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Časově závislé procesy v teorii strun
Jiroušek, Pavel ; Schnabl, Martin (advisor) ; Novotný, Jiří (referee)
String field theory is a second quantized approach to string theory able to describe off-shell processes like transition between vacua, so-called tachyon condensation, and its dynamics. In this work, we review the necessary tools and ingredients needed to formulate such theory for open bosonic string theory. Namely, we introduce elementary conformal field theory methods and some basics of perturbative string theory. We then recognize the kinetic term of string field action from on-shell condition of the perturbative string and axiomatically introduce an algebraic structure on its Hilbert space to construct interaction terms. We briefly discuss the connection of classical string field theory solutions with various string field backgrounds and present means of reconstructing the background from a given solution by calculating the boundary state. We show that such backgrounds act like a source of open string states. We conclude by calculation of such boundary state for a solution representing light-like tachyon rolling. Powered by TCPDF (www.tcpdf.org)
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Modified Gravity and Cosmic Acceleration: Now and in the Early Universe
Jiroušek, Pavel ; Vikman, Alexander (advisor) ; Mukohyama, Shinji (referee) ; Sawicki, Ignacy (referee)
We review our previous works which explored the problems of dark energy and cos- mological constant problem in the context of modified gravity. In the first part, we present extensions of mimetic dark matter. The latter is a Weyl-invariant scalar-tensor theory able to describe dark matter on cosmological scales. In our works we have extended the mimetic construction to vector fields and Yang-Mills gauge fields. The resulting theories provided a novel Weyl-invariant and higher derivative formulations of unimodular gravity. We also introduced a mixture of this mimetic dark energy with mimetic dark matter and showed that it results in k-essence like scalar theory. In the second part we reviewed a minimal modification of Einstein equations, in which their trace part is made trivial. This results in the Newton constant becoming a global degree of freedom. Consequently, the Newton constant is subjected to quantum fluctuations and uncertainty relations. We find that the same applies to the effective Planck constant in a certain classically equivalent formulation of this gravity modification. Finally, we present an analysis of tensor perturbations in the recently proposed models of minimally varying cosmological constant. In these theories, extending Einstein-Cartan gravity, the cosmological constant is allowed to...
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Rozšířená mimetická gravitace
Jiroušek, Pavel ; Vikman, Alexander (advisor)
We consider a novel extension of the recently proposed mimetic gravity. The latter is a scalar-tensor theory which is able to describe dark matter on cosmological scales. Moreover, this theory can be considered as a low energy limit of the projectable Horava-Lifshitz gravity. The proposed novel extension directly couples gradients of the mimetic scalar field to the curvature tensor. These couplings introduce into the energy momentum tensor an anisotropic stress which is non-vanishing even at the first order in perturbations around a cosmological background. Further we show that such terms modify the formula for the speed of sound of scalar perturbations and even more importantly change the speed of propagation for the gravitational waves. The appearance of the anisotropic stress and the consequent nontrivial speed of propagation of the gravity waves are new phenomena which were not present in the previously studied mimetic models. Furthermore, we demonstrate that the effective Newton's gravitational constant in the background Friedmann equations is shifted in the presence of the novel couplings of the mimetic scalar field. We calculate the quadratic action for scalar and tensor perturbations and briefly discuss possible instabilities. Finally we consider the current observational bounds on the model....
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Rozšířená mimetická gravitace
Jiroušek, Pavel ; Vikman, Alexander (advisor)
We consider a novel extension of the recently proposed mimetic gravity. The latter is a scalar-tensor theory which is able to describe dark matter on cosmological scales. Moreover, this theory can be considered as a low energy limit of the projectable Horava-Lifshitz gravity. The proposed novel extension directly couples gradients of the mimetic scalar field to the curvature tensor. These couplings introduce into the energy momentum tensor an anisotropic stress which is non-vanishing even at the first order in perturbations around a cosmological background. Further we show that such terms modify the formula for the speed of sound of scalar perturbations and even more importantly change the speed of propagation for the gravitational waves. The appearance of the anisotropic stress and the consequent nontrivial speed of propagation of the gravity waves are new phenomena which were not present in the previously studied mimetic models. Furthermore, we demonstrate that the effective Newton's gravitational constant in the background Friedmann equations is shifted in the presence of the novel couplings of the mimetic scalar field. We calculate the quadratic action for scalar and tensor perturbations and briefly discuss possible instabilities. Finally we consider the current observational bounds on the model....
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Rozšířená mimetická gravitace
Jiroušek, Pavel ; Vikman, Alexander (advisor) ; Novotný, Jiří (referee)
We consider a novel extension of the recently proposed mimetic gravity. The latter is a scalar-tensor theory which is able to describe dark matter on cosmological scales. Moreover, this theory can be considered as a low energy limit of the projectable Horava-Lifshitz gravity. The proposed novel extension directly couples gradients of the mimetic scalar field to the curvature tensor. These couplings introduce into the energy momentum tensor an anisotropic stress which is non-vanishing even at the first order in perturbations around a cosmological background. Further we show that such terms modify the formula for the speed of sound of scalar perturbations and even more importantly change the speed of propagation for the gravitational waves. The appearance of the anisotropic stress and the consequent nontrivial speed of propagation of the gravity waves are new phenomena which were not present in the previously studied mimetic models. Furthermore, we demonstrate that the effective Newton's gravitational constant in the background Friedmann equations is shifted in the presence of the novel couplings of the mimetic scalar field. We calculate the quadratic action for scalar and tensor perturbations and briefly discuss possible instabilities. Finally we consider the current observational bounds on the model....
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Časově závislé procesy v teorii strun
Jiroušek, Pavel ; Schnabl, Martin (advisor) ; Novotný, Jiří (referee)
String field theory is a second quantized approach to string theory able to describe off-shell processes like transition between vacua, so-called tachyon condensation, and its dynamics. In this work, we review the necessary tools and ingredients needed to formulate such theory for open bosonic string theory. Namely, we introduce elementary conformal field theory methods and some basics of perturbative string theory. We then recognize the kinetic term of string field action from on-shell condition of the perturbative string and axiomatically introduce an algebraic structure on its Hilbert space to construct interaction terms. We briefly discuss the connection of classical string field theory solutions with various string field backgrounds and present means of reconstructing the background from a given solution by calculating the boundary state. We show that such backgrounds act like a source of open string states. We conclude by calculation of such boundary state for a solution representing light-like tachyon rolling. Powered by TCPDF (www.tcpdf.org)
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