Anisotropy /ˌænɪˈsɒtrəpi/, /ˌænaɪˈsɒtrəpi/ is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties (absorbance, refractive index, conductivity, tensile strength, etc.)
Anisotropic filtering (AF) is a method of enhancing the image quality of textures on surfaces that are far away and steeply angled with respect to the point of view. Older techniques, such as bilinear and trilinear filtering, do not take into account the angle a surface is viewed from, which can result in aliasing or blurring of textures. By reducing detail in one direction more than another, these effects can be reduced.
A chemical anisotropic filter, as used to filter particles, is a filter with increasingly smaller interstitial spaces in the direction of filtration so that the proximal regions filter out larger particles and distal regions increasingly remove smaller particles, resulting in greater flow-through and more efficient filtration.
In NMR spectroscopy, the orientation of nuclei with respect to the applied magnetic field determines their chemical shift. In this context, anisotropic systems refer to the electron distribution of molecules with abnormally high electron density, like the pi system of benzene. This abnormal electron density affects the applied magnetic field and causes the observed chemical shift to change.
In fluorescence spectroscopy, the fluorescence anisotropy, calculated from the polarization properties of fluorescence from samples excited with plane-polarized light, is used, e.g., to determine the shape of a macromolecule. Anisotropy measurements reveal the average angular displacement of the fluorophore that occurs between absorption and subsequent emission of a photon.
Images of a gravity-bound or man-made environment are particularly anisotropic in the orientation domain, with more image structure located at orientations parallel with or orthogonal to the direction of gravity (vertical and horizontal).
Physicists from University of California, Berkeley reported about their detection of the cosine anisotropy in cosmic microwave background radiation in 1977. Their experiment demonstrated the Doppler shift caused by the movement of the earth with respect to the early Universe matter, the source of the radiation. Cosmic anisotropy has also been seen in the alignment of galaxies' rotation axes and polarisation angles of quasars.
Physicists use the term anisotropy to describe direction-dependent properties of materials. Magnetic anisotropy, for example, may occur in a plasma, so that its magnetic field is oriented in a preferred direction. Plasmas may also show "filamentation" (such as that seen in lightning or a plasma globe) that is directional.
An anisotropic liquid has the fluidity of a normal liquid, but has an average structural order relative to each other along the molecular axis, unlike water or chloroform, which contain no structural ordering of the molecules. Liquid crystals are examples of anisotropic liquids.
Some materials conduct heat in a way that is isotropic, that is independent of spatial orientation around the heat source. Heat conduction is more commonly anisotropic, which implies that detailed geometric modeling of typically diverse materials being thermally managed is required. The materials used to transfer and reject heat from the heat source in electronics are often anisotropic.
Many crystals are anisotropic to light ("optical anisotropy"), and exhibit properties such as birefringence. Crystal optics describes light propagation in these media. An "axis of anisotropy" is defined as the axis along which isotropy is broken (or an axis of symmetry, such as normal to crystalline layers). Some materials can have multiple such optical axes.
Seismic anisotropy is the variation of seismic wavespeed with direction. Seismic anisotropy is an indicator of long range order in a material, where features smaller than the seismic wavelength (e.g., crystals, cracks, pores, layers or inclusions) have a dominant alignment. This alignment leads to a directional variation of elasticity wavespeed. Measuring the effects of anisotropy in seismic data can provide important information about processes and mineralogy in the Earth; indeed, significant seismic anisotropy has been detected in the Earth's crust, mantle and inner core.
Geological formations with distinct layers of sedimentary material can exhibit electrical anisotropy; electrical conductivity in one direction (e.g. parallel to a layer), is different from that in another (e.g. perpendicular to a layer). This property is used in the gas and oil exploration industry to identify hydrocarbon-bearing sands in sequences of sand and shale. Sand-bearing hydrocarbon assets have high resistivity (low conductivity), whereas shales have lower resistivity. Formation evaluation instruments measure this conductivity/resistivity and the results are used to help find oil and gas in wells.
The hydraulic conductivity of aquifers is often anisotropic for the same reason. When calculating groundwater flow to drains or to wells, the difference between horizontal and vertical permeability must be taken into account, otherwise the results may be subject to error.
Most common rock-forming minerals are anisotropic, including quartz and feldspar. Anisotropy in minerals is most reliably seen in their optical properties. An example of an isotropic mineral is garnet.
