s 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 , occurs when the Electromagnetic Radiation , usually Visible Light , that is emitted from or reflected off an object is shifted toward the red end of the Electromagnetic 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 Blue Shift .
Any increase in wavelength is called "redshift", even if it occurs in electromagnetic radiation of non-optical wavelengths, such as Gamma Ray s, X-ray s and Ultraviolet . This nomenclature might be confusing since, at wavelengths longer than red (e.g., Infrared , Microwave s, 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 perceived frequency of Sound Waves . Although observing such redshifts, or complementary blue shifts, has several terrestrial applications (e.g., Doppler Radar and Radar Gun s),See Feynman, Leighton and Sands (1989) or any introductory undergraduate (and many high school) Physics Textbooks . See Taylor (1992) for a relativistic discussion. Spectroscopic astrophysics uses Doppler redshifts to determine the movement of distant astronomical objects.See Binney and Merrifeld (1998), Carroll and Ostlie (1996), Kutner (2003) for applications in astronomy. 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 .
Another redshift mechanism is the Expansion Of The Universe , which explains the famous observation that the spectral redshifts of distant Galaxies , Quasar s, and Intergalactic Gas Clouds increase in Proportion to their distance from the observer. This mechanism is a key feature of the Big Bang model of Physical Cosmology .See Misner, Thorne and Wheeler (1973) and Weinberg (1971) or any of the Physical Cosmology Textbooks
Yet a third type of redshift, the Gravitational Redshift , is a result of the Time Dilation that occurs near massive objects, according to General Relativity .See Misner, Thorne and Wheeler (1973) and Weinberg (1971).
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 . 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 ).
, who first described the Doppler redshift]]
The history of the subject began with the development in the 19th century of s, and in particular suggested that the varying Color s of Star s 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 in 1848 by French physicist Armand-Hippolyte-Louis Fizeau , who pointed to the shift in Spectral Line s 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.William Huggins, " Further Observations on the Spectra of Some of the Stars and Nebulae, with an Attempt to Determine Therefrom Whether These Bodies are Moving towards or from the Earth , Also Observations on the Spectra of the Sun and of Comet II." (1868) ''Philosophical Transactions of the Royal Society of London'', Volume 158, pp. 529–564
In 1871, optical redshift was confirmed when the phenomenon was observed in verified optical redshift in the laboratory using a system of rotating mirrors.Bélopolsky, A., " On an Apparatus for the Laboratory Demonstration of the Doppler-Fizeau Principle " (1901) Astrophysical Journal, vol. 13, p.15
The earliest occurrence of the term "red-shift" in print (in this hyphenated form), appears to be by American astronomer .W. de Sitter, " On distance, magnitude, and related quantities in an expanding universe , (1934) ''Bulletin of the Astronomical Institutes of the Netherlands'', Vol. 7, p.205. He writes: "It thus becomes urgent to investigate the effect of the redshift and of the metric of the universe on the apparent magnitude and observed numbers of nebulae of given magnitude"
Beginning with observations in 1912, and the Big Bang theory.This was recognized early on by physicists and astronomers working in cosmology in the 1930s. The earliest layman publication describing the details of this correspondence was Sir Arthur Eddington 's book ''The Expanding Universe: Astronomy's 'Great Debate', 1900–1931'', published by Press Syndicate of the University of Cambridge in 1933.
A redshift can be measured by looking at the 's Swift Space Telescope that is researching Gamma-ray Burst s: "Measurements of the gamma-ray spectra obtained during the main outburst of the GRB have found little value as redshift indicators, due to the lack of well-defined features. However, optical observations of GRB afterglows have produced spectra with identifiable lines, leading to precise redshift measurements."
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''. 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:
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 . 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 blue shifts are associated with light entering a strong Gravitational Field while Einstein effect redshifts imply light is leaving the field.
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.
|
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 Angle s (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 blue shift and at higher speed the approaching source will be redshifted.See "
Photons, Relativity, Doppler shift " at the University of Queensland
See Also: 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 as an observable manifestation of the time-dependent cosmic
Scale Factor (
) in the following way:
:
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 blue shifted by an amount proportional to their distance instead of redshifted.This is only true in a universe where there are no
Peculiar Velocities . Otherwise, redshifts combine as
:
which yields solutions where certain objects that "recede" are blue shifted and other objects that "approach" are redshifted. For more on this bizarre result see Davis, T. M., Lineweaver, C. H., and Webb, J. K. "
Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blue shifted objects ", ''
American Journal Of Physics '' (2003), 358–364.
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 , 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 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.; 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.This is because the
Expansion of the
Spacetime Metric is describable by
General Relativity and dynamically changing measurements as opposed to a rigid
Minkowski Metric . Space, not being composed of any
Material can grow faster than the speed of light since, not being an object, it is not bound by the speed of light upper bound. More mathematically, the viewpoint that "distant galaxies are receding" and the viewpoint that "the space between galaxies is expanding" are related by changing
Coordinate System s. Expressing this precisely requires working with the mathematics of the
Friedmann-Robertson-Walker Metric .M. Weiss, What Causes the Hubble Redshift?, entry in the Physics
FAQ (1994), available via
John Baez 's
website
See Also: Gravitational redshift
due to a
Neutron Star ]]
In the theory of 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
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.
1
The effect is very small but measurable on Earth using the , 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 ).
2
The redshift observed in astronomy can be measured because the , while investigating the mystery of
The Nature Of Quasars , tried to develop alternative redshift mechanisms, and very few of their fellow scientists acknowledged let alone accepted their work.
Spectroscopy, as a measurement, is considerably more difficult than simple '', , p.476–492 (2000). 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).A pedagogical overview of the K-correction by David Hogg and other members of the
SDSS collaboration can be found at
astro-ph .
coronagraph. 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 s 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.Rybicki, G. B. and A. R. Lightman, ''Radiative Processes in Astrophysics'', John Wiley & Sons, 1979, p. 288 ISBN 0-471-82759-2 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.
The most distant objects exhibit larger redshifts corresponding to the experiment.
The luminous point-like cores of observed in a distant
Galaxy Cluster may indicate a galaxy with a redshift of
.Pelló, R., Schaerer, D., Richard, J., Le Borgne, J.-F., & Kneib, J.P., ISAAC/VLT observations of a lensed galaxy at z = 10.0, ''
Astronomy And Astrophysics '' (2004), 416, L35
{Link without Title} .
For galaxies more distant than the follows in part from the
Copernican Principle .Peebles (1993). 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 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.Peebles (1993). 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 .
3
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 Supernova e have suggested that in comparatively recent times the expansion rate of the universe has
Begun To Accelerate .
See Also: Redshift survey
With the advent of automated
Telescope s 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-year s wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.M. J. Geller & J. P. Huchra, ''Science'' , 897 (1989).
online
The first redshift survey was the s 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.
5
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 such physical phenomena as
Coherence Effects or the
Scattering of
Electromagnetic Radiation whether from
Charged Elementary Particle s, from particulates, or from fluctuations of the
Index Of Refraction in a
Dielectric Medium as occurs in the radio phenomenon of
Radio Whistlers . 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 .
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
Angle s, 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 Line s as well.
In
Interstellar Astronomy ,
Visible Spectra can appear
Red der 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 color. This phenomenon is distinct from red''shift''ing 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 .
''For a list of scattering processes, see
Scattering .''
- 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. (This article is useful for explaining the cosmological redshift mechanism as well as clearing up misconceptions regarding the physics of the expansion of space.)
