Flying Jazz (talk | contribs) bathochromic shifts ARE redshifts. They're just not the kind you like to talk about. |
Removing non-standard cosmologies as per Wikipedia:Neutral point of view#Giving "equal validity" and Wikipedia:Neutral point of view#Undue weight. See talk. |
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Redshifts of spectral lines are measured by assuming that the known [[physical law]]s have not changed, in particular the [[quantum physics]] that determines the position of an absorption or emission line on the [[electromagnetic spectrum]] is the same in all of [[spacetime]]. Some physicists including [[Paul Davies]] and the late [[John Bahcall]] have pointed out that variations in the absolute positions of the spectral lines could reveal gradual changes in the physics governing the universe, in particular, measurements of an absolute shift in spectral lines could indicate a change in ''α'', the [[fine structure constant]]. If this quantity had varied in time, some typical [[spectral line]]s would be shifted to different wavelengths and some multiple line features would have different spacing. Measuring such changes would be an optimal test of the constancy of '' α''. As of [[2005]], attempts to measure this have been inconclusive, and the issue remains open. |
Redshifts of spectral lines are measured by assuming that the known [[physical law]]s have not changed, in particular the [[quantum physics]] that determines the position of an absorption or emission line on the [[electromagnetic spectrum]] is the same in all of [[spacetime]]. Some physicists including [[Paul Davies]] and the late [[John Bahcall]] have pointed out that variations in the absolute positions of the spectral lines could reveal gradual changes in the physics governing the universe, in particular, measurements of an absolute shift in spectral lines could indicate a change in ''α'', the [[fine structure constant]]. If this quantity had varied in time, some typical [[spectral line]]s would be shifted to different wavelengths and some multiple line features would have different spacing. Measuring such changes would be an optimal test of the constancy of '' α''. As of [[2005]], attempts to measure this have been inconclusive, and the issue remains open. |
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== Redshift interpretations in non-standard cosmologies == |
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Proponents of some [[non-standard cosmology|non-standard cosmologies]] sometimes dispute the standard explanation of the cosmological redshift and the resulting [[Hubble Law]]. These alternative models have very little or no support in the professional [[scientific community]]. On the theoretical side one major failing of the alternatives is that they are not sufficiently developed to explain the phenomena the Big Bang explains. |
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Some proponents point to [[tired light]], a proposal first outlined by [[Fritz Zwicky]] as an explanation for redshift to be a viable alternative to the cosmological redshift. This idea is rejected by most cosmologists as discussed in the article on that subject. Processes can under certain circumstances allow for shifts in the energy of spectral features that may mimic redshifts, all of these shifts are frequency-dependent at least to some degree and do not conform to the definition of redshift outlined above. |
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[[Halton C. Arp]], one of the few remaining critics of the [[Big Bang]] in the astronomical community has claimed to find empirical support in the existence of apparently connected objects with very different redshifts. Arp has hypothesized that observed redshifts of these objects could be dominated by a non-cosmological (intrinsic) component, thus calling into question the accuracy of the [[Hubble Law]]. Conventional cosmological models regard these as chance alignments and Arp's hypothesis has very few supporters within the research community. |
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== See also == |
== See also == |
Revision as of 17:46, 9 December 2005
- This article is about the light phenomenon. For other uses of the phrase "Red Shift", see Red Shift
In physics and astronomy, redshift is an observed increase in the wavelength of electromagnetic radiation received by a detector compared to that emitted by the source. For visible light, red is the color with the longest wavelength, so colors experiencing redshift actually shift towards the red part of the electromagnetic spectrum. The phenomenon uses the same name even if it occurs at non-optical wavelengths. The corresponding shift to shorter wavelengths is called blueshift.
Astrophysical redshift occurs, for example, when a light source moves away from an observer analogous to the Doppler effect for sound waves. Redshift is used as a diagnostic in spectroscopic astrophysics to determine information about the dynamics and kinematics of distant objects. Most famously, redshifts are observed in the spectra from distant galaxies, quasars, and intergalactic gas clouds to increase proportionally with the distance to the object. This is considered by astronomers to be one of the major forms of evidence that the universe is expanding as a result of the Big Bang.
In photochemistry, redshift is an informal term for a bathochromic shift. This is an increase in the wavelength position of a spectral band associated with one molecular state compared to another. Blueshift is an informal term for a hypsochromic shift, a decrease in the wavelength position.
Mathematical definitions
In photochemistry, redshift may refer to a numerical value in spite of its informal usage. Redshift is synonymous with a wavelength increase due to a molecular physicochemical change and has units of length:
- where is the wavelength of the spectral band of interest.
The band may occur on an absorbance, reflectance, transmittance, or emission spectrum.
In physics and astronomy, redshift and blueshift are more formally defined as the proportionality between the change in wavelength and the wavelength emitted. They are represented by the dimensionless quantity z determined by the equation:
- where is the wavelength of electromagnetic radiation.
The increase in wavelength of a photon subjected to a redshift corresponds to a decrease in its frequency:
where is the frequency of electromagnetic radiation.
The ratio z may be used in a variety of situations that only apply to individual spectral lines or a subset of the spectrum without representing a proportionality. However, most references to redshift—including the remainder of this article—utilize a proportionality that is uniformly applicable at all wavelengths.
Sometimes it is preferrable to use the form:
Causes of redshift
Redshift according to various physical models can be due to three different effects:
- Movement of the source with respect to the observer, equivalent to the Doppler Effect
- Expansion of the universe which stretches the space between the source and the observer increasing the wavelength of a photon traveling between the two
- Gravitational effects of massive bodies
Movement of the source.
If a source of the light is moving directly 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 also called the Doppler redshift. If the source moves away from the observer with velocity v, then the redshift is given by:
Where c is the speed of light and is the Lorentz factor from special relativity. Since most velocities measured are small fractions of the speed of light, the Lorentz factor is very close to unity and the formula:
is correct to a very good approximation. For a more complete discussion on the origin of the frequency shift, see the article on the relativistic Doppler effect. Additionally, there is a special form of Doppler redshift due to the Lorentz factor where a redshift is seen even when the source is moving at right angles to the detector. This transverse redshift was first observed in the a 1938 experiment performed by Herbert E. Ives and G.R. Stilwell, called the Ives-Stilwell experiment [1].
Expansion of space
The current models of physical cosmology predict space on the largest scales to be expanding due to a time-dependent scale factor. Light will experience a redshift if it travels through expanding space, because the metric expansion of spacetime causes a coordinate shift in the electromagnetic wave to lower energies. Such an effect is exactly analogous to a redshift caused by a recessional velocity that increases with the distance away from the observer. Conversely, if the Universe were contracting instead of expanding, we would see distant galaxies blueshifted proportional to their distance instead of redshifted. While this redshift of distant galaxies appears as recessional velocities in the observed object, in general relativity stretching of spacetime is different from the physical movement of the source. For one, metric expansion of a vacuum can occur faster than the speed of light since there are no physical objects associated with spacetime but only measurements. As such, the galaxies are not moving outward; instead, the intervening space is increasing as described by the FRW metric. Nevertheless, astronomers sometimes refer to "recession velocity" in the context of the redshifting of distant galaxies from the expansion of the Universe, since the effect is Galilean invariant with physical movement of the galaxies. This type of redshift is also called the cosmological redshift or Hubble redshift.
Gravitational effects
The theory of general relativity holds that light moving through strong gravitational fields experiences a red- or blueshift. This is known as the gravitational redshift or Einstein Shift. 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. In the 1970s, science historians discovered a letter dated 1784, from John Michell a natural philosopher and geologist, to scientist Henry Cavendish, in which he considers the effect of a heavenly object massive enough to prevent light from escaping (see black hole), and using a prism to measure the gravitational weakening of starlight due to the surface gravity of the source. This letter has been considered to be the first prediction of gravitational redshift [2].
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 analyzing light from astronomical objects, often the absorption and emission features appear shifted to other frequencies in the proportional fashion predicted by 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. 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 also used in helioseismology to determine the precise movements of the photosphere of the Sun. Redshifts have also been used to measure the velocity of gas 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. As a diagnostic tool, measuring redshifts are one of the most important spectroscopic measurements made in astronomy.
Extragalactic observations
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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.
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 (in the context of conventional cosmological models). This was first proposed by Edwin Hubble and is known as Hubble's 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. Measuring the redshift is often 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 spread 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, an important tool for measuring dark matter.
Vesto Slipher was the first to discover galactic redshifts from ~1912, while Hubble correlated Slipher's measurements with distances he measured by other means to formulate his Law.
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:
- The gravitational attraction between galaxies has been acting to slow down the expansion of the Universe since then.
- The possible existence of a cosmological constant 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 first point, but accelerating (see accelerating universe). It is widely, though not quite universally, believed that this is because there is a form of the cosmological constant due to a scalar field dubbed dark energy. Such a cosmological constant also 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 zero size. 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 size becomes zero.
Redshift surveys
With the advent of automated telescopes and improvements in spectroscopes, a number of collaborations have been made to map the universe in redshift space,
Redshift and the variation of the physical constants
Redshifts of spectral lines are measured by assuming that the known physical laws have not changed, in particular the quantum physics that determines the position of an absorption or emission line on the electromagnetic spectrum is the same in all of spacetime. Some physicists including Paul Davies and the late John Bahcall have pointed out that variations in the absolute positions of the spectral lines could reveal gradual changes in the physics governing the universe, in particular, measurements of an absolute shift in spectral lines could indicate a change in α, the fine structure constant. If this quantity had varied in time, some typical spectral lines would be shifted to different wavelengths and some multiple line features would have different spacing. Measuring such changes would be an optimal test of the constancy of α. As of 2005, attempts to measure this have been inconclusive, and the issue remains open.
See also
- Blue shift
- Fingers of God - cosmological effect causing elongation of galaxies in redshift space
- Phase shift
- Relativistic Doppler Effect
- False Doppler
References
- . ISBN 0201547309.
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