The process of matter re ionization in the universe
Re ionization isbig bangThe dark period of cosmology (seeDark Age of the Universe）Later, the process of re ionization of matter in the universe is the two main processes of gas in the universephase transitionThe second time in.When the mainbaryonic matter In the form of hydrogen, re ionization usually refers tohydrogenIonization of gases.The original helium in the universe has also experienced the same phase transition, but it is different in the history of the universe, and is usually called helium re ionization.
stayBig BangAt the initial stage, the material is in a high temperature and densityPlasmaState, with the expansion of the universe and constantly cooling.Protons and electrons recombined into hydrogen atoms, and the almost completely neutral universe entered a relatively quiet "dark period".And in our universe today,Intergalactic mediumThe gas in is highly ionized.In between, the universe experienced a very important stage of evolution from neutral to ionization - re ionization.The cosmic re ionization began when the first generation stars formed and emitted the first ray of dawn of the universe (about 400 million years after the Big Bang). Some of the high-energy photons emitted by these stars and galaxies leaked out, ionizing the relatively thin gas around the galaxy.With the continuous formation of galaxies, the ionization regions gradually expand and connect with each other.When the ionization zone covers the intergalactic medium in the whole universe, re ionization is completed.The re ionization of the universe is a key stage in the formation and evolution of galaxies, and is also an important blank in the history of the evolution of the universe that has been recognized by mankind so far. Therefore, it has become a very active research direction in cosmology and astrophysics in recent years.[1]
Detection method
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Reviewing the universe so far has brought some observational challenges.However, there are several observation methods used to study re dissociation.
Quasars and Gunn Peterson Trough
An important research on re dissociation is to use remotea quasarSpectrum of.Quasars emit a great deal of energy, which means that they are one of the brightest objects in the universe.Some quasars can even detect the early stage of re ionization.Quasars also have relatively consistent spectral characteristics, regardless of their position in the sky andearthDistance.Therefore, it can be inferred that any differences in the spectra of quasars are caused by the interaction with atoms in the line of sight direction.Lehmann conversionIn visible lightwavelengthIn terms of energyScattering cross section, which means that even if there is only a small amount of neutral hydrogen in theIntergalactic medium(IGM) at these wavelengthsabsorbIt will still be obvious.
In the celestial bodies near quasars, the absorption line of the spectrum is very sharp, because even if the energy of photons only causes the conversion of one atom, it can also cause this change.However, the distance between quasars and the telescope used for detection is very large, which means thatCosmic expansionCauses the received light to redden.This means that when the light of quasars redshifts during the journey and passes through the intergalactic medium (IGM).Since the wavelength of components that were shorter than Lyman α has been redshifted, the wavelength of some radiation components after redshift just corresponds to the wavelength of Lyman α, so from the corresponding wavelength of Lyman α line at the redshift of the quasar, there will be continuous absorption towards its short wavelength direction.This means that the obvious spectral line is replaced by the continuous spectrum, and the neutral hydrogen distributed in different regions of the quasar light through a wide space showsGunn Peterson slot。
The appearance of these redshifts allows us to capture fragments of information about the period of re ionization.Because the redshift of the celestial body corresponds to the time when the light we see radiates, it may be able to establish the time point when the period of re ionization ends.Quasars with redshifts below a certain value do not exhibit the Guann Peterson trough (although they may exhibitLyman-alpha forest ）, because the re ionization that will show the characteristics of Gunn Peterson trough is earlier than the light emitted by these quasars.In 2001,Sloan Digital Sky SurveyFound the redshift inz=5.82 toz=4 quasars between 6.28, of whichz=6 shows Gunn Peterson trough, which shows that at least part of IGM at this place is neutral hydrogen, and those below this value do not.It is speculated that the re ionization takes place in a relatively short time scale, and this result shows that the universe is approachingz=Re ionization ended in time of 6.This, in fact, shows that the universezThe moment of>10 has almost been neutralized.
Anisotropy and polarization of cosmic microwave background radiation
Cosmic microwave background radiationAnisotropy at different angles can also be used to study re ionization.When the photon is still a free photon, there is aThomson scatteringProcess.However, when the universe expands, the density of free electrons will decrease, and the frequency of scattering will also decrease.In the period after re ionization, but before the electron density will be sufficiently low and significant expansion occurs, the light from the calm CMB will experience observable Thomson scattering.These scattering will mark the anisotropy map of CMB and import the second anisotropy (import after recombinationanisotropy)。The overall effect removes the anisotropy that occurs on smaller scales.Although the anisotropy on a small scale will be deleted, because re ionization will indeed importpolarizationThe anisotropy of.Note that the electron train density in the period of re ionization can be determined by observing the anisotropy of CMB and comparing it with the area that does not appear to have re ionization.On this basis, the cosmic age at the time of re ionization can be calculated.
Wilkinson Microwave Anisotropy DetectorThis phenomenon can be compared.The initial observation, released in 2003, believed that re ionization occurred at 11zBut the results of the study on the spectral observations of quasars are obviously inconsistent with the range of these redshifts.However, the data observed by WMAP for three years give different results, and re ionization starts fromz=11 and cosmic ionizationz=7. This is in good agreement with the quasar data.
21cm line
Ionization degree at different redshifts
Even though the data of quasars are roughly consistent with the anisotropic data of the cosmic microwave background radiation, there are still some problems, especially with regard to the energy sources and effects of re ionization, as well as the role played in the structure formation during re ionization.Hydrogenic21cm lineIt may be an important tool for studying this period and the "dark period" before re ionization.
