How can the redshift be described


The cosmic time

The cosmological redshift z can be derived very easily from observations of spectra. If you are interested in how old the object in question is with a given redshift, you need a cosmological model. From the observation of the cosmic background radiation, a set of cosmological parameters is derived that describes our universe very well. These parameters include the proportion of dark energy, the proportion of dark and baryonic matter, the Hubble parameters and the curvature parameters. If you know it, you can assign an age to a redshift cosmic time (engl. cosmic time) is called. Cosmic time corresponds to the age of the universe since the Big Bang. It can be extracted from the Friedmann equations and shows a dependence on the cosmological parameters.

The diagram above shows the relationship between redshift and age of the universe as a graph. At z = 0 is the local universe, i.e. our immediate surroundings. A redshift z = 1100 (not shown here) marks the limit of the electromagnetically observable universe. Because at this redshift the occurred Recombination. For redshifts greater than about a thousand, the universe is not transparent (optically thick), because radiation cannot penetrate the primordial plasma of electrons and protons. During the recombination, the plasma was cool enough that one electron could be captured by one proton. Neutral hydrogen (HI) was formed, and the universe became transparent to radiation (optically thin). As you can see in the diagram under the entry Recombination, the universe was just about 400,000 years old when it became transparent. Our local universe is already 13.7 billion years old (left edge of the diagram above).

Milestones in Cosmology

The Distance information of distant, astronomical objects contain great uncertainties. That is why astronomers resort to cosmological redshift z back. It is much easier to determine and less prone to error than distance. The astronomers measure redshifts spectroscopically (spectro-z) or photometric (photo-z). Here are a few milestones in cosmology:

  • At z = 0 is the immediate cosmological neighborhood, that local universe. It was only in this late phase that the cosmos formed Life.
  • At z = 1.0 begins by definition the domain of highly redshifted Objects.
  • At z = 2.0 to 3.0 we observe the maximum of the abundance of the quasars. The quasar density was about 1000 higher at these times because the universe was smaller and more quasars formed.
  • At z = 5.0 was the age of Reionization of helium. This epoch is the 'preview' of the reionization of hydrogen.H preview') because astronomers observe them at shorter distances.
  • At z = 6.0 was the age of reionization of hydrogen, through intense, high-energy radiation sources, the first stars (Population III). In this area are very distant objects that have been observed: Hu et al. (2002) found with the Keck II 10m telescope behind the clusterAbell 370 a galaxy called HCM 6A. For them, a redshift of z = 6.56 (paper: astro-ph / 0203091).
    The most distant gamma-ray bursts are also included z ~ 6.
    In February 2004, a galaxy even further away was discovered in a similar way with the Keck telescope: The galaxy in question has a distance of 6.6 < z <7.1. The weak radiation is amplified by a factor of about 25 by a gravitational lens: the spatial upstream galaxy cluster Abell 2218 (z = 0.175) creates a triple image of the galaxy. At the time when the galaxy emitted its radiation, the universe was just 750 million years old according to current cosmological parameters! The galaxy is significantly smaller than, for example, the Milky Way (diameter about 100,000 light years), because it measures a maximum of 1.2 kpc, corresponding to about 4,000 ly. star formation rate, SFR) can be estimated from the (red-shifted) UV continuum. It is afflicted with a certain uncertainty, but 2.6 solar masses per year are quite high (source: Kneib et al., astro-ph / 0402319).
  • At z = 10.0 is the current distance record holderwho just two weeks after the galaxy with z = 7 was discovered. A collaboration of astronomers from Switzerland, France and California observed with the infrared camera ISAAC (Infrared Spectrometer And Array Camera) of Very Large Telescope (VLT) in Chile an infrared source in the galaxy cluster Abell 1835. This galaxy called Abell 1835 IR 1916 has a gigantic redshift of z = 10! The radiation of this galaxy reaches us from a time when the universe was only about 480 million years old (compare diagram that assigns a cosmic time to a given redshift). A gravitational lens in the foreground also helped with this observation, which led to an increase in brightness by a factor of 25 to 100. The total mass of this very young galaxy was estimated to be 500 million solar masses (including dark matter in the halo). With this observation, professional astronomy is moving ever closer to the Dark Age, which was ended by the first radiation sources in the universe (paper: Pello et al. 2004, astro-ph / 0403025).
  • The first cosmic radiation sources or the first elementary building blocks, which must have formed before the reionization era, are located at redshifts of 15 < z <30 on. Those first light sources put an end to that Dark ages (engl. dark ages) of cosmology. The first stars are assigned to population III. They were still relatively low in metal and were largely composed of hydrogen and helium.
  • At z = 1100 was the epoch of Recombinationwhere the radiation decoupled from matter and the universe became transparent. This is due to the fact that the atoms were formed (protons recombined with electrons) and thus the scattering cross-section for photons suddenly decreased. An observation of the universe with electromagnetic waves therefore leads back at most to this epoch. The cosmic background radiation in the field of microwaves, as used by the COBE (Cosmic Background Explorer) and WMAP (Wilkinson Microwave Anisoptropy Sample) was able to measure is precisely the relic that reaches us from this era of the young universe. With current cosmological parameters (flat universe, proportions of dark energy and matter, value of the Hubble constant), an age of the universe of about 400,000 years can be assigned to a redshift of 1100 (see diagram). Astronomers could only look deeper into space, at even earlier phases of development of the universe, with gravitational waves. So far, these distortions of space-time, which propagate at the speed of light, could not be detected. However, physicists are convinced that they must exist.

Spacious structure

With this information you can cosmography operate: you get a map of the large-scale structure of the universe. This is what astronomers have in the survey, for example 2dF done. They used the Anglo-Australia-Observatory (AAO) in Australia and determined the spectra of almost 250,000 objects. For the most part, these were galaxies, all of which together cover around 1500 square degrees of the sky. The following picture shows a result of 2dF, which shows very nicely the large-scale structure in Komos. It is the spatial distribution of galaxies in a fan-shaped section of the sky (Credit: 2dF Galaxy Redshift Survey, 2003; large version):

Every blue point is a galaxy! In total, this section shows almost 63,000 galaxies. The earthly observer sits at the intersection in the middle of the picture. The further you move to the edge of the fan, the higher the redshift and distance - maximum z ~ 0.2. Both dates were given on the right of the axes. The time is an indication of the angle of the position in the sky. As you can see, the distribution of the galaxies is not even and homogeneous, but very irregular. This photo reveals one honeycomb structure, which consists of individual galaxies, galaxy clusters and super galaxy clusters at the nodes of the honeycombs. Inside the honeycomb there are huge 'empty spaces' that Voids. The irregular distribution is mainly a result of gravity, because it causes masses to attract and clump together over time. As shown in the entry Dark Matter, the clumping of dark matter (increasing towards small redshifts) could recently also be directly observed.
The picture also shows that as the redshift increases, fewer and fewer blue dots can be seen. This is essentially an instrumental effect and not a cosmological one: the further away the galaxy is, the more the brightness decreases, so that many galaxies simply cannot be observed in the survey. The aim of such studies is therefore to improve the observation technique to such an extent that astronomers can look deeper into space (to even higher z), so that the large-scale structure can also be studied there.

astronomical measurement of redshift

Experimentally one can gain access to the important astronomical quantity Redshift by detecting spectra (Spectrography) and identified known spectral lines there. The astronomer Maarten Schmidt interpreted the emission lines of the in 1963 in this way Quasars as lines of particularly high redshift, so that quasars (then as now) are among the most distant objects. Due to the great distance, the spectra of active galactic nuclei (AGN) are particularly relevant because they can still be detected at all due to their enormous luminosity. Particularly characteristic and easy to recognize in these objects is the Lyman edge of hydrogen (Lyman-α, Lyα). Due to the cosmic expansion, this edge is shifted towards the red end. If one compares this observed wavelength with the laboratory wavelength, i.e. the wavelength in the terrestrial laboratory at z = 0, the quotient just provides the redshift factor G and thus also the redshift z.