Anisotropy is also a well-known property in medical ultrasound imaging describing a different resulting echogenicity of soft tissues, such as tendons, when the angle of the transducer is changed. Tendon fibers appear hyperechoic (bright) when the transducer is perpendicular to the tendon, but can appear hypoechoic (darker) when the transducer is angled obliquely. This can be a source of interpretation error for inexperienced practitioners.
Anisotropy, in Material Science, is a material's directional dependence of a physical property. Most materials exhibit anisotropic behavior. An example would be the dependence of Young's modulus on the direction of load. In such a case anisotropy could be effectively measured directly from its stiffness tensor independently of its origin which may for instance be its texture, randomness of internal composition or defects.  Texture patterns are often produced during manufacturing of the material. In the case of rolling, "stringers" of texture are produced in the direction of rolling, which can lead to vastly different properties in the rolling and transverse directions. Some materials, such as wood and fibre-reinforced composites are very anisotropic, being much stronger along the grain/fibre than across it. Metals and alloys tend to be more isotropic, though they can sometimes exhibit significant anisotropic behaviour. This is especially important in processes such as deep-drawing.
Wood is a naturally anisotropic (but often simplified to be transversely isotropic) material. Its properties vary widely when measured with or against the growth grain. For example, wood's strength and hardness is different for the same sample measured in different orientations.
In the Mechanics of Continuum Materials, isotropy and anisotropy are rigorously described through the symmetry group of the constitutive relation.
Anisotropic etching techniques (such as deep reactive ion etching) are used in microfabrication processes to create well defined microscopic features with a high aspect ratio. These features are commonly used in MEMS and microfluidic devices, where the anisotropy of the features is needed to impart desired optical, electrical, or physical properties to the device. Anisotropic etching can also refer to certain chemical etchants used to etch a certain material preferentially over certain crystallographic planes (e.g., KOH etching of silicon  produces pyramid-like structures)
Diffusion tensor imaging is an MRI technique that involves measuring the fractional anisotropy of the random motion (Brownian motion) of water molecules in the brain. Water molecules located in fiber tracts are more likely to be anisotropic, since they are restricted in their movement (they move more in the dimension parallel to the fiber tract rather than in the two dimensions orthogonal to it), whereas water molecules dispersed in the rest of the brain have less restricted movement and therefore display more isotropy. This difference in fractional anisotropy is exploited to create a map of the fiber tracts in the brains of the individual.
Radiance fields (see BRDF) from a reflective surface are often not isotropic in nature. This makes calculations of the total energy being reflected from any scene a difficult quantity to calculate. In remote sensing applications, anisotropy functions can be derived for specific scenes, immensely simplifying the calculation of the net reflectance or (thereby) the net irradiance of a scene. For example, let the BRDF be where 'i' denotes incident direction and 'v' denotes viewing direction (as if from a satellite or other instrument). And let P be the Planar Albedo, which represents the total reflectance from the scene.
It is of interest because, with knowledge of the anisotropy function as defined, a measurement of the BRDF from a single viewing direction (say, ) yields a measure of the total scene reflectance (Planar Albedo) for that specific incident geometry (say, ).
The Yuan-Tseh Lee Array for Microwave Background Anisotropy, also known as the Array for Microwave Background Anisotropy (AMiBA), is a radio telescope designed to observe the cosmic microwave background and the Sunyaev-Zel'dovich effect in clusters of galaxies.
After completion of the SZE campaigns, the telescope has been repurposed to study the evolution of molecular gas throughout the history of the Universe. It is now referred to as the Yuan-Tseh Lee Array (YTLA).
It is located on Mauna Loa in Hawaii, at 3,396 metres (11,142 ft) above sea level.
AMiBA was originally configured as a 7-element interferometer atop a hexapod mount. Observations at a wavelength of 3 mm (86–102 GHz) started in October 2006, and the detections of six clusters by the Sunyaev-Zel'dovich effect were announced in 2008. In 2009 the telescope was upgraded to 13 elements, and it is capable of further expansion to 19 elements. AMiBA is the result of a collaboration between the Academia Sinica Institute of Astronomy and Astrophysics, the National Taiwan University and the Australia Telescope National Facility, and also involves researchers from other universities.Birefringence
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.
