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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. 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. It is named for Siegfried Adolf Wouthuysen and George B. Field. Background The period after recombination occurred and before stars 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 the triplet spin state, or de-excited as the electron and proton spins go to the singlet state. The energy differenc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Excitation Temperature
The excitation temperature (T_) is defined for a population of particles via the Boltzmann factor. It satisfies : \frac = \frac \exp, where ''n''u and ''n''l represent the number of particles in an upper (''e.g.'' excited) and lower (''e.g.'' ground) state, and ''g''u and ''g''l their statistical weights respectively. Thus the excitation temperature is the temperature at which we would expect to find a system with this ratio of level populations. However it has no actual physical meaning except when in local thermodynamic equilibrium. The excitation temperature can even be negative for a system with inverted levels (such as a maser). In observations of the 21 cm line The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ... of hydrogen, the apparent value of the excitation temperat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hyperfine
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nucleus and electron clouds. In atoms, hyperfine structure arises from the energy of the nuclear magnetic dipole moment interacting with the magnetic field generated by the electrons and the energy of the nuclear electric quadrupole moment in the electric field gradient due to the distribution of charge within the atom. Molecular hyperfine structure is generally dominated by these two effects, but also includes the energy associated with the interaction between the magnetic moments associated with different magnetic nuclei in a molecule, as well as between the nuclear magnetic moments and the magnetic field generated by the rotation of the molecule. Hyperfine structure contrasts with ''fine structure'', which results from the interact ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dark Ages Radio Explorer (DARE)
The Dark Ages Radio Explorer (DARE) mission is a proposed concept lunar orbiter intended to identify redshifted emanations from primeval hydrogen atoms just as the first stars began to emit light. DARE will use the precisely redshifted 21-cm transition line from neutral hydrogen (1420.00 MHz emissions) to view and pinpoint the formation of the first illuminations of the universe and the period ending the Dark Ages of the universe. The orbiter will explore the universe as it was from around 80 million years to 420 million years after the Big Bang. The mission will deliver data pertaining to the formation of the first stars, the initial black hole accretions, and the reionization of the universe. Computer models of galaxy formation will also be tested. This mission might also add to research on dark matter decay. The DARE program will also provide insight for developing and deploying lunar surface telescopes that add to refined exoplanet exploration of nearby stars. It is e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Square Kilometer Array
The Square Kilometre Array (SKA) is an intergovernmental international radio telescope project being built in Australia (low-frequency) and South Africa (mid-frequency). The combining infrastructure, the Square Kilometre Array Observatory (SKAO), and headquarters, are located at the Jodrell Bank Observatory in the United Kingdom. The SKA cores are being built in the southern hemisphere, where the view of the Milky Way galaxy is the best and radio interference at its least. Conceived in the 1990s, and further developed and designed by the late-2010s, when completed a total collecting area of approximately one square kilometre. It will operate over a wide range of frequencies and its size will make it 50 times more sensitive than any other radio instrument. If built as planned, it should be able to survey the sky more than ten thousand times faster than before. With receiving stations extending out to a distance of at least from a concentrated central core, it will exploit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Large Aperture Experiment To Detect The Dark Ages
The Large-Aperture Experiment to Detect the Dark Ages (LEDA) is designed to detect the spectrum of the 21 cm Hydrogen line from the Intergalactic Medium (IGM) at redshifts of 15-30, when the Universe was just ~1% of its present age. It is located at the Long Wavelength Array site, adjacent to the Very Large Array. LEDA principally comprises a "large-N" array correlator (512 inputs over ~ 60 MHz), calibration & imaging system, and instrumentation for measurement of calibrated total-power. These systems will use the station 1 of the Long Wavelength Array as an aperture. LEDA is one of several efforts seeking to study cosmological reionization and the preceding Dark Ages. Others include the Precision Array for Probing the Epoch of Reionization (PAPER), Low Frequency Array (LOFAR), Murchison Widefield Array (MWA), and Giant Metrewave Radio Telescope (GMRT). LEDA will feature array-based calibration to improve the accuracy of foreground subtraction from the total-power signal. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Murchison Widefield Array
