FACTOID # 89: In the 1990's, nearly half of all arms exported to developing countries came from the United States of America.
 
 Home   Encyclopedia   Statistics   Countries A-Z   Flags   Maps   Education   Forum   FAQ   About 
 
WHAT'S NEW
RELATED ARTICLES
People who viewed "Reionization" also viewed:
RECENT ARTICLES
More Recent Articles »
 

FACTS & STATISTICS    Simple view

  1. Select countries to view: (hold down Control key and click to select several)

     

     

    Compare:

     

     
  1. Select fact or statistic: (* = graphable)

     

     

     

  2. (OPTIONAL) Compare to statistic: (both need to be graphable)

     

     

     

  3. View result as:

     

       
(OR) SEARCH ALL encyclopedia, stats & forums:   

Encyclopedia > Reionization
Schematic timeline of the universe, depicting reionization's place in cosmic history.
Schematic timeline of the universe, depicting reionization's place in cosmic history.

In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages." It is the second of two major phase changes of hydrogen gas in the universe. The universe is currently ionized, in this sense. For other uses, see Big Bang (disambiguation). ... This article is about the physics subject. ... For other uses, see Universe (disambiguation). ... A graphical timeline is available here: Graphical timeline of the Big Bang This box:      This timeline of the Big Bang describes the events that have occurred and will occur according to the scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates. ... In its most common usage, the term phase change indicates that a substance has changed among the three classical phases of matter: solid, liquid and gas. ... This article is about the chemistry of hydrogen. ... Gas phase particles (atoms, molecules, or ions) move around freely Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape and without a definite volume. ...


The first phase change was recombination, which occurred at a redshift 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 and proton to form neutral hydrogen was higher than the ionization rate of hydrogen. When electrons are found in neutral hydrogen (or other atoms or molecules), they can absorb energy in the form of photons by going to an excited state. Thus, a universe full of neutral hydrogen will be relatively opaque at certain wavelengths. A graphical timeline is available here: Graphical timeline of the Big Bang This box:      This timeline of the Big Bang describes the events that have occurred and will occur according to the scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates. ... This article is about the physical phenomenon. ... For other uses, see Electron (disambiguation). ... For other uses, see Proton (disambiguation). ... ... The Lyman series is the series of transitions and resulting emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron). ...


The second phase change occurred once objects started to form in the early universe energetic enough to ionize neutral hydrogen. As these objects formed and radiated energy, the universe went from being neutral back to being an ionized plasma, between 150 million and one billion years after the Big Bang (at a redshift 6 < z < 20). When protons and electrons are separate, they cannot capture energy in the form of photons. Photons may be scattered, but scattering interactions are infrequent if the density of the plasma is low. Thus, a universe full of low density ionized hydrogen will be relatively translucent, as is the case today. For other uses, see Universe (disambiguation). ... This box:      Electromagnetic (EM) radiation is a self-propagating wave in space with electric and magnetic components. ... For other uses, see Plasma. ... In physics, Thomson scattering is the scattering of electromagnetic radiation by a charged particle. ...

Contents

Detection methods

Looking back so far in the history of the universe presents some observational challenges. There are, however, a few main tools that have been used so far to learn more about how reionization occurred.


Quasars and the Gunn-Peterson trough

One means of studying reionization use the spectra of distant quasars. Quasars release an extraordinary amount of energy, making them detectable even as far back as the epoch of reionization. They also happen to have relatively uniform spectral features, regardless of position in sky or distance from Earth. Thus it can be inferred that any major differences between quasar spectra will be caused by interaction with molecules along the line of sight. For wavelengths of light at the energies of one of the Lyman transitions in hydrogen, the scattering cross-section is large, meaning that even for low levels of neutral hydrogen in the intergalactic medium (IGM), absorption at those wavelengths is highly likely. In most modern usages of the word spectrum, there is a unifying theme of between extremes at either end. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... This article is about Earth as a planet. ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... For other uses, see Wavelength (disambiguation). ... The Lyman series is the series of transitions and resulting emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron). ... Intergalactic space is the physical space between galaxies. ... In physics, absorption is the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make transition between two electronic energy levels. ...