Birefringence is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking slightly different paths. This effect was first described by the Danish scientist Rasmus Bartholin in 1669, who observed it in calcite, a crystal having one of the strongest birefringences. However it was not until the 19th century that Augustin-Jean Fresnel described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarizations (perpendicular to the direction of the wave vector).Charles L. Bennett
Charles L. Bennett (born November 1956) is an American observational astrophysicist. He is a Bloomberg Distinguished Professor, the Alumni Centennial Professor of Physics and Astronomy and a Gilman Scholar at Johns Hopkins University. He is the Principal Investigator of NASA's highly successful Wilkinson Microwave Anisotropy Probe (WMAP).His National Academy of Sciences (NAS) membership citation states, "As leader of the Wilkinson Microwave Anisotropy Probe (WMAP) mission, Bennett has helped quantify, with unprecedented precision and accuracy, many key properties of the universe, including its age, the dark and baryonic matter content, the cosmological constant, and the Hubble constant." Membership is a great honor bestowed upon the most distinguished scholars in engineering and the sciences.
He was awarded the National Academy of Sciences Henry Draper Medal in 2005 and the Comstock Prize in Physics in 2009, both for his leadership of WMAP. Bennett received the Harvey Prize in 2006 for, "the precise determination of the age, composition and curvature of the universe." Bennett shared the 2010 Shaw Prize in astronomy with Lyman A. Page,Jr. and David N. Spergel, both of Princeton University, for their work on WMAP.
The 2012 Gruber Cosmology Prize was awarded to "Charles L. Bennett and the WMAP Team" for
"transforming our current paradigm of structure formation from appealing scenario into precise science." "By observing the relic radiation from the early universe, Charles L. Bennett and the WMAP team established the Standard Cosmological Model."
Bennett was named the 2013 Karl G. Jansky Prize Lecturer.
In 2015 Bennett was awarded the Caterina Tomassoni and Felice Pietro Chisesi Prize "For Dr. Bennett's leadership in two experiments on the Cosmic Microwave Background that literally changed our view of the Universe: COBE-DMR, leading to the discovery of primordial spatial fluctuations in the CMB, and WMAP, leading to precise measurements of the cosmological parameters and establishing -de facto- the Standard Cosmological Model". Bennett was named for the 2017 Isaac Newton Medal and Prize: "Professor Charles L Bennett, the leader of the Wilkinson Microwave Anisotropy Probe (WMAP), has had a transformative effect in cosmology. WMAP has, through its incredibly precise measurements of temperature fluctuations in the cosmic microwave background (CMB), revolutionized our understanding of the universe. It transformed cosmology from an order-of-magnitude game to a precision experimental science."Bennett is a Fellow of both the American Association for the Advancement of Science and the American Physical Society. In 2002, ISI named him the most Highly Cited Researcher in space science worldwide. He is an author of the top two "Super Hot Papers in Science" published since 2003. In 2004, he was elected a Fellow of the American Academy of Arts and Sciences.Before leading WMAP, Bennett was the Deputy Principal Investigator for the Differential Microwave Radiometers (DMR) instrument on the Cosmic Background Explorer (COBE) mission that discovered the anisotropy of the cosmic microwave background radiation. Bennett led the effort to rebuild the radiometer front-end microwave components that succeeded in significantly enhancing the sensitivity of the DMR instrument. The Cosmic Background Explorer (COBE) Science Team also precisely measured the spectrum of the cosmic microwave background radiation.
Prior to 2005, Bennett was a Senior Scientist for Experimental Cosmology, Goddard Senior Fellow, and Infrared Astrophysics Branch Head at the NASA Goddard Space Flight Center.
Bennett was at the Carnegie Institution of Washington's Department of Terrestrial Magnetism during the summers from 1976 to 1978.Cosmic Anisotropy Polarization Mapper
CAPMAP is an experiment at Princeton university to measure the polarization of the Cosmic microwave background.Cosmic Anisotropy Telescope
The Cosmic Anisotropy Telescope (CAT) was a three-element interferometer for cosmic microwave background radiation (CMB/R) observations at 13 to 17 GHz, based at the Mullard Radio Astronomy Observatory. In 1995, it was the first instrument to measure small-scale structure in the cosmic microwave background. When the more sensitive Very Small Array came online, the CAT telescope was decommissioned in a ceremonial bonfire.Cosmic Background Explorer
The Cosmic Background Explorer (COBE ), also referred to as Explorer 66, was a satellite dedicated to cosmology, which operated from 1989 to 1993. Its goals were to investigate the cosmic microwave background radiation (CMB) of the universe and provide measurements that would help shape our understanding of the cosmos.
