|
|
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.
|
|
|
Formation and evolution of dynamical binaries
Pavlík, Václav ; Šubr, Ladislav (advisor) ; Heyrovský, David (referee)
The aim of this work is the study of hard binary stars in the star cluster and their time of life before they crossed the energetic boundary, which separates the soft and hard binaries. We used our own method of identification of binary stars within the model of star cluster which is based on the given positions, velocities and masses of stars. This method uses three main criteria -- the proximity of two stars, the time that this pair was found together and negative total energy. The results that we obtained using our algorithm helped us to determie more precise boundary between the hard and soft binary than the analytical estimates based on the evaluation of the typical kinetic energy of the free stars within the cluster. We also studied the lifetime of the binaries before they become hard and we reached a correspondence with the results of Tanikawa et al. (2011), who studied similar topic. Due to a large set of numerical realizations, which we analysed, we got better statistics than previous works. We also found that the examined lifetime of binaries seems to be represented by exponential distribution. For better analysis of this distribution it would be necessary to use about five times higher time resolution than we worked with.
|
|
| |
|
| |
|
|
Gravitational microlensing by a few-body system
Daněk, Kamil ; Heyrovský, David (advisor) ; Dovčiak, Michal (referee)
In the present work we study gravitational microlenses consisting of a few point masses. We present the theoretical framework of n-point-mass lensing, and followup by a more detailed description of binary and triple lenses. We demonstrate the analysis of critical curves and caustics of the bojary lens and apply the same approach to the case of triple lenses. We divided several two-dimensional cuts of the parameter space into areas with different topology of critical curves. We studied the amplification of a source located in the vicinity of interesting parts of the caustic. We tested the applicability of an approximation for the amplification at the intersection of two folds. Finally, we applied the formalism to the case of the observed triple-lens microlensing event OGLE-2006-BLG-109.
|
|
|
Measuring accretion disk properties from quasar microlensing
Ledvina, Lukáš ; Heyrovský, David (advisor) ; Dovčiak, Michal (referee)
When we observe a~distant quasar through an intervening galaxy we may see a~number of observable images caused by hte deflecton of light in galaxy's gravitational field. If an image passes through the stellar population of the galaxyit can be additionally microlensed by individual stars. In this work we first compute microlensing magnification maps for a~point source. We than use these maps to determine light curves for extended source, namely for a~quasar accretion disc.In the second part we compare obtained light curves for different discs and we will study the influence of inclination, orientation and surface brightness distribution of an accretion disk on the light curve during an individual crossing of a~cusp and fold of microlensing custic network
|
|
| |
|
| |
|
|
Quasar accretion disk mapping by gravitational microlensing
Ledvina, Lukáš ; Heyrovský, David (advisor)
Quasar microlensing is a relatively newly explored phenomenon, which is ideally suited for studying the spatial distribution of emission from the innermost accretion disc. By now we know many macrolensed quasars, in which we can observe multiple images formed by the deflection of light in the gravitational field of an intervening galaxy. In case one of these images passes directly through the stellar population of the galaxy, it can be additionally microlensed by individual stars. The gravitational field of these stars forms a caustic network for light passing by. When a quasar accretion disc crosses behind this network, induced changes can be observed in the light curve as well as in the spectrum. In the first part of this thesis we study the statistics of the time intervals between successive caustic crossings. In the second part we use a fully relativistic Kerr-metric thin-disc model for studying the light curve of a fold-caustic crossing and its dependence on the accretion- disc parameters. In the last part we simulate changes in the X-ray iron-line profile during a fold-caustic crossing. We find characteristic spectral features formed on the line, and derive their analytical description. Finally, we map the maximum strength of microlensing-generated peaks on the spectral line for different...
|
|
|
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...
|
guest ::
