1,790,637 research outputs found
F & R, Ludmilla, Schutzmarke registriert
No full textF & R, LUDMILLA, SCHUTZMARKE REGISTRIERT
F & R, Ludmilla, Schutzmarke registriert ( -
Cosmological consequences of f(R) gravity
No full textreservedIn this thesis, we are focusing on a theory called f(R) gravity, obtained from modifications of General Relativity (GR) by adding higher powers of the Ricci scalar into the standard GR action. By introducing additional terms to the action, we aim to replicate the observed evolution of the Universe without being hindered by the problems of the existence of dark matter and dark energy and address some other interesting phenomena in the universe such as gravitational waves and cosmological perturbations.In this thesis, we are focusing on a theory called f(R) gravity, obtained from modifications of General Relativity (GR) by adding higher powers of the Ricci scalar into the standard GR action. By introducing additional terms to the action, we aim to replicate the observed evolution of the Universe without being hindered by the problems of the existence of dark matter and dark energy and address some other interesting phenomena in the universe such as gravitational waves and cosmological perturbations
F & R, Randl Ia Ia, Schutzmarke registriert
No full textF & R, RANDL IA IA, SCHUTZMARKE REGISTRIERT
F & R, Randl Ia Ia, Schutzmarke registriert ( -
QCD-Scale f(R) Theories: Local and Cosmological Constraints
Full text linkIn this Thesis we investigate the phenomenology of two QCD-scale modified gravity models. The first f(R) modified gravity model is the square-root modified model , which is motivated by q-theory. The second f(R) model which is investigated in this Thesis is a modified gravity model that is relevant in the high-curvature regime, motivated by the form of the one-loop effective action arising from gluons in curved spacetime
Cosmological Models in Modified f(R) Gravity Theories
No full textLa actual expansión acelerada del universo sigue siendo un gran misterio para la física. Varios modelos se han propuesto para su explicación, entre ellos el de la energía oscura, que sin embargo posee problemas tanto de carácter teórico como experimental. Sin embargo, otras aproximaciones, como la de las teorías de gravedad modificada son una interesante alternativa para resolver este problema. Motivados en este enfoque realizamos un estudio de los modelos cosmológicos en las teorías f(R) que son extensiones naturales de la Relatividad General con funciones arbitrarias del escalar de Ricci. Un capítulo esta dedicado a la obtención de las ecuaciones de campo en el formalismo métrico de las teorías f(R), incluyendo la discusión sobre los términos de frontera. Aplicamos después tales ecuaciones para escribir las ecuaciones dinámicas del universo, aceptando para esto la validez del modelo estándar de la Cosmología, es decir, usando como espacio-tiempo el de Robertson-Walker. La aproximación de sistema dinámico nos permite encontrar criterios de viabilidad para las funciones f(R), exigiéndoles que cumplan con una transición de época de dominio de materia, a una de expansión acelerada. Centramos gran atención en el problema de las distancias cosmológicas en teorías f(R), obteniendo expresiones generales a partir del estudio de la Ecuación de Desvío Geodésico. Finalmente se muestra la teoría de perturbaciones lineales y el problema de Cauchy en estas teorías. / Abstract. The actual accelerated expansion of the universe continues being a mystery in physics. Some models had been proposed for this explanations, among them the dark energy, which however has problems of experimental character as well as theoretical. Other approximations, like modified gravity theories are an interesting alternative for this problem. Motivated in this approach we study cosmological models in f(R) theories which are natural extension of General Relativity with arbitrary functions of the Ricci scalar. One chapter has dedicated to obtain the modified field equations in the metric formalism of f(R) theories, including the discussion about boundary terms in the action. Later, we apply these equations in order to describe the dynamics of the universe, using for this as space-time, the Robertson- Walker metric. The Dynamical system approach for this equations allow us to find viability criteria for the functions f(R), demanding them to satisfy a transition between a mater dominated epoch to an accelerated dominating epoch. We focus our study in the problem of cosmological distances in f(R) theories, obtaining general expressions from the Geodesic Deviation Equation (GDE). Finally we show the perturbation theory to first order and the Cauchy problem in these theories.Maestrí
