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Infinite Derivative Gravity
Infinite derivative gravity is a theory of gravity which attempts to remove cosmological and black hole singularities by adding extra terms to the Einstein–Hilbert action, which weaken gravity at short distances. History In 1987, Krasnikov considered an infinite set of higher derivative terms acting on the curvature terms and showed that by choosing the coefficients wisely, the propagator would be ghost-free and exponentially suppressed in the ultraviolet regime. Tomboulis (1997) later extended this work. By looking at an equivalent scalar-tensor theory, Biswas, Mazumdar and Siegel (2005) looked at bouncing FRW solutions. In 2011, Biswas, Gerwick, Koivisto and Mazumdar demonstrated that the most general infinite derivative action in 4 dimensions, around constant curvature backgrounds, parity invariant and torsion free, can be expressed by: :S = \int \mathrm^4x \sqrt \left(M^2_P R+ R F_1 (\Box) R + R^ F_2 (\Box) R_ + C^ F_3 (\Box) C_ \right) where the F_i (\Box)=\sum^\infty_ f_ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theory Of Gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong interaction, 1036 times weaker than the electromagnetic force and 1029 times weaker than the weak interaction. As a result, it has no significant influence at the level of subatomic particles. However, gravity is the most significant interaction between objects at the macroscopic scale, and it determines the motion of planets, stars, galaxies, and even light. On Earth, gravity gives weight to physical objects, and the Moon's gravity is responsible for sublunar tides in the oceans (the corresponding antipodal tide is caused by the inertia of the Earth and Moon orbiting one another). Gravity also has many important biological functions, helping to guide the growth of plants through the process of gravitropism and influencing the circulatio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gravitational Singularity
A gravitational singularity, spacetime singularity or simply singularity is a condition in which gravitational field, gravity is so intense that spacetime itself breaks down catastrophically. As such, a singularity is by definition no longer part of the regular spacetime and cannot be determined by "where" or "when". Gravitational singularities exist at a junction between general relativity and quantum mechanics; therefore, the properties of the singularity cannot be described without an established theory of quantum gravity. Trying to find a complete and precise definition of singularities in the theory of general relativity, the current best theory of gravity, remains a difficult problem. A singularity in general relativity can be defined by the Curvature invariant (general relativity), scalar invariant Curvature of Riemannian manifolds, curvature becoming Infinity, infinite or, better, by a Geodesics in general relativity, geodesic being Geodesic manifold#Non-examples, incom ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Einstein–Hilbert Action
The Einstein–Hilbert action (also referred to as Hilbert action) in general relativity is the action that yields the Einstein field equations through the stationary-action principle. With the metric signature, the gravitational part of the action is given as :S = \int R \sqrt \, \mathrm^4x, where g=\det(g_) is the determinant of the metric tensor matrix, R is the Ricci scalar, and \kappa = 8\pi Gc^ is the Einstein gravitational constant (G is the gravitational constant and c is the speed of light in vacuum). If it converges, the integral is taken over the whole spacetime. If it does not converge, S is no longer well-defined, but a modified definition where one integrates over arbitrarily large, relatively compact domains, still yields the Einstein equation as the Euler–Lagrange equation of the Einstein–Hilbert action. The action was first proposed by David Hilbert in 1915. Discussion Deriving equations of motion from an action has several advantages. First, it allows ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Anupam Mazumdar
Anupam Mazumdar is a theoretical physicist at the University of Groningen specializing in cosmology and quantum gravity. Together with Sougato Bose, Mazumdar has proposed a bonafide test for the existence of the graviton in a table-top experiment, via witnessing gravitationally-mediated entanglement between two macroscopic superpositions of masses. A positive test of this phenomenon would establish experimentally that gravity is quantum mechanical in nature, and establish the existence of the graviton. The test crucially depends on the quantum nature of gravity, creating non-classical states of matter, and local operation and quantum communication (LOQC). He has previously been affiliated to the Higgs Centre, at the University of Edinburgh, and the Discovery Center at the Niels Bohr Institute, Copenhagen. His work has focused on multi field theories of inflation, such as ''assisted inflation'', visible sector inflation such as ''MSSM inflation''. He has worked on the ghost-free ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Warren Siegel
