| For other uses of the phrase "red shift" or "redshift", see redshift (disambiguation).In physics and astronomy, redshift occurs when the electromagnetic radiation, usually visible light, that is emitted from or reflected off an object is shifted toward the (less energetic) 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 nomenclature might be confusing since, at wavelengths longer than red (e.Although observing such redshifts, or complementary blue shifts, has several terrestrial applications (e.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.Gravitational redshift is observed if the receiver is located at higher gravitational potential than the source.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.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.The effect is named after Christian Andreas Doppler, who offered the first known physical explanation for the phenomenon in 1842.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.Louis Fizeau, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect.In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.In 1901 Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors.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 corroborated Alexander Friedman's 1922 work, in which he derived the famous Friedmann equations.They are today considered strong evidence for an expanding universe and the Big Bang theory.To determine the redshift, features in the spectrum such as absorption lines, emission lines, or other variations in light intensity, are searched for.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.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.Redshift (and blue shift) may be characterized by the relative difference between the observed and emitted wavelengths (or frequency) of an object.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.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.Mechanisms
A single photon propagated through a vacuum can redshift in several distinct ways.Cosmological redshift
General relativistic tr.Gravitational redshift
General relativistic tr.This is true for all electromagnetic waves and is explained by the Doppler effect.Consequently, this type of redshift is called the Doppler redshift.Doppler redshift requires considering relativistic effects associated with motion of sources close to the speed of light.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.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.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.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.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 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.Doppler redshifts and blue shifts.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.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 the theory of general relativity, there is time dilation within a gravitational well.This is known as the gravitational redshift or Einstein Shift.Schwarzschild coordinate), and
is the speed of light.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.However, it is significant near a black hole, and as an object approaches the event horizon the red shift becomes infinite.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 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.However, photometry does allow at least for a qualitative characterization of a redshift.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 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.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.Doppler and gravitational redshifts.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 Hubble flow of the universe.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.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.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.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 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.Type Ia supernovae have suggested that in comparatively recent times the expansion rate of the universe has begun to accelerate.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.The first redshift survey was the CfA Redshift Survey, started in 1977 with the initial data collection completed in 1982.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.DEEP1, DEEP2 is designed to measure faint galaxies with redshifts 0.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 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.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.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 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.See Taylor (1992) for a relativistic discussion.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."Philosophical Transactions of the Royal Society of London, Volume 158, pp.Fizeau Principle" (1901) Astrophysical Journal, vol.Carnegie Institution of Washington, vol.Sitter, "On distance, magnitude, and related quantities in an expanding universe, (1934) Bulletin of the Astronomical Institutes of the Netherlands, Vol.This was recognized early on by physicists and astronomers working in cosmology in the 1930s.However, optical observations of GRB afterglows have produced spectra with identifiable lines, leading to precise redshift measurements."Cosmology: The Science of the Universe (New York: Cambridge University Press).Doppler)(1 + zexpansion)
which yields solutions where certain objects that "recede" are blue shifted and other objects that "approach" are redshifted.Weiss, What Causes the Hubble Redshift?When cosmological redshifts were first discovered, Fritz Zwicky proposed an effect known as tired light.While usually considered for historical interests, it is sometimes, along with intrinsic redshift suggestions, utilized by nonstandard cosmologies.Reboul summarised many alternative redshift mechanisms that had been discussed in the literature since the 1930s.Burbidge and Halton Arp, 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.For a review of the subject of photometry, consider Budding, E.Photometric redshifts based on standard SED fitting procedures, Astronomy and Astrophysics, 363, p.The Exoplanet Tracker is the newest observing project to use this technique, able to track the redshift variations in multiple objects at once, as reported in Ge, Jian et al.The final published temperature of 2."Cosmic microwave background dipole spectrum measured by the COBE FIRAS instrument", Astrophysical Journal, 420, 445.The most accurate measurement as of 2006 was achieved by the WMAP experiment.The Astronomical Journal (2003), v.Huchra, Science 246, 897 (1989)."Science objectives and early results of the DEEP2 redshift survey".This article is useful further reading in distinguishing between the 3 types of redshift and their causes.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.Book references
Binney, James; and Michael Merrifeld (1998).Einstein's General Theory of Relativity.Astronomy: A Physical Perspective.Wheeler, John Archibald (1992).Spacetime Physics: Introduction to Special Relativity (2nd ed.See also physical cosmology textbooks for applications of the cosmological and gravitational redshifts.Ned Wright's Cosmology tutorial
Article on redshift from SPACE.Companies who are driving heavy Internet traffic.This includes popular web portals like Google, Yahoo, AOL and MSN.PC, mobile phone or other device over a network, there must exist computing systems to send it over the network.Companies doing complex simulations such as weather, stock market or drug design simulations.Companies aggregating traditional computing applications and offer them as services, typically in the form of Software as a Service (SaaS).Moore's Law, but companies that aggregate CRM solutions and offer them as a service, such as Salesforce.Traditional Computing Markets (Blueshifting)
Redshift theory suggests that traditional computing markets, such as those serving Enterprise Resource Planning (ERP) or Customer Relationship Management (CRM) applications, have reached relative saturation in industrialized nations.Given that Moore's Law continues to predict accurately the rate of computing transistor growth, which roughly translates into computing power doubling every two years, the Redshift theory suggests that traditional computing markets will ultimately contract as a percentage of computing expenditures over time.Industry analysts are also beginning to quantify Redshifting and Blueshifting markets.According to International Data Corporation (IDC) vice president Matthew Eastwood, "IDC believes that the IT market is in a period of hyper segmentation...This a class of customers that is Moore's law driven and as price performance gains continue, IDC believes that these organizations will accelerate their consumption of IT infrastructure.Nomenclature
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