Our universe is cosmic due to the vastness of astronomical things. Our universe consists of matter and energy due to which galaxies, stars, and planets formed. An important event in the history of the universe is the Big Bang which is related to the cosmological development of the universe. Here in this guide, we will study about cosmic reionization and its implications in detail.
During the formation of our universe, two major processes, recombination, and reionization, occurred. To study the reionization process we have to understand cosmic history and reionization models. The age of the universe is 13.7 billion years ago.
Our universe is made up of a hot dense state of gas and it is continuously expanding due to the Big Bang. One of the end products of the Big Bang was cosmic microwave background radiations and these radiations traveled freely to space and gave the first light to the universe.
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1. The Cosmic History
The universe became opaque at shorter wavelengths due to atomic hydrogen absorption after the cosmic microwave impression. These changes are the sources of galaxy formation. Primordial universe study is possible with the help of cosmic microwave background. A.Penzias and R.W. Wilson discovered CMB in 1965. The intergalactic medium became clumpy due to the process of reionization and reheating started in the universe.
The temperature of CMB is approximately 2.726 kelvin. The cosmic microwave background has been considered monoenergetic with photon energy. Cosmic structure originated from hot dark matter. Along with dark matter, the other constituents that made the universe are CMB radiation, massless neutrinos, dark matter, massive neutrinos, and dark energy. Cosmic structure growth is differentiated into three phases-
1.1. The First Phase
The first phase is the duration between the Big Bang and the formation of the cosmic microwave background- After the Big Bang in the universe, there was the emission of radiation which is lately known as CMB.
Along with CMB, there was an emission of different particles like quarks, neutrinos, photons, electrons, dark matter, etc. During this time Quarks teamed up in trios and formed protons and neutrons. These protons and neutrons made atomic nuclei of hydrogen and helium.
1.2. The Second Phase
The second phase is the duration when the universe starts cooling after the Big Bang- In this period the temperature of the universe reaches 3000 kelvin and provides conditions for the formation of stars and galaxies.
At that time the protons and electrons combined and formed atoms of neutral hydrogen. This recombination of hydrogen in the Intergalactic medium marked the beginning of the Dark Ages. At that time photons were transparent and free to propagate across the universe and observed as a cosmic microwave background. Dark matter particles constitute the cosmic web, later they formed stars and galaxies.
1.3. The Third Phase
The third phase is the time when the Dark Ages ended- In this phase, the light starts passing through the universe with the formation of stars and galaxies.
The first stars were formed of hydrogen and helium from which light was able to pass out. The life duration of the first star was short, they exploded soon after their formation and released their material in the surroundings. These materials trigger the birth of large-scale structures in the universe.
2. The Formation of Galaxies
Galaxies are massive groupings of billions of stars and other materials. All stars are gravitationally attracted to each other and are found in clusters of dozens to hundreds. The most giant galaxy is Alcyoneus, which is 100 times bigger than our galaxy, the Milky Way.
It is believed that after one billion years of the Big Bang, the first galaxy took shape. According to research, it is believed that after 400 million years of the Big Bang, GN-Z11 came into existence. There are two theories about galaxy formation.
According to the first theory, galaxies were formed by vast clouds of gas and dust that collapsed under their gravitational pull. According to the second theory, after the explosion of the universe, there were small lumps of matter that joined together with time to form galaxies.
2.1. Shapes of the Galaxies
Astronomical observations verified three shapes of galaxies: elliptical, spiral, and irregular. The shape of our galaxy Milky Way is spiral because of excessive dust and gas which are still forming new stars in their arms. The process of star formation and degradation with the interaction of interstellar medium is constantly happening in galaxies.
The space between stars and galaxies is filled with matter (gas in ionic, atomic, and molecular form, dust, and cosmic rays) and radiation that part is called the interstellar medium. The study of the galaxy and the life cycle of stars is possible with the help of interstellar medium.
3. The Formation of Stars
The aggregation of stars forms a galaxy. The fundamental unit of the galaxy is a star. Cold dense gas is the major requirement for star formation. There are different techniques to study the formation of the first star and galaxy. One of the techniques is an observation of infrared astronomy. James Webb’s space telescope helps to calculate infrared astronomy.
Stars are formed due to gas clouds over the galaxy. It is believed that early galaxies would have contained primordial gasses and clumps of these gasses condensed and formed the first star. The primordial gas clouds contained only two elements hydrogen and helium. When the newly born stars start to ionize and heat their surroundings, they emit lots of hydrogen-ionizing photons.
These energetic photons convert the hydrogen into its HII form. The Orion Nebula is known as the HII region. This region is of any shape because of the irregular distribution of stars and gas. Thousands of stars over several million years can be produced by the HII region’s Cosmic Expansion.
The age of our solar system is approximately 9 billion years. According to NASA, the growth of the universe will continue. The universe expansion is related to redshift. When the universe expanded light traveling through space stretched that phenomenon was called cosmological redshift.
The distance traveled by light is directly proportional to redshift. The phenomenon in which electromagnetic radiation increases the wavelength of the object is called redshift. It is denoted by z. The redshift z is the shift in wavelength in the spectra.
3.1. The Hubble Constant
The Hubble constant tells us, about the expansion of the universe. In 1929, the Hubble constant was calculated by Edwin Hubble by measurements of stars. The Hubble constant is believed to be around 70 km/s/Mpc, but there is still a long way to go. It is said that the value of this constant is uncertain because of the expansion of the universe constantly. This constant tells us about the rate of moving distant galaxies from nearby galaxies.
The invention of Hubble Law is important for studying dark matter and energy as well as predicting the exact age of the universe. The cosmic scale factor is the procedure by which the expansion of the universe can be explored. Redshift and cosmic scale are interrelated to each other. with the help of this scale, the changing separation of two points can be measured.
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4. Models of the Reionization Process
4.1. Reionization History
About 500000, years after the Big Bang reionization began. The beginning of reionization process started with the formation of the first star. The lifespan of the reionization process was between the formation of the first star and the ionizing of the intergalactic medium.
After the end of the Dark Ages, the neutral atoms of hydrogen were formed by reionization. The reionization of intergalactic hydrogen made the universe transparent. Throughout the first ten years of the universe’s history intergalactic hydrogen was generated by introducing and transferring Lyc radiation from intergalactic space into the atmosphere. It’s an easy story about UV sources and sinks.
4.2. The Physics of Intergalactic Gas
The IGM houses most cosmic ray baryons in redshift. It forms a filamentary network facilitating galaxy formation and gives birth to an Oxford of Lyman-A () absorption line, the sign of a high fluctuation medium at temperatures typical of photoionizing gases.
It is dark matter that provides the backbone of large-scale structures in the Universe, a web-like pattern present in embryonic form in the over-density motif of the initial fluctuation field but not sharpened by nonlinear gravitational dynamics. The intergalactic medium is dependent upon redshift z.
4.3. Epoch of Reionization
The Epoch of Reionization is the period between the dark ages and the formation of the first star and galaxy. Before this era, the universe was dark, dense, and covered with a fog of primordial gas. After the 400 million years of the Big Bang, there was the beginning of an epoch of reionization. During the epoch of reionization light enters deeply into space due to the pushing of gas away by stars and galaxies.
The contribution of dwarf galaxies is nearly 30% of the ultraviolet light during reionization. To understand the reionization process it is necessary to study recombination which balances the photoionization. Recombination is the process when photons are released due to the attraction of ionized hydrogen and free electrons at high temperatures by coulomb force. The first change in hydrogen in the universe was due to recombination at redshift.
5. Models of Reionization
The universe is very vast and there are various large- and small-scale structures. previously studying the local universe was difficult. After much research, the toy model and the Tanh-based model were discovered for observation and theoretical studies of the epoch of reionization.
5.1. Toy Model of Reionization
This model helps to describe the reionization signal and also explains the properties of the ionizing sources. This model captured the actual evolution of reionization signals.
5.2. The Tanh-Based Model of Reionization
During Tanh’s parameterization, two parameters describe the 2 key characteristics of the reionization. The same Tanh-based fitting function has been used in the PL 21-mm signal using somewhat different parametrization. There are different models of reionization based upon different properties like hydrodynamical simulations, and numerical, analytical, and semi-analytical methods.
5.3. Hydrodynamical Simulations
Reionization of neutral hydrogen is related to ultraviolet radiations emitted by the first galaxies and Quasars. There are three models based on reionization and ultraviolet background hydrodynamical simulations.
5.3.1. Homogeneous Reionization
Excursion set-based models while able to characterize reionization in homogeneous processes where over-dense regions are photoionized after void formation the PDF-based model’s implicit assumption that there is uniform light reflected in the universe is inadequate.
