Wouthuysen-Field coupling
Encyclopedia
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. This coupling plays a role in producing a difference in the temperature of neutral hydrogen and the cosmic microwave background at the end of the dark ages and the beginning of the epoch of reionization
.
occurred and before star
s and galaxies formed is known as the "dark ages". During this time, the majority of matter
in the universe is neutral hydrogen. This hydrogen has yet to be observed, but there are experiments underway to detect the hydrogen line
produced during this era. The hydrogen line is produced when an electron
in a neutral hydrogen atom
is excited
to a state where the electron and proton
have aligned spin
s, or de-excited as the electron and proton spins go from being aligned to anti-aligned. The energy difference between these two hyperfine states is
electron volts, with a wavelength
of 21 centimeters. At times when neutral hydrogen is in thermodynamic equilibrium
with the photons in the cosmic microwave background (CMB), the neutral hydrogen and CMB are said to be "coupled", and the hydrogen line is not observable. It is only when the two temperatures differ, or decoupled, that the hydrogen line can be observed.
s. Neutral hydrogen absorbs Lyman-alpha photons, and then re-emits Lyman-alpha photons, and may enter either of the two spin states. This process causes a redistribution of the electrons between the hyperfine states, decoupling the neutral hydrogen from the CMB photons.
The coupling between Lyman-alpha photons and the hyperfine states depends not on the intensity of the Lyman-alpha radiation, but on the shape of the spectrum
in the vicinity of the Lyman-alpha transition. That this mechanism might effect the population of the hyperfine states in neutral hydrogen was first suggested in 1952 by S. A. Wouthuysen in 1952, and then further developed by George B. Field
in 1959.
The effect of Lyman-alpha photons on the hyperfine levels depends upon the relative intensities of the red and blue wings of the Lyman-alpha line, reflecting the very small difference in energy of the hyperfine states relative to the Lyman-alpha transition. At a cosmological redshift of
, Wouthuysen-Field coupling is expected to raise the spin temperature of neutral hydrogen above that of the CMB, and produce emission in the hydrogen line.
, the Precision Array for Probing the Epoch of Reionization
, the Murchison Widefield Array
, and the Square Kilometer Array.
Excitation temperature
The Excitation Temperature is defined for a population of particles via the Boltzmann factor...
, also called the spin temperature, of neutral 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...
to Lyman-alpha
Lyman-alpha line
In physics, the Lyman-alpha line, sometimes written as Ly-\alpha line, is a spectral line of hydrogen, or more generally of one-electron ions, in the Lyman series, emitted when the electron falls from the n = 2 orbital to the n = 1 orbital, where n is the principal quantum number...
radiation. This coupling plays a role in producing a difference in the temperature of neutral hydrogen and the cosmic microwave background at the end of the dark ages and the beginning of the epoch of reionization
Reionization
In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages," and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of...
.
Background
The period after recombinationRecombination
Recombination may refer to:* Recombination , the process by which genetic material is broken and joined to other genetic material* Recombination , in semiconductors, the elimination of mobile charge carriers...
occurred and before 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 and galaxies formed is known as the "dark ages". During this time, the majority of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
in the universe is neutral hydrogen. This hydrogen has yet to be observed, but there are experiments underway to detect the hydrogen 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...
produced during this era. The hydrogen line is produced when 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...
in a neutral hydrogen 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...
is excited
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
to a state where the electron 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....
have aligned spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
s, or de-excited as the electron and proton spins go from being aligned to anti-aligned. The energy difference between these two hyperfine states is
electron volts, with a wavelengthWavelength
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...
of 21 centimeters. At times when neutral hydrogen is in thermodynamic equilibrium
Thermodynamic equilibrium
In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. The word equilibrium means a state of balance...
with the photons in the cosmic microwave background (CMB), the neutral hydrogen and CMB are said to be "coupled", and the hydrogen line is not observable. It is only when the two temperatures differ, or decoupled, that the hydrogen line can be observed.
Coupling mechanism
Wouthuysen-Field coupling is a mechanism that couples the spin temperature of neutral hydrogen to Lyman-alpha radiation, which decouples the neutral hydrogen from the CMB. The energy of the Lyman-alpha transition is 10.2 eV—this energy is approximately two million times greater than the hydrogen line, and is produced by astrophysical sources such as stars and quasarQuasar
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. Neutral hydrogen absorbs Lyman-alpha photons, and then re-emits Lyman-alpha photons, and may enter either of the two spin states. This process causes a redistribution of the electrons between the hyperfine states, decoupling the neutral hydrogen from the CMB photons.
The coupling between Lyman-alpha photons and the hyperfine states depends not on the intensity of the Lyman-alpha radiation, but on the shape of the spectrum
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...
in the vicinity of the Lyman-alpha transition. That this mechanism might effect the population of the hyperfine states in neutral hydrogen was first suggested in 1952 by S. A. Wouthuysen in 1952, and then further developed by George B. Field
George B. Field
George B. Field is an American astrophysicist.- Education and career :Field became interested in astronomy at an early age, but at the urging of his father he studied chemical engineering at the Massachusetts Institute of Technology. Disliking engineering, he later switched to astrophysics...
in 1959.
The effect of Lyman-alpha photons on the hyperfine levels depends upon the relative intensities of the red and blue wings of the Lyman-alpha line, reflecting the very small difference in energy of the hyperfine states relative to the Lyman-alpha transition. At a cosmological redshift of
, Wouthuysen-Field coupling is expected to raise the spin temperature of neutral hydrogen above that of the CMB, and produce emission in the hydrogen line.Observational prospects
A hydrogen line signal produced by Wouthuysen-Field coupling has not yet been observed. There are multiple experiments and radio observatories that aim to detect the neutral hydrogen line the dark ages and epoch of reionization, the time at which Wouthuysen-Field coupling is expected to be important. These include the Giant Metrewave Radio TelescopeGiant 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...
, 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....
, the 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...
, and the Square Kilometer Array.