COBE's measurements provided two key pieces of evidence that supported the Big Bang theory of the universe: that the CMB has a near-perfect black-body spectrum, and that it has very faint anisotropies. Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006 for their work on the project. According to the Nobel Prize committee, "the COBE-project can also be regarded as the starting point for cosmology
as a precision science".COBE was followed by two more advanced spacecraft: WMAP operated from 2001-2010 and Planck from 2009-2013.Cosmic microwave background
All-sky mollweide map of the CMB, created from 9 years of WMAP data
The cosmic microwave background (CMB, CMBR) is electromagnetic radiation as a remnant from an early stage of the universe in Big Bang cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation (CMBR) or "relic radiation". The CMB is a faint cosmic background radiation filling all space that is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, a sufficiently sensitive radio telescope shows a faint background noise, or glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize in Physics.
The discovery of CMB is landmark evidence of the Big Bang origin of the universe. When the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with a uniform glow from a white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral hydrogen atoms. Unlike the uncombined protons and electrons, these newly conceived atoms could not absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. Cosmologists refer to the time period when neutral atoms first formed as the recombination epoch, and the event shortly afterwards when photons started to travel freely through space rather than constantly being scattered by electrons and protons in plasma is referred to as photon decoupling. The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck's relation). This is the source of the alternative term relic radiation. The surface of last scattering refers to the set of points in space at the right distance from us so that we are now receiving photons originally emitted from those points at the time of photon decoupling.
Precise measurements of the CMB are critical to cosmology, since any proposed model of the universe must explain this radiation. The CMB has a thermal black body spectrum at a temperature of 2.72548±0.00057 K. The spectral radiance dEν/dν peaks at 160.23 GHz, in the microwave range of frequencies, corresponding to a photon energy of about 6.626 × 10−4 eV. Alternatively, if spectral radiance is defined as dEλ/dλ, then the peak wavelength is 1.063 mm (282 GHz, 1.168 x 10−3 eV photons). The glow is very nearly uniform in all directions, but the tiny residual variations show a very specific pattern, the same as that expected of a fairly uniformly distributed hot gas that has expanded to the current size of the universe. In particular, the spectral radiance at different angles of observation in the sky contains small anisotropies, or irregularities, which vary with the size of the region examined. They have been measured in detail, and match what would be expected if small thermal variations, generated by quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today. This is a very active field of study, with scientists seeking both better data (for example, the Planck spacecraft) and better interpretations of the initial conditions of expansion. Although many different processes might produce the general form of a black body spectrum, no model other than the Big Bang has yet explained the fluctuations. As a result, most cosmologists consider the Big Bang model of the universe to be the best explanation for the CMB.
The high degree of uniformity throughout the observable universe and its faint but measured anisotropy lend strong support for the Big Bang model in general and the ΛCDM ("Lambda Cold Dark Matter") model in particular. Moreover, the fluctuations are coherent on angular scales that are larger than the apparent cosmological horizon at recombination. Either such coherence is acausally fine-tuned, or cosmic inflation occurred.Diffusion MRI
Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data, that uses the diffusion of water molecules to generate contrast in MR images. It allows the mapping of the diffusion process of molecules, mainly water, in biological tissues, in vivo and non-invasively. Molecular diffusion in tissues is not free, but reflects interactions with many obstacles, such as macromolecules, fibers, and membranes. Water molecule diffusion patterns can therefore reveal microscopic details about tissue architecture, either normal or in a diseased state. A special kind of DWI, diffusion tensor imaging (DTI), has been used extensively to map white matter tractography in the brain.Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials (such as iron) form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished. Ferromagnetism (along with the similar effect ferrimagnetism) is the strongest type and is responsible for the common phenomena of magnetism in magnets encountered in everyday life. Substances respond weakly to magnetic fields with three other types of magnetism, paramagnetism, diamagnetism, and antiferromagnetism, but the forces are usually so weak that they can only be detected by sensitive instruments in a laboratory. An everyday example of ferromagnetism is a refrigerator magnet used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".Permanent magnets (materials that can be magnetized by an external magnetic field and remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are the materials that are noticeably attracted to them. Only a few substances are ferromagnetic. The common ones are iron, nickel, cobalt and most of their alloys, and some compounds of rare earth metals.