The Murchison Widefield Array (MWA) is a joint project between an international consortium of organisations to construct and operate a low-frequency radio array. 'Widefield' refers to its very large field of view (on the order of 30 degrees across). Operating in the frequency range 70–300 MHz, the main scientific goals of the MWA are to detect neutral atomic Hydrogen emission from the cosmological Epoch of Reionization (EoR), to study the sun, the heliosphere, the Earth's ionosphere, and radio transient phenomena, as well as map the extragalactic radio sky. It is located at the Murchison Radio-astronomy Observatory (MRO). Along with the Australian Square Kilometre Array Pathfinder (ASKAP), also at the MRO, and two radio telescopes in South Africa, the Hydrogen Epoch of Reionization Array (HERA) and MeerKAT, the MWA is one of four precursors to the international project known as the Square Kilometre Array (SKA). Development The MWA was to be situated at Mileura S ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Precision Array For Probing The Epoch Of Reionization
The Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) is a radio interferometer funded by the National Science Foundation to detect 21 cm hydrogen (HI) fluctuations occurring when the first galaxies ionized intergalactic gas at around 500 Million years after the Big Bang. PAPER is a focused experiment aimed toward making the first statistical detection of the 21 cm reionization signal. Given the stringent dynamic range requirements for detecting reionization in the face of foregrounds that are five orders of magnitude brighter, the PAPER project is taking a carefully staged engineering approach, optimizing each component in the array to mitigate, at the outset, any potentially debilitating problems in subsequent data calibration and analysis. This staged approach addresses the observational challenges that arise from very-wide-field, high-dynamic-range imaging over wide bandwidths in the presence of transient terrestrial interference. PAPER ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Giant Metrewave Radio Telescope
The Giant Metrewave Radio Telescope (GMRT), located near Pune, Junnar, near Narayangaon at khodad in India, is an array of thirty fully steerable parabolic radio telescopes of 45 metre diameter, observing at metre wavelengths. It is operated by the National Centre for Radio Astrophysics (NCRA), a part of the Tata Institute of Fundamental Research, Mumbai. It was conceived and built under the direction of Late Prof. Govind Swarup during 1984 to 1996. It is an interferometric array with baselines of up to . It was recently upgraded with new receivers, after which it is also known as the Upgraded Giant Metrewave Radio Telescope (uGMRT). Location The GMRT Observatory is located about 80 km north of Pune at Khodad. A nearby town is Narayangaon which is around 9 km from the telescope site. The office of NCRA is located in the Savitribai Phule Pune University campus. Science and observations One of the aims for the telescope during its development was to search for the highl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cosmological Redshift
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum. Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model. The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is described by the equation , with ''H''0 the constant of proportionality—the Hubble constant—between the "proper distance" ''D'' to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation ''v'', i.e. the derivative of proper distance with respect to t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors in visible light after passing through a prism. As scientific understanding of light advanced, it came to apply to the entire electromagnetic spectrum. It thereby became a mapping of a range of magnitudes (wavelengths) to a range of qualities, which are the perceived "colors of the rainbow" and other properties which correspond to wavelengths that lie outside of the visible light spectrum. Spectrum has since been applied by analogy to topics outside optics. Thus, one might talk about the " spectrum of political opinion", or the "spectrum of activity" of a drug, or the " autism spectrum". In these uses, values within a spectrum may not be associated with precisely quantifiable numbers or definitions. Such uses imply a broad range of co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quasar
A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin. The term originated as a contraction of "quasi-stellar '' tar-like' radio source"—because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin—and when ident ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, (or about ). The photon belongs to the class of bosons. As with other elementary particles, photons are best explained by quantum mechanics and exhibit wave–particle duality, their behavior featuring properties of both waves and particles. The modern photon concept originated during the first two decades of the 20th century with the work of Albert Einstein, who built upon the research of Max Planck. While trying to explain how matter and electromagnetic radiation could be in thermal equilibrium with one another, Planck proposed that the energy stored within a material object should be regarded as composed of an integer number of discrete, equal-sized parts. To explain the photoelectric eff ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