The successes with extended Press – Schechter formalisms for defining the correlation of peak distributions in the primordial mass distribution have inspired research to create an analogy approach for studying peaks in mass.
5.3.2. Inhomogeneous Reionization
In this method, each resolution element is given a specific reionization redshift. The ultraviolet background of the resolution element becomes zero due to a smaller reionization redshift of the resolution element as compared to the redshift of the simulation.
5.3.3. Flash Reionization
In this method, the heat is injected in a fixed reionization redshift for all resolution elements in the simulation.
6. The Numerical Methods
These methods are based upon the relationship between hydrogen density field and 3-dimensional volume and are called numerical methods of reionization. Numerical algorithms describing the dynamics of collisions and baryons are well-based and mature. Four different numerical models help study reionization.
Volume averaged analytical model, This model is based upon the relation between the number of ionizing photons and hydrogen atoms at a particular volume. Arons and McCray studied this model for the first time.
Spatially dependent semi-numeric model, this model is based upon the assumption that the region must be ionized if the number of baryons is increased by several ionizing photons of recombination. This technique does not require accurate numerical calculations on the full nonlinear matrix.
Information about the spatial distribution of the H II region is directly generated by the Gaussian random field in the initial condition’s initial derivative 21cm fast. This is the relationship between the distribution of ionized gas at a particular redshift. It uses the excursion set-based formation.
In radiative transfer in an expanding universe, in an expanding Universe, it’s not possible to detect scattering in the beam; the evolution of the specific intensity is described through radiative waves. This assumes that gas combines with dark matter and then occupies the ring of light with galaxies that produce ionizing radiations. This model calculates the arrangement of dark matter.
6.1 Full Radiation Hydrodynamic Galaxy Formation Simulations-
This is the most accurate calculation. There are two methods of this model.
6.2 Moment Method-
The moments method uses a radiatively transferred equation (15) as a method generating a hierarchy of moments of radiation intensity.
Angular structure describes the angular moments of the radiations because it depends upon energy, flux, and radiation pressure. In rayPart tracing, in this method, radiation and rays are propagated that extend through a computational grid or particle set.
7. The Analytical Models of Reionization
Typically to zeroth orders, the tracing of cosmic re-ionization involves photon counting where the H II regions’ growth rate corresponds to the production rate minus the consumption rate. Despite their appearance, they use in some sense similar bookkeeper algorithms on different spatial scales.
Dark matter observation by the semi-analytical method, in this category, we place a technique that relies on dark matter.
Cold Dark Matter-dominated galaxy formation model, this section focuses on a toy model used previously for observation and theoretical studies. However, the astrophysics approach used for toy models is not unbiased and problematic.
The particular model assumed can lead to systematic biased results when it is not reasonably approximative to the real ionization. It may be also difficult to determine the underlying properties of toy models.
8. The Future Observations
The universe’s expansion will continue forever and all the galaxies we have observed will disappear one day. The prediction says that if the universe continues expansion after 100 billion years, we can see only our galaxy and other galaxies will go out of reach from us. After trillions of years from now star formation will slow down and cease. The stars will die due to gamma-ray bursts.
A black hole is the result of a gamma-ray burst, usually, this happens when the center of a massive star runs out of nuclear fuel and collapses under its weight. The reionization of the universe can be studied by these methods.
It is believed that the last light will go out from the universe in the future by 100 trillion years. The universe’s life is determined by the amount and density of dark energy, which is the major cause of the universe’s expansion. The study of dark energy in the early universe is possible with the help of the square kilometer array (SKA).
The square kilometer array is a large radio telescope being built in the southern hemisphere. In this Array, several antennas are assembled which work as a telescope. the future predictions say that the coming years will mark the golden age of the early universe and the epoch of reionization.
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9. Conclusion
Our universe follows cosmological principles. The Epoch of reionization is considered an important principle for the formation of the universe. After the lapse of the dark ages the process in which the formation of neutral hydrogen atoms formed is called reionization.
Different reionization models help understand the formation of stars and galaxies and predict the future and lifespan of the universe. Edwin Hubble who discovered the cosmos explained the expansion of the universe. It is believed that cosmic microwave background radiations are the cause of the expansion of the universe. We have to go a long way to explore about uncertainties of the beginning and fate of the universe.
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