Ferromagnetism is very important in industry and modern technology, and is the basis for many electrical and electromechanical devices such as electromagnets, electric motors, generators, transformers, and magnetic storage such as tape recorders, and hard disks, and nondestructive testing of ferrous materials.Fluorescence anisotropy
Fluorescence anisotropy or fluorescence polarization is the phenomenon where the light emitted by a fluorophore has unequal intensities along different axes of polarization. Early pioneers in the field include Aleksander Jablonski, Gregorio Weber, and Andreas Albrecht. The principles of fluorescence polarization and some applications of the method are presented in Lakowicz's book.Fractional anisotropy
Fractional anisotropy (FA) is a scalar value between zero and one that describes the degree of anisotropy of a diffusion process. A value of zero means that diffusion is isotropic, i.e. it is unrestricted (or equally restricted) in all directions. A value of one means that diffusion occurs only along one axis and is fully restricted along all other directions. FA is a measure often used in diffusion imaging where it is thought to reflect fiber density, axonal diameter, and myelination in white matter. The FA is an extension of the concept of eccentricity of conic sections in 3 dimensions, normalized to the unit range.Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek isos (ἴσος, "equal") and tropos (τρόπος, "way"). Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary systematically, dependent on direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented.Magnetic anisotropy
Magnetic anisotropy is the directional dependence of a material's magnetic properties. The magnetic moment of magnetically anisotropic materials will tend to align with an easy axis, which is an energetically favorable direction of spontaneous magnetization. The two opposite directions along an easy axis are usually equivalent, and the actual direction of magnetization can be along either of them (see spontaneous symmetry breaking).
In contrast, a magnetically isotropic material has no preferential direction for its magnetic moment unless there is an applied magnetic field.
Magnetic anisotropy is a prerequisite for hysteresis in ferromagnets: without it, a ferromagnet is superparamagnetic.Magnetocrystalline anisotropy
In physics, a ferromagnetic material is said to have magnetocrystalline anisotropy if it takes more energy to magnetize it in certain directions than in others. These directions are usually related to the principal axes of its crystal lattice. It is a special case of magnetic anisotropy.Millimeter Anisotropy eXperiment IMaging Array
The Millimeter Anisotropy eXperiment IMaging Array (MAXIMA) experiment was a balloon-borne experiment funded by the U.S. NSF, NASA and Department of Energy, and operated by an international collaboration headed by the University of California, to measure the fluctuations of the cosmic microwave background. It consisted of two flights, one in August 1998 and one in June 1999. For each flight the balloon was started at the Columbia Scientific Balloon Facility in Palestine, Texas and flew to an altitude of 40,000 metres for over 8 hours. For the first flight it took data from about 0.3 percent of the sky of the northern region near the Draco constellation. For the second flight, known as MAXIMA-II, twice the area was observed, this time in the direction of Ursa Major.
Initially planned together with the BOOMERanG experiment, it split off during the planning phase to take a less risky approach by reducing flying time as well as launching and landing on U.S. territory.Mobile Anisotropy Telescope
MAT is an experiment to measure the anisotropy of the Cosmic microwave background at angular scales of 50 < l < 400.RELIKT-1
RELIKT-1 (sometimes RELICT-1 from Russian: РЕЛИКТ-1) - a Soviet cosmic microwave background anisotropy experiment on board the Prognoz 9 satellite (launched 1 July 1983) gave upper limits on the large-scale anisotropy. A reanalysis of the data in the later years claimed a confident blackbody form and anisotropy of the cosmic microwave background radiation. Results have been reported in January 1992 at the All-Moscow Astronomy Seminar held at Sternberg Astronomical Institute, and published, for example, in issue 4/1992 of the "Science in USSR" journal and in Soviet Astronomy Letters in May–June 1992. Nevertheless, the Nobel Prize in Physics for 2006 was awarded to a team of American scientists, who announced the fact on April 23, 1992 based on data taken by the COBE spacecraft.
This experiment was prepared by the Space Research Institute of the USSR Academy of Sciences and supervised by Dr. Igor Strukov.
A map of the sky at 37 GHz was built using an 8 mm band Dicke-type modulation radiometer. The radiometer could not conduct multi-band astronomical observations. The entire sky was observed in 6 months. The angular resolution was 5.5 degrees, with a temperature resolution of 0.6 mK.
