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Lambda-CDM Model
The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda (Greek Λ) associated with dark energy; second, the postulated cold dark matter (abbreviated CDM); and third, ordinary matter. It is frequently referred to as the ''standard model'' of Big Bang cosmology because it is the simplest model that provides a reasonably good account of the following properties of the cosmos: * the existence and structure of the cosmic microwave background * the large-scale structure in the distribution of galaxies * the observed abundances of hydrogen (including deuterium), helium, and lithium * the accelerating expansion of the universe observed in the light from distant galaxies and supernovae The model assumes that general relativity is the correct theory of gravity on cosmological scales. It emerged in the late 1990s as a con ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Parameter
A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when identifying the system, or when evaluating its performance, status, condition, etc. ''Parameter'' has more specific meanings within various disciplines, including mathematics, computer programming, engineering, statistics, logic, linguistics, and electronic musical composition. In addition to its technical uses, there are also extended uses, especially in non-scientific contexts, where it is used to mean defining characteristics or boundaries, as in the phrases 'test parameters' or 'game play parameters'. Modelization When a system is modeled by equations, the values that describe the system are called ''parameters''. For example, in mechanics, the masses, the dimensions and shapes (for solid bodies), the densities and the viscosit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Quintessence (physics)
In physics, quintessence is a hypothetical form of dark energy, more precisely a scalar field, postulated as an explanation of the observation of an accelerating rate of expansion of the universe. The first example of this scenario was proposed by Ratra and Peebles (1988) and Wetterich (1988). The concept was expanded to more general types of time-varying dark energy, and the term "quintessence" was first introduced in a 1998 paper by Robert R. Caldwell, Rahul Dave and Paul Steinhardt. It has been proposed by some physicists to be a fifth fundamental force. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic; that is, it changes over time, unlike the cosmological constant which, by definition, does not change. Quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Those working with this postulate believe that quintessence became repulsive about ten billion years ago, ab ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Rotation Curve
The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy. Theories involving dark matter are the main postulated solutions to account for the variance. The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. In contrast, the orbital velocities of planets in planetary syst ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Dark Matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect. Various astrophysical observationsincluding gravitational effects which cannot be explained by currently accepted theories of gravity unless more matter is present than can be seenimply dark matter's presence. For this reason, most experts think that dark matter is abundant in the universe and has had a strong influence on its structure and evolution. The primary evidence for dark matter comes from calculations showing that many galaxies would behave quite differently if they did not contain a large amount of unseen matter. Some galaxies would not have formed at all and others would not move as they currently do. Other lines of evidence include obse ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Planck (spacecraft)
''Planck'' was a space observatory operated by the European Space Agency (ESA) from 2009 to 2013, which mapped the anisotropies of the cosmic microwave background (CMB) at microwave and infrared frequencies, with high sensitivity and small angular resolution. The mission substantially improved upon observations made by the NASA Wilkinson Microwave Anisotropy Probe (WMAP). ''Planck'' provided a major source of information relevant to several cosmological and astrophysical issues, such as testing theories of the early Universe and the origin of cosmic structure. Since the end of its mission, ''Planck'' has defined the most precise measurements of several key cosmological parameters, including the average density of ordinary matter and dark matter in the Universe and the age of the universe. The project was started around 1996 and was initially called COBRAS/SAMBA: the Cosmic Background Radiation Anisotropy Satellite/Satellite for Measurement of Background Anisotropies. It was l ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Type Ia Supernova
A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf. Physically, carbon–oxygen white dwarfs with a low rate of rotation are limited to below 1.44 solar masses (). Beyond this "critical mass", they reignite and in some cases trigger a supernova explosion; this critical mass is often referred to as the Chandrasekhar mass, but is marginally different from the absolute Chandrasekhar limit, where electron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, or merges with a second white dwarf, the general hypothesis is that a white dwarf's core will reach the ignition temperature for carbon fusion as it approaches the Chandrasekhar mass. Within a few seconds of initiation of nuclear fusion, a substant ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Dark Energy Survey
The Dark Energy Survey (DES) is an astronomical survey designed to constrain the properties of dark energy. It uses images taken in the near-ultraviolet, visible, and near-infrared to measure the expansion of the universe using Type Ia supernovae, baryon acoustic oscillations, the number of galaxy clusters, and weak gravitational lensing. The collaboration is composed of research institutions and universities from the United States,DES Collaboration Page DES Collaborators. Australia, Brazil,DES-Brazil , DES-Brazil Consortium. the United Kingdom, Germany, Spain, and Switzerland. The collaboration is divided into several scientific working groups. The director of DES is [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Stress–energy Tensor
The stress–energy tensor, sometimes called the stress–energy–momentum tensor or the energy–momentum tensor, is a tensor physical quantity that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and non-gravitational force fields. This density and flux of energy and momentum are the sources of the gravitational field in the Einstein field equations of general relativity, just as mass density is the source of such a field in Newtonian gravity. Definition The stress–energy tensor involves the use of superscripted variables (''not'' exponents; see tensor index notation and Einstein summation notation). If Cartesian coordinates in SI units are used, then the components of the position four-vector are given by: , , , and , where ''t'' is time in seconds, and ''x'', ''y'', and ''z'' are distances in meters. The stress–energy tensor is defined as the tensor ''T ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Doppler Shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842. A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession. The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an inc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Red Shift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum. In astronomy and cosmology, the three main causes of electromagnetic redshift are # The radiation travels between objects which are moving apart (" relativistic" redshift, an example of the relativistic Doppler effect) #The radiation travels towards an object in a weaker gravitational potential, i.e. towards an object in less strongly curved (flatter) spacetime (gravitational redshift) #The radiation travels through expanding space ( cosmological redshift). The observation that all sufficiently distant light sources show redshift corresponding to their distance from ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Pavel Kroupa
Pavel Kroupa (born 24 September 1963 in Jindřichův Hradec, Czechoslovakia) is a Czech-Australian astrophysicist and professor at the University of Bonn. Biography and career After the 1968 failure of Prague spring, Kroupa's family fled from Czechoslovakia losing all possessions; as a consequence Kroupa grew up in Germany and South Africa. He acquired in 1983 his Abitur final exams in Göttingen and afterwards studied physics at The University of Western Australia in Perth. In 1988 he won the Isaac Newton scholarship at the University of Cambridge and in 1992 the senior Rouse Ball research scholarship at Trinity College, Cambridge and attained a doctorate in England in 1992 with a dissertation on the distribution of low mass stars in the Milky Way. Afterwards Kroupa worked until 2000 in astronomical research groups at Heidelberg University and at the Max Planck Institute for Astronomy, before he went to the University of Kiel and earned his Habilitation there. He was awarded ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |