Redshift

As redshift of electromagnetic waves extending the measured wavelength compared to the originally emitted radiation is called. The degree of redshift z is given as a ratio of wavelength change to the original wavelength:

The name is derived from the corresponding ratios in the visible spectrum, in which the longest wavelengths correspond to the red light. At even longer wavelength radiation, therefore, no shift to red out takes place, but away from it, and thus in the infrared radiation passing over.

The red shift is usually measured by means of the displacement of spectral lines, ie Emissions or absorptions atomic or molecular downer frequencies.

Of importance is the effect, inter alia, in astronomy, where the light from distant galaxies appears to be shifted to the red, and in molecular spectroscopy, where after elastic scattering with energy transfer photons occur correspondingly lower energy.

Causes

Causes the red shift can be:

The first three of the above-mentioned Causes are discussed in greater detail below.

Red shift due to relative movement

Emits an object electromagnetic radiation and it is absorbed by a second, this is relatively removed object, increases the measured at the moment of absorption against wavelength of the emitted, rising to the escape velocity of the two objects. This relativistic Doppler effect follows from the constancy of the speed of light: electromagnetic radiation moves at both the emission and in absorption, no matter how fast the source and target are moving relative to each other.

With a movement along a line (not tangential ) of the connection:

Gravitational redshift

The gravitational redshift is a direct result of gravitational time dilation. It is not strictly an effect of general relativity, but it is clear from the special theory of relativity and the equivalence principle of general relativity. Light (that is, the center of gravity away) emitted from a light source with a given frequency according to above is therein measured at a lower frequency. Thus, this means in particular that for a light signal having a predetermined number of oscillations of the time interval between the beginning and the end of the signal at the receiver is greater than the sender. This can be understood by the gravitational time dilation.

Due to the gravitational time dilation, the time interval between the beginning and end of the light wave is longer, the further up you move in a gravitational field, because the time passes faster and faster. This means that the shaft is always measured as it moves more upwardly. Therefore, the distance between wave peaks is always more to grow, so that the light that is always long wavelength, so appears less energy.

The gravitational redshift predicted by Einstein in 1911 before the completion of the general theory of relativity and can already be derived from the conservation of energy, so that their experimental confirmation is indeed a necessary condition for the validity of the general theory of relativity, but on the other hand, has not very great significance. From WS Adams, 1925, the redshift at the white dwarf Sirius B was detected. The measurement of the gravitational redshift of white dwarfs is difficult to distinguish from the red shift by the proper motion, and the accuracy is limited. Robert Pound and Glen Rebka reported in 1962 using the Mössbauer effect, the gravitational redshift of the radiation of a gamma source in the earth's gravitational field at an elevation of only 25 m with sufficient accuracy by (Pound - Rebka experiment). Subsequent improvements (Pound - Rebka Snider experiment) reached an accuracy of about 1.5%. The gravitational redshift was detected by space probes for the Sun and Saturn. The planned satellite OPTIS should, in addition to other tests for special and general relativity, gravitational redshift test with an accuracy of 10-5.

The development of atomic clocks has made it possible to measure the influence of gravity directly to the time. In principle, this measurement is a variation of the proof of the gravitational redshift. 1971 was clearly demonstrated by J. Hafele and R. Keating ( Hafele - Keating experiment) with Caesiumuhren in aircraft caused by the gravitational retardation of clocks at different heights according to the general theory of relativity with about 10 % accuracy. By a similar experiment by C. Alley (Maryland experiment), the accuracy could be increased to 1% in 1976. Vessot R. and M. Levine published 1979 results of a similar experiment with the help of rockets and gave an accuracy of 0.02 % at. At today's satellite-based GPS navigation system, both corrections in accordance with the special and the general theory of relativity must be considered, with the effects of general relativity predominate. Conversely, this can also be regarded as a confirmation of these theories.

Cosmological redshift

The expansion of the universe must not be taken to mean that galaxies in space-time apart from each other ( relative movement ). It is the space-time itself, which expands, the galaxies are moved. Gravitationally bound objects such as galaxies or clusters of galaxies do not expand, because they are by their self-gravity of the general movement of expansion ( described by the Friedmann equations) decoupled. This is particularly true for objects gravitationally bound within such systems are (stars, planets), and also for electromagnetically bound systems such as atoms and molecules. An electromagnetic wave, however, that propagates freely through an expanding space-time, the expansion motion is imparted directly: enlarges the space-time during the term by a factor, it also happens to the wavelength of light.

This so-called cosmological redshift is fundamentally different from the redshift by the Doppler effect, which depends only on the relative velocity of the galaxies in emission and absorption. The derived from the cosmological redshift escape velocities of distant galaxies are thus directly attributable to the expansion of space-time. Starting from distances of a few 100 megaparsecs, the proportion of the Doppler effect is negligible. It also follows from the general theory of relativity, that the observed flight speeds do not cause relativistic time effects, as described by the special theory of relativity for motions in space. A cosmological time dilation takes place nevertheless, since the later photons emitted an object must travel a greater distance due to expansion. Therefore physical processes appear at redshift objects ( in our view ) are increasingly slowed expire.

Redshift, blue shift and Cosmology

The light from galaxies is in most cases a red shift (already among the nearest 1000 is around 75 percent). The farther away a galaxy is, the greater the red shift in the mean. Only a few relatively nearby galaxies show due to additional " own " motion relative to the earth to us to an overall blue shift. An example is the Andromeda Galaxy.

Vesto Slipher conducted from 1912 spectroscopic observations of galaxies and their radial velocities determined from the line shifts. He soon realized that most of the galaxies observed by him had a redshift. 1929 Edwin Hubble discovered the relationship between redshift and distance of galaxies and led him back to a cosmological expansion. First, the effect has been interpreted as a Doppler effect. It increases with the distance of galaxies according to the Hubble constant, which is why you can estimate the distance by measuring the redshift.

The higher the redshift of an astronomical object, the longer was the emitted light from him on the road and the farther back in time we see it. From the redshift and the distance of the object can be determined, but this is not clearly defined in an expanding space-time. There are several distance measures that can be derived from the redshift. In cosmology, observations and calculations are therefore always employed in redshift space.

In October 2010, astronomers have been able to prove with the help of the Very Large Telescope, that the light of previously discovered with the Hubble Space Telescope Galaxy UDFy - 38,135,539 was 13.1 billion years to us on the go. With a Rotverschiebungsrekord of thus reaching us now for the first time observed light was emitted just 700 million years after the Big Bang; the galaxy was thus at a time when the universe was still small, fully transparent and by a factor of 9.6.

With the discovery of the galaxy UDFj - 39546284 in the Hubble Ultra Deep Field 09 shooting ( HUDF09 ) a cosmological redshift could be determined. The observed age Most shifts so that more 120 million years towards 580 million years after the Big Bang. The newly discovered galaxy with their age of 13.2 billion years would provide for a confirmation of the redshift is an important observation block to the development of the first galaxies after the Big Bang.

Relativistic derivation

The cosmological redshift is already beginning to dominate at distances of a few hundred megaparsecs over the redshift caused by relative motion (Doppler effect).

Consider a photon emitted by a galaxy with comoving distance (see also the relativistic derivation of the Friedmann equations), and absorbed by the observers. Both the galaxy and the observer follow the cosmic expansion. Oriented to the descriptive coordinate system so that the photon along its polar axis is running, then is the line element of the photon

Wherein the speed of light is the expansion factor, and is moved along the radial coordinate. Two successive maxima of the light wave is absorbed to the cosmological times and sent forth, and at the times and again. The wavelength of the photon in the time of emission and absorption are then

Moved along the distance traveled by the two peaks is the same, by definition. Integrating the line element of the photon, we obtain

Swapping the limits of integration is obtained for infinitesimally small intervals between emission ( absorption) of the two maxima

Using the emitted and absorbed wavelengths as given above, one can derive the ratio,

Finally, one then defines the cosmological redshift to

Since for most purposes the absorption time point coincides with the present time and, results in the simplified

Conversely, results results directly in the scale factor of the universe at time of issuance compared to the current value,

If one observes, for example, a galaxy with redshift, so had the universe at the time of emission of the light received by us only a quarter of its size. All physical processes in this galaxy run from the perspective of the observer from slowing down by a factor, since the distance between two successively emitted photons correspondingly increased, and thus their arrival at the observer ( cosmological time dilation ). A well-known example of this is the increasing extension of the light curves of supernovae of type Ia, the existence of which is well understood, with increasing redshift.

Measurement methods

In astronomy, the redshift is measured by methods of spectral analysis; they have become much more accurately by digital rather than photographic recording today. But to be able to detect spectral lines well, the galaxies must have a certain minimum brightness. Redshifts of galaxies are regularly determined on the basis of surveys such as the Sloan Digital Sky Survey new.

The gravitational redshift was observed in laboratory experiments on Earth with the help of Mößbauereffekts (see Mössbauer spectroscopy ).