Warren Siegel ( ) is a theoretical physicist specializing in supersymmetric quantum field theory and string theory. He is a professor at the C. N. Yang Institute for Theoretical Physics at Stony Brook University in New York. Background Siegel did his undergraduate and graduate work at the University of California, Berkeley, graduating with a PhD in 1977. Following his graduation he worked at several postdoctoral appointments at Harvard (7/77-7/79), Brandeis University (3/79-6/79), the Institute for Advanced Study (8/79-8/80), Caltech (8/80-8/82) and University of California, Berkeley (8/82-8/85). He served as an assistant professor at the University of Maryland, College Park from 1985 to 1987 before becoming a professor at Stony Brook University in 1987. Research His early work involved the use of superspace to treat supersymmetric theories, including supergravity. Along with S.J. Gates, M.T. Grisaru, and M. Rocek he discovered methods for both deriving classical actions, and per ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
D'Alembert Operator
In special relativity, electromagnetism and wave theory, the d'Alembert operator (denoted by a box: \Box), also called the d'Alembertian, wave operator, box operator or sometimes quabla operator (''cf''. nabla symbol) is the Laplace operator of Minkowski space. The operator is named after French mathematician and physicist Jean le Rond d'Alembert. In Minkowski space, in standard coordinates , it has the form : \begin \Box & = \partial^\mu \partial_\mu = \eta^ \partial_\nu \partial_\mu = \frac \frac - \frac - \frac - \frac \\ & = \frac - \nabla^2 = \frac - \Delta ~~. \end Here \nabla^2 := \Delta is the 3-dimensional Laplacian and is the inverse Minkowski metric with :\eta_ = 1, \eta_ = \eta_ = \eta_ = -1, \eta_ = 0 for \mu \neq \nu. Note that the and summation indices range from 0 to 3: see Einstein notation. We have assumed units such that the speed of light = 1. (Some authors alternatively use the negative metric signature of , with \eta_ = -1,\; \eta_ = \eta_ = \ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gibbons–Hawking–York Boundary Term
In general relativity, the Gibbons–Hawking–York boundary term is a term that needs to be added to the Einstein–Hilbert action when the underlying spacetime manifold has a boundary. The Einstein–Hilbert action is the basis for the most elementary variational principle from which the field equations of general relativity can be defined. However, the use of the Einstein–Hilbert action is appropriate only when the underlying spacetime manifold \mathcal is closed, i.e., a manifold which is both compact and without boundary. In the event that the manifold has a boundary \partial\mathcal, the action should be supplemented by a boundary term so that the variational principle is well-defined. The necessity of such a boundary term was first realised by York and later refined in a minor way by Gibbons and Hawking. For a manifold that is not closed, the appropriate action is :\mathcal_\mathrm + \mathcal_\mathrm = \frac \int_\mathcal \mathrm^4 x \, \sqrt R + \frac \int_ \mathrm^3 y ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Friedmann–Lemaître–Robertson–Walker Metric
The Friedmann–Lemaître–Robertson–Walker (FLRW; ) metric is a metric based on the exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic, expanding (or otherwise, contracting) universe that is path-connected, but not necessarily simply connected. The general form of the metric follows from the geometric properties of homogeneity and isotropy; Einstein's field equations are only needed to derive the scale factor of the universe as a function of time. Depending on geographical or historical preferences, the set of the four scientists – Alexander Friedmann, Georges Lemaître, Howard P. Robertson and Arthur Geoffrey Walker – are customarily grouped as Friedmann or Friedmann–Robertson–Walker (FRW) or Robertson–Walker (RW) or Friedmann–Lemaître (FL). This model is sometimes called the ''Standard Model'' of modern cosmology, although such a description is also associated with the further developed Lambda-CDM model. T ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theories Of Gravity
A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be scientific, belong to a non-scientific discipline, or no discipline at all. Depending on the context, a theory's assertions might, for example, include generalized explanations of how nature works. The word has its roots in ancient Greek, but in modern use it has taken on several related meanings. In modern science, the term "theory" refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with the scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that scientific tests should be able to provide empirical support for it, or empirical contradiction ("falsify") of it. Scientific theories are the most reliable, rigorous, and compre ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
General Relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the ' is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of second order partial differential equations. Newton's law of universal gravitation, which describes classical gravity, can be seen as a prediction of general relativity for the almost flat spacetime geometry around stationary mass distributions. Some predictions of general relativity, however, are beyond Newton's law of universal gravitat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
