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Black hole

Black hole
A black hole is defined as a region of spacetime from which gravity prevents anything, including light, from escaping.[1] The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.[2] Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return. The hole is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[3][4] Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater. Objects whose gravity fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. History General relativity

Galaxy Galaxies contain varying numbers of planets, star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Supermassive black holes reside at the center of most galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. Galaxies have been historically categorized according to their apparent shape, usually referred to as their visual morphology. Etymology[edit] The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας, "milky one"), or kyklos ("circle") galaktikos ("milky")[11] for its appearance as a lighter colored band in the sky. In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our galaxy, the Milky Way, to distinguish it from the billions of other galaxies. "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." Nomenclature[edit] Observation history[edit]

Loop quantum gravity More precisely, space can be viewed as an extremely fine fabric or network "woven" of finite loops. These networks of loops are called spin networks. The evolution of a spin network over time is called a spin foam. The predicted size of this structure is the Planck length, which is approximately 10−35 meters. According to the theory, there is no meaning to distance at scales smaller than the Planck scale. Today LQG is a vast area of research, developing in several directions, which involves about 50 research groups worldwide.[1] They all share the basic physical assumptions and the mathematical description of quantum space. Research into the physical consequences of the theory is proceeding in several directions. History[edit] The canonical version of the dynamics was put on firm ground by Thomas Thiemann, who defined an anomaly-free Hamiltonian operator, showing the existence of a mathematically consistent background-independent theory. LQG is formally background independent. and . . .

Cosmic microwave background radiation Temperature of the cosmic background radiation spectrum as determined with the COBE satellite: uncorrected (top), corrected for the dipole term due to our peculiar velocity (middle), and corrected for contributions from the dipole term and from our galaxy (bottom). The Sunyaev–Zel'dovich effect shows the phenomena of radiant cosmic background radiation interacting with "electron" clouds distorting the spectrum of the radiation. There is also background radiation in the infrared, x-rays, etc., with different causes, and they can sometimes be resolved into an individual source. See cosmic infrared background and X-ray background. See also cosmic neutrino background and extragalactic background light. History of significant events[edit] 1896: Charles Édouard Guillaume estimates the "radiation of the stars" to be 5.6 K. 1926: Sir Arthur Eddington estimates the non-thermal radiation of starlight in the galaxy has an effective temperature of 3.2 K. [1] See also[edit] References[edit]

Event horizon In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms, it is defined as "the point of no return", i.e., the point at which the gravitational pull becomes so great as to make escape impossible. The most common case of an event horizon is that surrounding a black hole. More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Existence and evolution of the particle horizon[edit] Note: This section is based mainly on two relatively new articles (2012 and 2013) and is not yet accepted as part of the standard discussion of Black Holes. A simple example of event horizon emerges from some cases of the FLRW cosmological model. , partial pressure and state equation , such that they add up to the total density and total pressure .[1] Let us now define the following functions: (or equivalently ). where . is the lowest (possibly ). ) is:[2] (natural units). by

Gravitational singularity A gravitational singularity or spacetime singularity is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system. These quantities are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. The two most important types of spacetime singularities are curvature singularities and conical singularities.[2] Singularities can also be divided according to whether they are covered by an event horizon or not (naked singularities).[3] According to general relativity, the initial state of the universe, at the beginning of the Big Bang, was a singularity. Interpretation[edit] Many theories in physics have mathematical singularities of one kind or another. Some theories, such as the theory of loop quantum gravity suggest that singularities may not exist. Types[edit] Curvature[edit] , which is diffeomorphism invariant, is infinite. Conical[edit] Naked[edit] Entropy[edit]

Nebula Portion of the Carina nebula A nebula (from Latin: "cloud";[1] pl. nebulae or nebulæ, with ligature, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. Observational history The "Pillars of Creation" from the Eagle Nebula. On November 26, 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. In 1715, Edmund Halley published a list of six nebulae.[8] This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. The number of nebulae was then greatly expanded by the efforts of William Herschel and his sister Caroline Herschel. Formation The Triangulum Emission Garren Nebula NGC 604 Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium or ISM. Other nebulae may form as planetary nebulae.

String theory String theory was first studied in the late 1960s[3] as a theory of the strong nuclear force before being abandoned in favor of the theory of quantum chromodynamics. Subsequently, it was realized that the very properties that made string theory unsuitable as a theory of nuclear physics made it a promising candidate for a quantum theory of gravity. Five consistent versions of string theory were developed until it was realized in the mid-1990s that they were different limits of a conjectured single 11-dimensional theory now known as M-theory.[4] Many theoretical physicists, including Stephen Hawking, Edward Witten and Juan Maldacena, believe that string theory is a step towards the correct fundamental description of nature: it accommodates a consistent combination of quantum field theory and general relativity, agrees with insights in quantum gravity (such as the holographic principle and black hole thermodynamics) and has passed many non-trivial checks of its internal consistency.

Cosmic string Cosmic strings are hypothetical 1-dimensional (spatially) topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking was not simply connected. It is expected that at least one string per Hubble volume is formed. Their existence was first contemplated by the theoretical physicist Tom Kibble in the 1970s. The formation of cosmic strings is somewhat analogous to the imperfections that form between crystal grains in solidifying liquids, or the cracks that form when water freezes into ice. Theories containing cosmic strings[edit] The prototypical example of a quantum field theory with cosmic strings is the Abelian Higgs model. Dimensions[edit] Cosmic strings, if they exist, would be extremely thin with diameters of the same order of magnitude as that of a proton, i.e. ~ 1 fm, or smaller. Gravitation[edit] Negative Mass Cosmic String[edit] Observational evidence[edit]

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