The galactic microwave flux was measured and the CMB dipole observed. A quadrupole moment was found between 17 and 95 microkelvins rms, with 90% confidence level.
The heat radiation map of the Universe served as the emblem of the 1989 international conference "The Cosmic Wave Background: 25 Years Later" in L'Aquila, Italy.
The discovery of anisotropy by the RELIKT-1 spacecraft was first reported officially in January 1992 at the All-Moscow Astronomy Seminar held at Sternberg Astronomical Institute.
As a follow-up to RELIKT-1, it was decided in 1986 to study the anisotropy of CMB as part of the Relikt-2 project. The sensitivity of the equipment had greatly increased. The spacecraft was scheduled to launch in 1993-1994, but the launch never took place because of the Soviet Union's break-up and lack of funding.Reionization
In the field of Big Bang theory, and cosmology, reionization is the process that caused the matter in the universe to reionize after the lapse of the "dark ages".
Reionization is the second of two major phase transitions of gas in the universe. While the majority of baryonic matter in the universe is in the form of hydrogen and helium, reionization usually refers strictly to the reionization of hydrogen, the element.
It's believed that the primordial helium also experienced the same phase of reionization changes, but at different points in the history of the universe. This is usually referred to as helium reionization.Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP), was a spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic microwave background (CMB) – the radiant heat remaining from the Big Bang. Headed by Professor Charles L. Bennett of Johns Hopkins University, the mission was developed in a joint partnership between the NASA Goddard Space Flight Center and Princeton University. The WMAP spacecraft was launched on June 30, 2001 from Florida. The WMAP mission succeeded the COBE space mission and was the second medium-class (MIDEX) spacecraft in the NASA Explorers program. In 2003, MAP was renamed WMAP in honor of cosmologist David Todd Wilkinson (1935–2002), who had been a member of the mission's science team. After nine years of operations, WMAP was switched off in 2010, following the launch of the more advanced Planck spacecraft by ESA in 2009.
WMAP's measurements played a key role in establishing the current Standard Model of Cosmology: the Lambda-CDM model. The WMAP data are very well fit by a universe that is dominated by dark energy in the form of a cosmological constant. Other cosmological data are also consistent, and together tightly constrain the Model. In the Lambda-CDM model of the universe, the age of the universe is ±0.059 billion years. The WMAP mission's determination of the age of the universe is to better than 1% precision. The current expansion rate of the universe is (see Hubble constant) 13.772±0.80 km·s−1·Mpc−1. The content of the universe currently consists of 69.32±0.093% ordinary baryonic matter; 4.628%+0.88%
−0.87% cold dark matter (CDM) that neither emits nor absorbs light; and 24.02%+0.95%
−0.96% of dark energy in the form of a cosmological constant that accelerates the expansion of the universe. Less than 1% of the current content of the universe is in neutrinos, but WMAP's measurements have found, for the first time in 2008, that the data prefer the existence of a cosmic neutrino background with an effective number of neutrino species of 71.35%±0.35. The contents point to a Euclidean flat geometry, with curvature ( 3.26) of +0.0039
−0.0038. The WMAP measurements also support the cosmic inflation paradigm in several ways, including the flatness measurement. −0.0027
The mission has won various awards: according to Science magazine, the WMAP was the Breakthrough of the Year for 2003. This mission's results papers were first and second in the "Super Hot Papers in Science Since 2003" list. Of the all-time most referenced papers in physics and astronomy in the INSPIRE-HEP database, only three have been published since 2000, and all three are WMAP publications. Bennett, Lyman A. Page, Jr., and David N. Spergel, the latter both of Princeton University, shared the 2010 Shaw Prize in astronomy for their work on WMAP. Bennett and the WMAP science team were awarded the 2012 Gruber Prize in cosmology. The 2018 Breakthrough Prize in Fundamental Physics was awarded to Bennett, Gary Hinshaw, Norman Jarosik, Page, Spergel and the WMAP science team.
As of October 2010, the WMAP spacecraft is derelict in a heliocentric graveyard orbit after 9 years of operations. All WMAP data are released to the public and have been subject to careful scrutiny. The final official data release was the nine-year release in 2012.
Some aspects of the data are statistically unusual for the Standard Model of Cosmology. For example, the largest angular-scale measurement, the quadrupole moment, is somewhat smaller than the Model would predict, but this discrepancy is not highly significant. A large cold spot and other features of the data are more statistically significant, and research continues into these.