan Digital Sky Survey (SDSS), is ongoing
   as of 2005 and aims to obtain measurements on around 100 million
   objects. SDSS has recorded redshifts for galaxies as high as 0.4, and
   has been involved in the detection of quasars beyond z = 6. 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 0.7 and above, and it is
   therefore planned to provide a complement to SDSS and 2dF.

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. 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 coherence effects (see Wolf effect) or due
   to the scattering of electromagnetic radiation whether from charged
   elementary particles, from particulates, or from fluctuations in a
   dielectric medium. While such phenomena are sometimes referred to as
   "redshifts" and "blueshifts", 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.

   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). 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. Furthermore,
   scattering from random media generally occurs at many angles, and z is
   a function of the scattering angle. If multiple scattering occurs, or
   the scattering particles have relative motion, then there is generally
   distortion of spectral lines as well.

   In interstellar astronomy, visible spectra can appear redder due to
   scattering processes in a phenomenon referred to as interstellar
   reddening — 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 colour. 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.
   Retrieved from " http://en.wikipedia.org/wiki/Redshift"
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