The 21cm line is the hyperfine structure spectral line of the ground state of neutral hydrogen atom, which is directly connected with neutral hydrogen in the universe.On the one hand, because the spontaneous transition probability of the 21cm line is very small (it takes about 10 million years for each hydrogen atom to spontaneously transition once on average), it is difficult to saturate in a large neutral degree, or even completely neutral environment, so it is very suitable for detecting the neutral structure in the period of cosmic re ionization.On the other hand, the 21cm line is a spectral line with a certain frequency. The 21cm spectral line observed in different radio bands corresponds to signals at different redshifts, so we can get three-dimensional information about the evolution of the cosmic structure and the ionization process of intergalactic media.[1]The 21cm line occurs when the electron spin of neutral hydrogen is converted between parallel and antiparallel, and this conversion is prohibited, which means that it is difficult to occur, and this conversion also requireshigh temperature, which means formed in the "dark period" and radiatedphotonHeated the surrounding neutral hydrogen atoms, causing the surrounding area to radiate more 21 cm lines.
There are two main methods to detect cosmic re ionization using 21cm spectral lines.The most discussed method is the 21 cm tomography method, that is, using the cosmic microwave background radiation as the background source, to observe the signal generated by the absorption or emission of 21 cm photons of background radiation by the interstellar medium at different redshifts.The 21cm spectral line of the hydrogen atom has a characteristic temperature - spin temperature. According to the relative height of the spin temperature and the brightness temperature of the cosmic microwave background radiation, the hydrogen atom in the interstellar medium will emit or absorb 21cm photons, making the brightness temperature of the microwave background slightly increase or decrease, thus making the brightness temperature of the cosmic microwave background fluctuate to a certain extent.The second method is "21 cm forest" observation.Such observations are quasars with very high redshifts (redshifts above 6) orgamma-ray bursts As the background radio radiation source, the afterglow of is used to detect the 21 cm absorption line generated by various structures in the line of sight direction.The structures with different redshifts produce absorption lines at different frequencies on the afterglow spectra of quasars or gamma bursts, forming a "forest" like spectral structure.Similarly, the strength of the 21cm absorption line reflects the temperature, density, ionization degree of the absorber and the radiation status of the ionization source.Different from the 21 cm tomography method, the 21 cm forest signal is more sensitive to the temperature of the interstellar medium, and can more effectively extract the information about the evolution of the cosmic temperature.
By studying the 21 cm line radiation, we can learn more about the formation of early structures.Although there is no result yet, there are several projects in progress21 cm line array(21CMA（21 Centimeter Array）)、Low-frequency array(LOFAR（Low Frequency Array）)、Murchison wide-angle array(MWA (Murchison Wide Field Array) andRice wave radio telescope(GMRT (Giant Meterwave Radio Telescope)) is expected to make progress in this field in the near future.
Energy source
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Although the data obtained from observation narrowed the scope of the re ionization era, it is still uncertain which celestial body provided photons to re ionize IGM.Ionize neutral hydrogen, only 13.6Electron voltThis is equivalent to photons of 91.2 nm or shorter wavelength.This isElectromagnetic spectrumThe middle part is the ultraviolet part, which means that all the main candidates are ultraviolet rays and photons with higher energy.There are many sources that must be considered, such as long-lived protons and electrons, but they will recombine if they do not continue to supply energy to separate them.At the same time, the key parameter to consider any source is "the emissivity of hydrogen ionized photons per unit of cosmological volume".Due to these limitations, quasars and the first generationfixed starIs the main source of these energies.
a quasar
Quasars are good candidates because they can efficientlyqualityConvert toenergyAnd radiates a large amount of light with energy above the ionized hydrogen threshold.However, it is not known how many quasars existed before re ionization.When re ionization is in progress, only the brightest quasars can be detected, which means that there is no direct data about the existence of darker quasars.However, by looking at the quasars that are easy to observe in the nearby cosmic region, and assuming the brightness function of the period of re ionization (the number of quasarsbrightnessFunction) is roughly the same as today's distribution, which will allow us to estimate the number of early quasars.Such research found that there is not enough quasars to cause the re ionization of IGM alone, that is, "only when the low brightness active galactic nucleus (AGN) dominated by the re ionization background is also a quasar, can it provide enough ionized photons".Note that quasars are a type ofActive galactic nucleus, or AGN.
The third star family
Simulating the image of the first star 400 million years after the Big Bang
The third star familyIt is a star made of nothing heavier than helium.WhenPrimordial nucleosynthesisExcept trace traceablelithiumIn addition, helium is the only element synthesized from hydrogen.However, the spectra of quasars show that there are heavy elements in the early IGM.SupernovaThese heavy elements can be produced by the explosion of the third star group, which is hot and huge and can form supernovae, may be the mechanism of re ionization.Although they are not directly observed, they are consistent with the simulation and observation of the numerical and qualitative analogy model.Gravity lensGalaxies also provide indirect evidence of the third star family.Even if the third star group is not directly observed, it is still a convincing source.They can radiate more photons than the stars of the second family of stars, cause re ionization more efficiently, andInitial mass functionCorresponding to some of their own re ionization models, hydrogen can be re ionized.As a result, the third star group is considered to be the most likely energy source to launch the cosmic re ionization.
