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Cosmology on Small Scales 2022: Dark Energy and the Local Hubble Expansion Problem
Křížek, Michal ; Dumin, Y. V.
One hundred years ago, Russian mathematician and physicist Alexander A. Friedmann applied the system of Einstein equations to the three-dimensional sphere with a time varying radius. In this way, he obtained a nonlinear ordinary differential equation which is called the Friedmann equation after him and serves now as a cornerstone of the standard cosmological model. Unfortunately, it is well known that this model exhibits a number of paradoxes. Thus, the main goal of the CSS 2022 Conference Proceedings is to discuss whether and how the Friedmann equation can be applied at the various spatial scales, from our local cosmic neighborhood up to the whole Universe, and if the existence of dark matter and dark energy are merely artifacts of the excessive extrapolations.
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Určování parametrů temné energie a modifikované gravitace v rámci projektu LSST
Vraštil, Michal ; Prouza, Michal (advisor) ; Heyrovský, David (referee)
Temnáenergietvořícípřibližně70%hmotyvesmíruz·stávájednouznejvětšch záhad moderní fyziky. K pochopení její podstaty jsou potřeba přesná kosmolog- ická měření. Jedním z projekt· zkoumající tuto exotickou formu hmoty bude i Large Synoptic Survey Telescope, který pom·že potvrdit či vyvrátit standardní kosmologický model (ΛCDM). Pro úspěch projektu je potřeba prozkoumat r·zné teorie temné energie. Jednou z alternativních teoríí vysvětlujících urychlenou expanzi vesmíru je tzv. chameleoní gravitace. Chameleon je nové skalární pole s hmotou závisející na okolní hustotě. V hustých prostředích jako je například Sluneční soustava získává pole velkou hmotu a propaguje se pouze na malých vzdálenostech díky čemuž m·že uniknout standardním test·m gravitace. V prá- ci mimo jiné studujeme chování chameleoního pole v okolí hvězd a v galaxiích. Ukazujeme také za jakých okolností je toto pole možné detekovat pomocí spek- troskopických měření a slabého čočkování.
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Study of dark energy and modified gravity and their influence on the cosmological parameters of the universe
Vraštil, Michal ; Prouza, Michael (advisor) ; Heitmann, Katrin (referee) ; Heyrovský, David (referee)
Title: Study of dark energy and modified gravity and their influence on the cosmological parameters of the universe Author: Michal Vraštil Institute: Institute of Physics of the Czech Academy of Sciences Supervisor: RNDr. Michael Prouza, Ph.D., Institute of Physics of the Czech Academy of Sciences Abstract: Discovery of the accelerated expansion of the Universe poses a major theoretical puzzle. Although the assumption of a non-zero cosmological constant provides a minimal extension of general relativity that is consistent with observational data, many theories of modified gravity have been suggested as possible alternatives due to serious problems connected with the cosmological constant. Numerical predictions of structure formation for these models in the fully non-linear regime are very expensive and it is difficult, if not impossible, to explore such a huge space of models and parameters using high-resolution N-body simulations. Even in the mildly nonlinear regime, perturbative methods can become extremely complex. We explore whether simplified dynamical approximations, applicable for a certain set of cosmological probes, can be used to investigate models of modified gravity with acceptable accuracy in the latter instance. For the case of chameleon gravity, we found that it is screened away on scales...
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Dynamical systems in cosmology
Knob, Lukáš ; Acquaviva, Giovanni (advisor) ; Loukes Gerakopoulos, Georgios (referee)
The main aim of this thesis is the analysis of different cosmological models from the standpoint of dynamical systems theory. We consider mostly spatially curved FLRW metric with different source terms, some of them possible candidates for dark matter and dark energy, particularly linear barotropic fluids, Chaplygin gas and canonical scalar field with exponential and general form of potential. We rewrite the cosmological equations as the system of the first order differential equations in dimensionless variables and study globally their phase space and the stability of the critical points. We also present few interesting features of models with interactions between two cosmic fluid constituents and mention dynamical properties of orthogonal Bianchi I models. 1
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Určování parametrů temné energie a modifikované gravitace v rámci projektu LSST
Vraštil, Michal ; Prouza, Michal (advisor) ; Heyrovský, David (referee)
Temnáenergietvořícípřibližně70%hmotyvesmíruz·stávájednouznejvětšch záhad moderní fyziky. K pochopení její podstaty jsou potřeba přesná kosmolog- ická měření. Jedním z projekt· zkoumající tuto exotickou formu hmoty bude i Large Synoptic Survey Telescope, který pom·že potvrdit či vyvrátit standardní kosmologický model (ΛCDM). Pro úspěch projektu je potřeba prozkoumat r·zné teorie temné energie. Jednou z alternativních teoríí vysvětlujících urychlenou expanzi vesmíru je tzv. chameleoní gravitace. Chameleon je nové skalární pole s hmotou závisející na okolní hustotě. V hustých prostředích jako je například Sluneční soustava získává pole velkou hmotu a propaguje se pouze na malých vzdálenostech díky čemuž m·že uniknout standardním test·m gravitace. V prá- ci mimo jiné studujeme chování chameleoního pole v okolí hvězd a v galaxiích. Ukazujeme také za jakých okolností je toto pole možné detekovat pomocí spek- troskopických měření a slabého čočkování.
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Inhomogeneous cosmology and averaging methods
Kašpar, Petr ; Svítek, Otakar (advisor) ; Balek, Vladimír (referee) ; Carloni, Sante (referee)
In this work we have examined different methods of averaging in general relativity and cosmology. We developed the method based on Cartan scalars. We computed the backreaction term for a flat LTB model with a special ansatz for the radial function. We found out that it behaves as a positive cosmological constant. In the next part of this thesis we were interested in averaging inside LRS class II dust model. For this family we averaged all the Einstein equations and the resulting system generalizes the Buchert equations. We numerically worked out two concrete examples where deceleration parameter changes its sign from positive to negative. Powered by TCPDF (www.tcpdf.org)
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Částicový model pro Higgsův kondensát a anomální geometrická difrakce
Souček, Jiří
In this paper we propose a particle model for the Higgs’ condensate: we propose that this condensate is the set of (infinite velocity) non-local tachyons. We show that then there exists the anomalous geometrical diffraction (which contradicts to quantum mechanics). We show that there exists a universal time constant which defines the limits of the validity of quantum mechanics. We propose an experiment testing the existence of the anomalous geometrical diffraction. We proposed the dark energy conjecture which enables to make an estimate of the time constant. We assume certain (“Feynman”) interaction between standard particle and the nonlocal tachyon. All this is related to the new (finite) form of the Feynman integral.
Fulltext:  PDF
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On the Friedmann equation for the three-dimensional hypersphere
Křížek, Michal ; Mészáros, A.
The present standard cosmological model of the evolution of our universe, is based on the Friedmann equation, which was published by Alexander Friedmann in 1922. He applied Einstein’s equations to an expanding threedimensional sphere which enabled him to avoid boundary conditions. However, his description was very brief. Therefore, the main objective of this article is to detailed a derivation of the Friedmann equation for an unknown expansion function a = a(t) representing the radius of the universe. Furthermore, we present serious arguments showing why the validity of Einstein’s equations should not be extrapolated to the entire universe.
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