Reionization

In Big Bang cosmology, the Reionisierungsepoche corresponds to the period in which the matter of the universe ionized ( reionisierte ) before the universe became transparent to visible light again. This period is the second major phase transition of hydrogen gas in the universe. In this sense, the universe is ionized today.

The first phase transition was the so-called Rekombinationsepoche that about 400,000 years ( redshift ) took place after the Big Bang. The universe cooled down it as far from ( below 3000 K), that an interaction of electrons and protons to form stable, neutral hydrogen was possible. The hydrogen production rate was higher than the ionization of hydrogen. Since the electrons in neutral hydrogen atoms (as well as in others) can absorb energy in the form of photons to reach an excited state, the universe for certain wavelengths, which account for the excitation of atoms, opaque.

The second phase transition began when objects formed in the early universe that were energetic enough to ionize hydrogen. While these objects formed and the radiated energy, changed the universe back from the neutral state to an ionized plasma. This period lasted approximately between 150 million to 1 billion years ( redshift ) after the Big Bang. If protons and electrons are separated from each other, they can not absorb energy in the form of photons. Although photon may be scattered, however, the scattering is always less frequently at a low density of the plasma. This creates a universe with ionized hydrogen at low density is relatively transparent to light, as our current universe.

Energy sources of the reionization

Although the range in which the reionization may have occurred, was limited by observations, it is uncertain which objects provided the energy for it. In order to ionize hydrogen is an energy 13.6 eV needed. This corresponds to photons with a wavelength of less than or equal to 91.2 nm, this radiation is in the ultraviolet region of the electromagnetic spectrum. Now all objects that use large amounts of energy in the ultraviolet range, and leave about come into question. It is also to consider the total number of objects, as well as their service life, because again occurs recombination of protons and electrons, if not enough power is provided to tell them apart. The critical parameters of these objects is thus the " emission rate of photons for the ionization of hydrogen per unit volume cosmological " ( " emission rate of hydrogen - ionizing photons per unit cosmological volume" ). With these restrictions, it is expected that quasars and the first generation of stars ready to put these energies.

Quasars

Quasars are good options for these energy sources as they are very efficient in converting mass into radiation and emit a lot of light with energies above the limit for the ionization of hydrogen. However, it raises the question whether there is enough quasars were present in this era of the universe. It is up to now only possible to detect the brightest quasars in the Reionisierungsepoche. That is, there is no information on weaker quasars that existed policy. However, it is possible to use the well to observable quasars in the local universe for an estimate. Assuming that the number of quasars as a function of luminosity during the Reionisierungsepoche was about the same as today, it is possible to determine the Quasarpopulation to earlier times. Such studies have shown that quasars do not occur in sufficient numbers to ionize the intergalactic medium alone. This would only be possible if the ionizing background would be dominated by low-luminosity active galactic nuclei. Quasars are among the most active galactic nuclei.

Population III stars

Population III stars are stars that consist of no heavier elements than hydrogen and helium. During the nucleosynthesis formed in addition to hydrogen and helium only small traces of lithium. Nevertheless, spectral analysis of quasars have revealed the presence of heavy elements in the intergalactic medium in the early Universe. Supernova explosions produce such elements, that are hot, big star of the third population, which end in supernovae, a potential source of reionization. Although they were not directly observed, they are consistent with models based on numerical simulation, as well as other observations. Another indirect proof is a galaxy that has been distorted by gravitational lensing. Even without direct observation of these stars appear to be a reliable source for the theory. They are more efficient and effective ionization sources as stars of the second population, since they emit large amounts of photons and are powerful enough to reionisieren hydrogen alone, they should have a reasonable initial mass function according to some models. Therefore, star of the third population as the most likely source of energy are considered, which could have started the reionization.