F. R. Leavis: The Creative University
No full textThis is a critical introduction to the educational thought of F. R. Leavis (1895-1978), the greatest English literary critic of the twentieth century, providing the first in-depth examination of Leavis's ideas in relation to contemporary mass higher education. During the course of a long, prolific and controversial academic career, which saw him take issue with figures such as Wittgenstein, T. S. Eliot and C. P. Snow, Leavis became one of the most articulate advocates for the idea of the university as 'a centre of consciousness and human responsibility' in the face of what he saw as the relentless technological drive of civilisation. With the journal Scrutiny which he co-founded, as well as his critical writings, Leavis became a decisive influence on generations of teachers in Britain and overseas. Widely misrepresented as narrowly elitist, his ideas about 'the creative university', with their radical, student-centred approach to teaching, constitute a powerful resource for a higher education system grappling with the contradictory demands of continuity and change. Based on original research, the study provides an overview of Leavis's life, work and heritage and his educational world view, and a comprehensive exploration of Leavis's pedagogy from theoretical and practical perspectives. It also includes a first-hand account by the author of being taught by Leavis in person
f(R,T)=f(R)+λT gravity models as alternatives to cosmic acceleration
No full textAbstract This article presents cosmological models that arise in a subclass of f(R,T)=f(R)+f(T) gravity models, with different f(R) functions and fixed T-dependence. That is, the gravitational lagrangian is considered as f(R,T)=f(R)+λT , with constant λ . Here R and T represent the Ricci scalar and trace of the stress-energy tensor, respectively. The modified gravitational field equations are obtained through the metric formalism for the Friedmann–Lemaître–Robertson–Walker metric with signature (+,-,-,-) . We work with f(R)=R+αR2-μ4R , f(R)=R+kln(γR) and f(R)=R+me[-nR] , with α,μ,k,γ,m and n all free parameters, which lead to three different cosmological models for our Universe. For the choice of λ=0 , this reduces to widely discussed f(R) gravity models. This manuscript clearly describes the effects of adding the trace of the energy-momentum tensor in the f(R) lagrangian. The exact solution of the modified field equations are obtained under the hybrid expansion law. Also we present the Om diagnostic analysis for the discussed models
Personal Papers (MS 80-0002)
No full textLetter from D. W. Kempner to F. R. Tripler & Co. regarding several recent orders that were mishandled
Parámetros de anisotropía en teoría f(R)
No full textSe presenta la teoría de perturbaciones cosmológicas a primer orden en Relatividad General basado en los trabajos desarrollados por Bardeen, H. Kodama, Mukhanov [13] [52], por medio de la cual se tratan las ecuaciones de campo en teoría f(R) en el formalismo métrico bajo el gauge Newtoniano en métrica FLRW para un fluido no perfecto, en donde se plantea la posibilidad de parametrizar las anisotropías inherentes a la construcción del modelo en teoría f(R), de forma similar a como se construyen en las parametrizaciones post-Friedmannianas [25], las cuales buscan por medio de parámetros hacer mímica del modelo ΛCDM. Para finalizar se hace una comparación de códigos para anisotropías de la radiación cósmica de fondo CAMB [11] y FRcamb [38] con el fin de observar en primera aproximación el comportamiento de un modelo f(R) que hace mímica al modelo ΛCDMAbstract. The first order cosmological perturbation theory for General Relativity is introduced, based on the work developed by Bardeen, H. Kodama, Mukhanov [13] [52], which is used to treat the field equations in the metric f(R) theory for the flat FLRW metric in a non-perfect fluid under the Newtonian Gauge, where is set the posibility to paramerterize the inerent anisotropies obtained from f(R) field equations in a similar way as done in the Post-Friedmannian Paramerterizations[25], which mimic the ΛCDM model by the introdution of phenomenological parameters. In the final part its shown a comparison between the codes for the anisotropies in the cosmic microwave background, CAMB [11] and the FRCAMB [38], with the purpose of observing in the first approximation the behavior of a model f(R) that mimics the ΛCDM model.Maestrí
- …