For nearby objects in the universe, spectral absorption lines are very sharp, as only photons with energies just sufficient to cause an atomic transition will do so. However, the distances between quasars and the telescopes which detect them are large, which means that the expansion of the universe 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.[1] The metric expansion of space is a key part of sciences current understanding of the universe, whereby space itself is described by a metric which changes over time. ... 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 (IGM). ...


The redshifting that occurs also 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. This is the case because quasars below a certain redshift will not show the Gunn-Peterson trough (though they may show the Lyman-alpha forest) that is evident in any object which emitted light before reionization had ended. In 2001, four quasars were detected by the Sloan Digital Sky Survey 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 at least 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.[2] This, in turn, indicates that the universe must still have been almost entirely neutral at z > 10. In astronomical spectroscopy, the Lyman alpha forest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lyman alpha line at 121. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... SDSS Logo The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2. ...


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. 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 map, introducing secondary anisotropies (anisotropies introduced after recombination).[3] 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.[4] 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. WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ... In physics, Thomson scattering is the scattering of electromagnetic radiation by a charged particle. ... Look up anisotropy in Wiktionary, the free dictionary. ... In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. ...


The Wilkinson Microwave Anisotropy Probe allowed just that comparison to be made. The initial observations, released in 2003, suggested that reionization took place from 11 < z < 30.[5]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.[6] This is in much closer agreement with the quasar data. Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ...


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 during reionization. The 21-cm line 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 dependent, meaning that as objects form in the "dark ages" and emit photons that heat the surrounding neutral hydrogen, it will cause more 21-cm line emission in the surrounding area.[7] 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 LOw Frequency ARray (LOFAR), Mileura Wide-Field Array (MWA), and the Giant Metrewave Radio Telescope (GMRT) It has been suggested that this article or section be merged into Large-scale structure of the cosmos. ... This article or section does not adequately cite its references or sources. ... A forbidden line is a spectral line emitted by atoms undergoing energy transitions not normally allowed by the selection rules of quantum mechanics. ... For other uses, see Temperature (disambiguation). ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... LOFAR is the LOw Frequency ARray for radio astronomy. ... Giant Metrewave Radio Telescope (GMRT), located near Pune in India, is the worlds largest radio telescope at metre wavelengths. ...


Energy sources of reionization

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 energy that fueled reionization. To ionize neutral hydrogen, an energy larger than 13.6 eV is required, which corresponds to photons with a wavelength of 91.2 nm or shorter. This is in the ultraviolet part of the electromagnetic spectrum, 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."[8] With these constraints, it is expected that quasars and first generation stars were the main sources of energy.[9] The electronvolt (symbol eV) is a unit of energy. ... A nanometre (American spelling: nanometer, symbol nm) (Greek: νάνος, nanos, dwarf; μετρώ, metrό, count) is a unit of length in the metric system, equal to one billionth of a metre (or one millionth of a millimetre), which is the current SI base unit of length. ... For other uses, see Ultraviolet (disambiguation). ... Legend γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High... This article is about the astronomical object. ...


Quasars

Quasars are a good candidate source because they are highly efficient at converting mass to energy, and emit a great deal of light above the threshold for ionizing hydrogen. This leaves the question of whether there were enough quasars during reionization. Unfortunately, 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) 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,[8] saying that "only if the ionizing background is dominated by low-luminosity AGNs can the quasar luminosity function provide enough ionizing photons."[10] Note that quasars are a class of active galactic nuclei, or AGN. For other uses, see Mass (disambiguation). ... This article does not cite any references or sources. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ...


