Reionization epoch

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Universe timeline showing the reionization era compared to the history of the universe

The reionization corresponds to the big bang cosmology the period in which the matter is the universe again ionized (reionisierte) before the universe was transparent to visible light. 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 recombination epoch , which took place around 400,000 years ( redshift ) after the Big Bang. The universe cooled down so far (below 3000 K) that an interaction of electrons and protons to form stable, neutral hydrogen was possible. The hydrogen formation rate was higher than the ionization rate of hydrogen. Since the electrons in neutral hydrogen atoms (as well as in others) can absorb energy in the form of photons in order to get into an excited state , the universe is opaque for certain wavelengths that make up the excitation of the atoms.

The second phase transition began when objects formed in the early universe that were energetic enough to ionize hydrogen. As these objects formed and radiated energy , the universe shifted from its neutral state back to an ionized plasma . This period lasted between 150 million to 1 billion years ( redshift ) after the Big Bang. When protons and electrons are separated from each other, they cannot absorb energy in the form of photons. Photons can be scattered , but the scattering is less and less common with a low density of the plasma. As a result, a universe with ionized hydrogen at low density is relatively translucent, like our universe today.

Reionization energy sources

Although the area in which the reionization could have occurred has been narrowed down by observation, it is uncertain which objects provided the energy for it. An energy above 13.6 eV is required to ionize hydrogen  . This corresponds to photons with a wavelength less than or equal to 91.2  nm . This radiation is in the ultraviolet part of the electromagnetic spectrum . This means that all objects that emit large amounts of energy in the ultraviolet range and above are eligible. The number of these objects must be considered as well as their lifespan, since a recombination of protons and electrons takes place again if enough energy is not provided to keep them apart. The critical parameter of one of these objects is the “emission rate of photons for ionizing hydrogen per unit cosmological volume” (“emission rate of hydrogen-ionizing photons per unit cosmological volume”). With these limitations, it is expected that quasars as well as the first generation of stars would provide these energies.

Quasars

Quasars are good options for these energy sources because they convert mass into radiation very efficiently and emit a lot of light with energies above the limit for ionization of hydrogen. The question arises, however, whether there were enough quasars in this epoch of the universe. So far it is only possible to detect the brightest of the quasars in the reionization epoch. That is, there is no information about any weaker quasars that might exist. However, it is possible to use the easily observable quasars in the near universe for an estimate. Assuming that the number of quasars as a function of luminosity during the reionization epoch was roughly the same as it is today, it is possible to determine the quasar population at earlier times. Such studies have shown that quasars are not present in sufficient numbers to ionize the intergalactic medium on their own. This would only be possible if the ionizing background were dominated by faint active galactic nuclei . Quasars belong to the active galactic nuclei.

Population III stars

Simulated image of the first stars, 400 million years after the Big Bang .

Population III stars are stars that are made up of no heavier elements than hydrogen and helium . During the nucleosynthesis , apart from hydrogen and helium, only small traces of lithium were formed . Even so, spectral analysis of quasars has revealed the presence of heavier elements in the intergalactic medium in the early Universe. Supernova explosions produce such elements, so hot, large, third population stars ending in supernovae are a possible source of reionization. Although 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 the gravitational lensing effect . Even without direct observation, these stars seem to be a reliable source for the theory. They are more efficient and effective sources of ionization than stars of the second population because they emit larger quantities of photons and, according to some models, are powerful enough to reionize hydrogen on their own if they have an adequate original mass function . Hence, third population stars are considered to be the most likely source of energy that could have started reionization.

Evidence from the observation at the 21 cm wavelength of neutral hydrogen

It is hoped that the observation of the 21 cm wavelength will provide evidence of the end of the Dark Age and the reionization epoch. The signal for these epochs is now strongly redshifted (at around 50 to 100 MHz). In 2018, the Edges collaboration (Experiment to Detect the Global Epoch of Reionization Signature) announced the observation of an absorption profile at 78 MHz, which points to the reionization epoch (around 180 million years after the Big Bang, i.e. z = ~ 20) . There are also indications from it that may point to dark matter. The signal was very difficult to observe as it is heavily obscured by terrestrial sources, galactic radiation and other sources. It is hoped that the planned square kilometer array will provide better data .

See also

Web links

Individual evidence

  1. ↑ a b Piero Madau et al .: Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Source . In: The Astrophysical Journal . 514, 1999, pp. 648-659. doi : 10.1086 / 306975 .
  2. ↑ Loeb and Barkana: In the Beginning: The First Sources of Light and the Reionization of the Universe . In: Physics Reports . 349, 2000, pp. 125-238. bibcode : 2001PhR ... 349..125B . doi : 10.1016 / S0370-1573 (01) 00019-9 .
  3. ^ Paul Shapiro & Mark Giroux: Cosmological H II regions and the photoionization of the intergalactic medium . In: The Astrophysical Journal . 321, 1987, pp. 107-112. bibcode : 1987ApJ ... 321L.107S . doi : 10.1086 / 185015 .
  4. ↑ Xiaohu Fan, et al .: A Survey of z> 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z ~ 6 . In: The Astronomical Journal . 122, 2001, pp. 2833-2849. bibcode : 2001AJ .... 122.2833F . doi : 10.1086 / 324111 .
  5. ^ Nickolay Gnedin and Jeremiah Ostriker: Reionization of the Universe and the Early Production of Metals . In: Astrophysical Journal . 486, 1997, pp. 581-598. bibcode : 1997ApJ ... 486..581G . doi : 10.1086 / 304548 .
  6. ^ Limin Lu et al .: The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium . In: Astrophysics . 1998. arxiv : astro-ph / 9802189 .
  7. ↑ RAE Fosbury et al .: Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357 . In: Astrophysical Journal . 596, No. 1, 2003, pp. 797-809. bibcode : 2003ApJ ... 596..797F . doi : 10.1086 / 378228 .
  8. ↑ Jason Tumlinson et al .: Cosmological Reionization by the First Stars: Evolving Spectra of Population III . In: ASP Conference Proceedings . 267, 2002, pp. 433-434. bibcode : 2002hsw..work..433T .
  9. ↑ Aparna Venkatesan et al .: Evolving Spectra of Population III Stars: Consequences for Cosmological Reionization . In: Astrophysical Journal . 584, 2003, pp. 621-632. bibcode : 2003ApJ ... 584..621V . doi : 10.1086 / 345738 .
  10. ^ Marcelo Alvarez et al .: The H II Region of the First Star . In: Astrophysical Journal . 639, 2006, pp. 621-632. bibcode : 2006ApJ ... 639..621A . doi : 10.1086 / 499578 .
  11. ^ Edges, MIT Haystack Observatory
  12. ^ Judd Bowman, Alan Rogers, Raul Monsalve, Thomas Mozdzen, Nivedita Mahesh: An absorption profile centered at 78 megahertz in the sky-averaged spectrum, Nature, Volume 555, 2018, pp. 67-70, abstract
  13. ↑ Joshua Kerrigan, First Detection of the 21cm Cosmic Dawn Signal , Astrobites, March 14, 2018
  14. ↑ Jennifer Chu, Astronomers detect earliest evidence yet of hydrogen in the universe emitted just 180 million years after Big Bang, signal universe indicates was much colder than expected , MIT News, February 21, 2018