The Experts below are selected from a list of 298140 Experts worldwide ranked by ideXlab platform
Parampreet Singh - One of the best experts on this subject based on the ideXlab platform.
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robustness of key features of loop quantum Cosmology
Physical Review D, 2008Co-Authors: Abhay Ashtekar, Alejandro Corichi, Parampreet SinghAbstract:A small simplification based on well-motivated approximations is shown to make loop quantum Cosmology of the $k=0$ Friedman-Robertson-Walker model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler-DeWitt theory approximates loop quantum Cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying loop quantum Cosmology is fundamental. Finally, the model is compared to analogous discussions in the literature and it is pointed out that some of their expectations do not survive a more careful examination. An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity.
E L Wright - One of the best experts on this subject based on the ideXlab platform.
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a Cosmology calculator for the world wide web
Publications of the Astronomical Society of the Pacific, 2006Co-Authors: E L WrightAbstract:A Cosmology calculator that computes times and distances as a function of redshift for user- defined cosmological parameters has been made available online. This paper gives the formulae used by the Cosmology calculator and discusses some of its implementation. A version of the calculator that allows one to specify the equation-of-state parameter w and , and one for converting the light-travel times usually given in � w the popular press into redshifts, is also located at the same site.
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a Cosmology calculator for the world wide web
arXiv: Astrophysics, 2006Co-Authors: E L WrightAbstract:A Cosmology calculator that computes times and distances as a function of redshift for user-defined cosmological parameters is available on the World Wide Web. This note gives the formulae used by the Cosmology calculator and discusses some of its implementation. A version of the calculator that allows one to specify the equation of state parameter w and w' and neutrino masses, and a version for converting the light travel times usually given in the popular press into redshifts are also available.
Francesca Vidotto - One of the best experts on this subject based on the ideXlab platform.
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General covariant transition amplitudes in quantum Cosmology
Scientifica Acta, 2011Co-Authors: Francesca VidottoAbstract:The path-integral approach to Cosmology consists in the computation of transition amplitudes between states of the quantum geometry of the universe. In the past, the concrete computation of these transitions amplitudes has been performed in a perturbative regime, breaking the full general covariance of the theory. Here I present how it is possible to define a general covariant path integral in quantum Cosmology, by relying on the most recent results of the canonical and covariant formulations of Loop Quantum Gravity. I present two strategies that have been implemented. The first starts from the full Spinfoam theory, i.e. the path-integral framework for Loop Quantum Gravity, and defines a cosmological system. This is not obtained from symmetry reduced variables that are successively quantized, but directly considering the approximations that are characteristic of the full theory. The Spinfoam Cosmology obtained in this way includes quantum fluctuations beyond standard perturbation theory. The second strategy exploits the Hamiltonian constraint of Loop Quantum Cosmology, that is exponentiated in the formal expression of the usual path integral. The result is a general covariant path integral, that reproduces the form of the amplitude in the full Spinfoam theory. Therefore, this procedure connects the canonical and the covariant formalisms.
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Spinfoam Cosmology: quantum Cosmology from the full theory
2010Co-Authors: Francesca VidottoAbstract:Quantum Cosmology is usually studied quantizing symmetry-reduced variables. Is it possible, instead, to define quantum Cosmology starting from the full quantum gravity theory? In Loop Quantum Gravity (LQG), it is possible to cut the degrees of freedom in a suitable way, in order to define a cosmological model. Such a model provides a tool for describing general fluctuations of the quantum geometry at the bounce that replaces the initial singularity. I focus on its simplest version, a "dipole" formed by two tetrahedra. This has been shown to describe a universe with anisotropic and inhomogenous degrees of freedom. Its dynamics can be given using the spinfoam formalism. I briefly review the present state of this approach.
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Stepping out of Homogeneity in Loop Quantum Cosmology
Classical and Quantum Gravity, 2008Co-Authors: Carlo Rovelli, Francesca VidottoAbstract:We explore the extension of quantum Cosmology outside the homogeneous approximation, using the formalism of loop quantum gravity. We introduce a model where some of the inhomogeneous degrees of freedom are present, providing a tool for describing general fluctuations of quantum geometry near the initial singularity. We show that the dynamical structure of the model reduces to that of loop quantum Cosmology in the Born-Oppenheimer approximation. This result corroborates the assumptions that ground loop Cosmology, sheds light on the physical and mathematical relation between loop Cosmology and full loop quantum gravity, and on the nature of the cosmological approximation. Finally, we show that the non-graph-changing Hamiltonian constraint considered in the context of algebraic quantum gravity provides a viable effective dynamics within this approximation.
Abhay Ashtekar - One of the best experts on this subject based on the ideXlab platform.
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robustness of key features of loop quantum Cosmology
Physical Review D, 2008Co-Authors: Abhay Ashtekar, Alejandro Corichi, Parampreet SinghAbstract:A small simplification based on well-motivated approximations is shown to make loop quantum Cosmology of the $k=0$ Friedman-Robertson-Walker model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler-DeWitt theory approximates loop quantum Cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying loop quantum Cosmology is fundamental. Finally, the model is compared to analogous discussions in the literature and it is pointed out that some of their expectations do not survive a more careful examination. An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity.
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mathematical structure of loop quantum Cosmology
arXiv: General Relativity and Quantum Cosmology, 2003Co-Authors: Abhay Ashtekar, Martin Bojowald, Jerzy LewandowskiAbstract:Applications of Riemannian quantum geometry to Cosmology have had notable successes. In particular, the fundamental discreteness underlying quantum geometry has led to a natural resolution of the big bang singularity. However, the precise mathematical structure underlying loop quantum Cosmology and the sense in which it implements the full quantization program in a symmetry reduced model has not been made explicit. The purpose of this paper is to address these issues, thereby providing a firmer mathematical and conceptual foundation to the subject.
B. Bolliet - One of the best experts on this subject based on the ideXlab platform.
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Observational exclusion of a consistent quantum cosmological scenario
Physical Review D, 2016Co-Authors: B. Bolliet, A. Barrau, J. Grain, S. SchanderAbstract:It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very \textit{ad hoc} scenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well motivated candidate quantum Cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the \textit{deformed algebra approach} to loop quantum Cosmology. We emphasize that neither loop quantum Cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.
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Some conceptual issues in loop quantum Cosmology
International Journal of Modern Physics D, 2016Co-Authors: A. Barrau, B. BollietAbstract:Loop quantum gravity is a mature theory. To proceed to explicit calculations in Cosmology, it is necessary to make assumptions and simplifications based on the symmetries of the cosmological setting. Symmetry reduction is especially critical when dealing with cosmological perturbations. The present article reviews several approaches to the problem of building a consistent formalism that describes the dynamics of perturbations on a quantum spacetime and tries to address their respective strengths and weaknesses. We also review the main open issues in loop quantum Cosmology.