Population III stars

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

Population III stars are stars which have no elements more massive than hydrogen or helium. During Big Bang nucleosynthesis, the only elements that formed aside from hydrogen and helium were trace amounts of lithium. Yet quasar spectra have revealed the presence of heavy elements in the IGM at an early era. Supernova explosions produce such heavy elements, so hot, large, Population III stars which will form supernova are a possible mechanism for reionization. While they have not been directly observed, they are consistent according to models using numerical simulation[11] and current observations[12]. A Gravitationally lensed galaxy also provides indirect evidence of Population III stars.[13] 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, [14] and are capable of reionizing hydrogen on their own in some reionization models with reasonable initial mass functions.[15] As a consequence, Population III stars are currently considered the most likely energy source to reionize the universe. Image File history File links NASA-WMAP-first-stars. ... Image File history File links NASA-WMAP-first-stars. ... For other uses, see Big Bang (disambiguation). ... The globular cluster M80. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). ... This article is about the chemical element named Lithium. ... For other uses, see Supernova (disambiguation). ... This article or section is in need of attention from an expert on the subject. ... The initial mass function (IMF) is a relationship that specifies the mass distribution of a newly formed stellar population, by giving the number of stars of mass per pc3 and per unit mass. ...

See also

For other uses, see Big Bang (disambiguation). ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

Notes and references

  1. ^ J.E. Gunn and B.A. Peterson (1965). "On the Density of Neutral Hydrogen in Intergalactic Space". The Astrophysical Journal 142: 1633-1641.
  2. ^ R.H. Becker, et. al (2001). "Evidence For Reionization at z ~ 6: Detection of a Gunn-Peterson Trough In A z=6.28 Quasar". Astronomical Journal 122: 2850-2857.
  3. ^ Manoj Kaplinghat et al. (2003). "Probing the Reionization History of the Universe using the Cosmic Microwave Background Polarization". The Astrophysical Journal 583: 24-32.
  4. ^ Olivier Dore et al. (2007). "The Signature of Patchy Reionization in the Polarization Anisotropy of the CMB". arXiv:astro-ph/0701784.
  5. ^ A. Kogut et al. (2003). "First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature-Polarization Correlation". The Astrophysical Journal Supplement Series 148: 161-173.
  6. ^ D.N. Spergel et al. (2006). "Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for Cosmology". The Astrophysical Journal Supplement Series 170: 377-408.
  7. ^ Rennan Barkana and Abraham Loeb (2005). "Detecting the Earliest Galaxies through Two New Sources of 21 Centimeter Fluctuations". The Astrophysical Journal 626: 1-11.
  8. ^ a b Piero Madau et al. (1999). "Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Source". The Astrophysical Journal 514: 648-659.
  9. ^ Loeb and Barkana (2000). "In the Beginning: The First Sources of Light and the Reionization of the Universe". Physics Reports 349: 125-238.
  10. ^ Xiaohu Fan, et al. (2001). "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". The Astronomical Journal 122: 2833-2849.
  11. ^ Nickolay Gnedin and Jeremiah Ostriker (1997). "Reionization of the Universe and the Early Production of Metals". Astrophysical Journal 486: 581-598.
  12. ^ Limin Lu et al. (1998). "The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium". arXiv:astro-ph/9802189.
  13. ^ R. A. E. Fosbury et al. (2003). "Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357". Astrophysical Journal 596 (1): 797-809.
  14. ^ Jason Tumlinson et al. (2002). "Cosmological Reionization by the First Stars: Evolving Spectra of Population III". ASP Conference Proceedings 267: 433-434.
  15. ^ Aparna Venkatesan et al. (2003). "Evolving Spectra of Population III Stars: Consequences for Cosmological Reionization". Astrophysical Journal 584: 621-632.

arXiv (pronounced archive, as if the X were the Greek letter χ) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and quantitative biology which can be accessed via the Internet. ... arXiv (pronounced archive, as if the X were the Greek letter χ) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and quantitative biology which can be accessed via the Internet. ...

External links


  Results from FactBites:
 
Reionization - definition of Reionization in Encyclopedia (507 words)
Reionization is a process that occurs after the epoch of galaxy formation begins, and is the second of two major phase changes of hydrogen gas in the universe.
The second phase change, reionization, is thought to have occurred when the first few generations of population III stars and quasars emitted radiation that reionized the universe, making it once again an ionized plasma (6 < z < 20; 150 million - one billion years after big bang).
Theoretical studies of reionization suggest that the universe should go from highly neutral to highly ionized in a relatively short period of time.
  More results at FactBites »


 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments
Please enter the 5-letter protection code

Want to know more?
Search encyclopedia, statistics and forums:
 


Lesson Plans | Student Area | Student FAQ | Reviews | Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms.