Reionization

Reionization

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In Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

, reionization is the process that reionized the matter in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 after the "dark ages," and is the second of two major phase changes of gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas. The primordial
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...

 helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 in the universe experienced the same phase changes, but at different points in the history of the universe, and is usually referred to as Helium reionization.

Background



The first phase change of hydrogen in the universe was recombination
Recombination (cosmology)
In cosmology, recombination refers to the epoch at which charged electrons and protons first became bound to form electrically neutral hydrogen atoms.Note that the term recombination is a misnomer, considering that it represents the first time that electrically neutral hydrogen formed. After the...

, which occurred at a redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

 z = 1100 (400,000 years after the Big Bang), due to the cooling of the universe to the point where the rate of combination of an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 and proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 to form neutral hydrogen was higher than the ionization rate of hydrogen. The universe was opaque before recombination because photons scatter
Thomson scattering
Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency are the same before and after the scattering...

 off free electrons (and, to a significantly lesser extent, free protons), but it became transparent as more and more electrons and protons combined to form hydrogen atoms. While electrons in neutral hydrogen (or other atoms or molecules) can absorb photons of some wavelengths by going to an excited state
Lyman series
In physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...

, a universe full of neutral hydrogen will be relatively opaque only at those wavelengths, and transparent over most of the spectrum. The Dark Ages start at that point, because there are no light sources yet other than the gradually darkening cosmic background radiation.

The second phase change occurred once objects started to form in the early universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 energetic enough to ionize neutral hydrogen. As these objects formed and radiated
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 energy, the universe went from being neutral back to being an ionized plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

, between 150 million and one billion years after the Big Bang (at a redshift 6 < z < 20). By now, however, matter has been diluted by the expansion of the universe, and scattering interactions are much less frequent than before recombination. Thus a universe full of low density ionized hydrogen will remain transparent, as is the case today.

Detection methods


Looking back so far in the history of the universe presents some observational challenges. There are, however, a few observational methods for studying reionization.

Quasars and the Gunn-Peterson trough


One means of studying reionization uses the spectra
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 of distant quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s. Quasars release an extraordinary amount of energy, meaning they are among the brightest objects in the universe. Some quasars are even detectable as far back as the epoch of reionization. Quasars also happen to have relatively uniform spectral features, regardless of position in sky or distance from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. Thus it can be inferred that any major differences between quasar spectra will be caused by interaction with atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s along the line of sight. For wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s of light at the energies of one of the Lyman transitions
Lyman series
In physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...

 in hydrogen, the scattering cross-section
Scattering cross-section
The scattering cross-section, σscat, is a hypothetical area which describes the likelihood of light being scattered by a particle. In general, the scattering cross-section is different from the geometrical cross-section of a particle, and it depends upon the wavelength of light and the...

 is large, meaning that even for low levels of neutral hydrogen in the intergalactic medium (IGM), absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

 at those wavelengths is highly likely.

For nearby objects in the universe, spectral absorption lines are very sharp, as only photons with energies just sufficient to cause an atomic transition can cause the transition. However, the distances between quasars and the telescopes which detect them are large, which means that the expansion of the universe
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

 causes light to undergo noticeable redshifting. This means that as light from the quasar travels through the IGM and is redshifted, wavelengths which had been above the Lyman Alpha limit are stretched, and will at some point be just equal to the wavelength needed for the Lyman Alpha transition. This means that instead of showing sharp spectral lines, a quasar's light which has traveled through a large, spread out region of neutral hydrogen will show a Gunn-Peterson trough
Gunn-Peterson trough
In astronomical spectroscopy, the Gunn-Peterson trough is a feature of the spectra of quasars due to the presence of neutral hydrogen in the Intergalactic Medium . The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the Lyman-alpha...

.

The redshifting that occurs allows for temporal information about reionization to be learned. Since an object's redshift corresponds to the time at which it emitted the light we see, it is possible to determine when reionization ended. Quasars below a certain redshift will not show the Gunn-Peterson trough (though they may show the Lyman-alpha forest
Lyman-alpha forest
In astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....

), while quasars emitting light prior to reionization will feature a Gunn-Peterson trough. In 2001, four quasars were detected by the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...

 with redshifts ranging from z = 5.82 to z = 6.28. While the quasars above z = 6 showed a Gunn-Peterson trough, indicating that the IGM was still at least partly neutral, the ones below did not. As reionization is expected to occur over relatively short timescales, the results suggest that the universe was approaching the end of reionization at z = 6. This, in turn, indicates that the universe must still have been almost entirely neutral at z > 10.

CMB anisotropy and polarization


The anisotropy of the cosmic microwave background on different angular scales can also be used to study reionization. Photons undergo scattering when there are free electrons present, in a process known as Thomson scattering
Thomson scattering
Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency are the same before and after the scattering...

. However, as the universe expands, the density of free electrons will decrease, and scattering will occur less frequently. In the period during and after reionization, but before significant expansion had occurred to sufficiently lower the electron density, the light that composes the CMB will experience observable Thomson scattering. This scattering will leave its mark on the CMB anisotropy
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...

 map, introducing secondary anisotropies (anisotropies introduced after recombination). The overall effect is to erase anisotropies that occur on smaller scales. While anisotropies on small scales are erased, polarization anisotropies are actually introduced because of reionization. By looking at the CMB anisotropies observed, and comparing with what they would look like had reionization not taken place, the electron column density at the time of reionization can be determined. With this, the age of the universe when reionization occurred can then be calculated.

The Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 allowed that comparison to be made. The initial observations, released in 2003, suggested that reionization took place from 11 <z < 30. This redshift range was in clear disagreement with the results from studying quasar spectra. However, the three year WMAP data returned a different result, with reionization beginning at z = 11 and the universe ionized by z = 7. This is in much better agreement with the quasar data.

21-cm line


Even with the quasar data roughly in agreement with the CMB anisotropy data, there are still a number of questions, especially concerning the energy sources of reionization and the effects on, and role of, structure formation
Structure formation
Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago...

 during reionization. The 21-cm line
Hydrogen line
The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...

 in hydrogen is potentially a means of studying this period, as well as the "dark ages" that preceded reionization. The 21-cm line occurs in neutral hydrogen, due to differences in energy between the parallel and anti-parallel spin states of the electron and proton. This transition is forbidden, meaning it occurs extremely rarely. The transition is also highly temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 dependent, meaning that as objects form in the "dark ages" and emit Lyman-alpha photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s that are absorbed and re-emitted by surrounding neutral hydrogen, it will produce a 21-cm line signal in that hydrogen through Wouthuysen-Field coupling
Wouthuysen-Field coupling
Wouthuysen-Field coupling, or the Wouthuysen-Field effect, is a mechanism that couples the excitation temperature, also called the spin temperature, of neutral hydrogen to Lyman-alpha radiation...

. By studying 21-cm line emission, it will be possible to learn more about the early structures that formed. While there are currently no results, there are a few projects underway which hope to make headway in this area in the near future, such as the Precision Array for Probing the Epoch of Reionization
Precision Array for Probing the Epoch of Reionization
The Precision Array for Probing the Epoch of Reionization is a radio interferometer designed to detect 21 cm Hydrogen fluctuations occurring when the first galaxies ionized intergalactic gas at around 500 Million years after the Big Bang....

 (PAPER), Low Frequency Array
LOFAR
LOFAR is the Low Frequency Array for radio astronomy, built by the Netherlands astronomical foundation ASTRON and operated by ASTRON's radio observatory....

 (LOFAR), Murchison Widefield Array
Murchison Widefield Array
The Murchison Widefield Array is a joint project between MIT, the Harvard-Smithsonian Center for Astrophysics, the Raman Research Institute and an Australian consortium of universities, to build a low-frequency radio array operating in the frequency range 80-300 MHz...

 (MWA), and the Giant Metrewave Radio Telescope
Giant Metrewave Radio Telescope
Giant Metrewave Radio Telescope , located near Pune in India, is the world's largest, array of radio telescopes at metre wavelengths. It is operated by the National Centre for Radio Astrophysics, a part of the Tata Institute of Fundamental Research, Mumbai.-Location:The GMRT is located around...

 (GMRT).

Energy sources


While observations have come in which narrow the window during which the epoch of reionization could have taken place, it is still uncertain which objects provided the photons that reionized the IGM. To ionize neutral hydrogen, an energy larger than 13.6 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...

 is required, which corresponds to photons with a wavelength of 91.2 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

 or shorter. This is in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

, which means that the primary candidates are all sources which produce a significant amount of energy in the ultraviolet and above. How numerous the source is must also be considered, as well as the longevity, as protons and electrons will recombine if energy is not continuously provided to keep them apart. Altogether, the critical parameter for any source considered can be summarized as its "emission rate of hydrogen-ionizing photons per unit cosmological volume." With these constraints, it is expected that quasars and first generation star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s were the main sources of energy.

Quasars


Quasars are a good candidate source because they are highly efficient at converting mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 to energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

, and emit a great deal of light above the threshold for ionizing hydrogen. It is unknown, however, how many quasars existed prior to reionization. Only the brightest of quasars present during reionization can be detected, which means there is no direct information about dimmer quasars that existed. However, by looking at the more easily observed quasars in the nearby universe, and assuming that the luminosity function (number of quasars as a function of luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

) during reionization will be approximately the same as it is today, it is possible to make estimates of the quasar populations at earlier times. Such studies have found that quasars do not exist in high enough numbers to reionize the IGM alone, saying that "only if the ionizing background is dominated by low-luminosity AGNs can the quasar luminosity function provide enough ionizing photons." Note that quasars are a class of active galactic nuclei
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...

, or AGN.

Population III stars


Population III stars are stars which have no elements more massive than hydrogen or helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. During Big Bang nucleosynthesis, the only elements that formed aside from hydrogen and helium were trace amounts of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

. Yet quasar spectra have revealed the presence of heavy elements in the IGM at an early era. Supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 explosions produce such heavy elements, so hot, large, Population III stars which will form supernovae are a possible mechanism for reionization. While they have not been directly observed, they are consistent according to models using numerical simulation and current observations. A gravitationally lensed
Gravitational lens
A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...

 galaxy also provides indirect evidence of Population III stars. Even without direct observations of Population III stars, they are a compelling source. They are more efficient and effective ionizers than Population II stars, as they emit more ionizing photons, and are capable of reionizing hydrogen on their own in some reionization models with reasonable initial mass function
Initial mass function
The initial mass function is an empirical function that describes the mass distribution of a population of stars in terms of their theoretical initial mass...

s. As a consequence, Population III stars are currently considered the most likely energy source to initiate the reionization of the universe.

See also

  • Big Bang
    Big Bang
    The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

  • List of most distant astronomical object record holders
  • Quasar
    Quasar
    A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

    s

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