arXiv subjects

Denitsa Staicova

Publications and source records attributed to Denitsa Staicova.

28 records · Page 2Linked to original sources

arXiv2019

Cosmological Solutions from a Multi-Measure Model with Inflaton Field

In a recent work, we demonstrated that a modified gravity model in which a scalar "darkon" field is coupled to both the standard Riemannian metric and to another non-Riemannian volume form is compatible with observational data from Supernovae Type Ia. Here, we investigate a more complicated model with an additional "inflaton" scalar field. We demonstrate numerically that the model can qualitatively reproduce the Universe inflation epoch, matter dominated epoch, and present accelerating expansion in a seamless way. We show that such solutions occur only when the model parameters are within a very particular range. The main numerical problem we are faced with is reproducing the extremely small time of the inflation epoch. Here, we present how the variation of some parameters affects this time.

gr-qc↗

arXiv2019

Cosmology from multimeasure multifield model

We consider the cosmological application of a (variant of) relatively newly proposed model \cite{1609.06915} unifying inflation, dark energy, dark matter, and the Higgs mechanism. The model was originally defined using additional non-Riemannian measures but it can be reformulated into effective quintessential model unifying inflation, dark energy and dark matter. Here we demonstrate numerically that it is capable of describing the entire Universe evolution in a seamless way, but this requires some revision of the model setup. The main reason is that there is a strong effective friction in the model, a feature which has been neglected in the pioneer work. This improves the model potential for proper description of the Universe evolution, because the friction ensures a finite time inflation with dynamically maintained low-value slow-roll parameters in the realistic scenarios. In addition, the model predicts the existence of a constant scalar field in late Universe.

gr-qc↗

arXiv2018

The multi-measure cosmological model and its peculiar effective potential

The multi-measures model applied to cosmology has been recently shown to reproduce qualitatively the expected stages of the Universe evolution, along with some unexpected features. In this article, we continue its exploration with a detailed study of the effective potential of the model. An analysis of the limits of applicability of the effective potential show that during most of the Universe evolution, the effective potential is a very good approximation of the actual potential term. There is, however, a deviation between the two occurring in the earliest moments of the evolution, which has an important role on the behavior of the inflaton scalar field. In the studied cases, during this initial time, the inflaton is increasing in absolute value, which seems consistent with "climbing up" the effective potential. To investigate this behavior, we use numerical integration to find the numerical potential and we show that during the early stage of the evolution, the shape of the numerical potential is very different from that of the effective one and instead of a left plateau followed by steep slope, one observes only a slope with additional local maximum and minimum. This result demonstrates that for complicated equations of motion of the inflaton, one should not rely only on the notion of kinetic and effective potential terms to describe the problem as they may not be accurate in the entire numerical domain.

gr-qc↗

arXiv2018

Cosmological solutions from models with unified dark energy and dark matter and with inflaton field

Recently, few cosmological models with additional non-Riemannian volume form(s) have been proposed. In this article we use Supernovae type Ia experimental data to test one of these models which provides a unified description of both dark energy via dynamically generated cosmological constant and dark matter as a "dust" fluid due to a hidden nonlinear Noether symmetry. It turns out that the model allows various scenarios of the future Universe evolution and in the same time perfectly fits contemporary observational data. Further, we investigate the influence of an additional inflaton field with a step like potential. With its help we can reproduce the Universe inflation epoch, matter dominated epoch and present accelerating expansion in a seamless way. Interesting feature is that inflaton undergoes a finite change during its evolution. It can be speculated that the inflaton asymptotic value is connected to the vacuum expectation value of the Higgs field.

gr-qc↗

arXiv2016

Cosmological aspects of a unified dark energy and dust dark matter model

Recently, a model of modified gravity plus single scalar field model was proposed, in which the scalar couples both to the standard Riemannian volume form given by the square root of the determinant of the Riemannian metric, as well as to another non-Riemannian volume form given in terms of an auxiliary maximal rank antisymmetric tensor gauge field. This model provides an exact unified description of both dark energy (via dynamically generated cosmological constant) and dark matter (as a "dust" fluid due to a hidden nonlinear Noether symmetry). In this paper we test the model against Supernovae type Ia experimental data and investigate the future Universe evolution which follows from it. Our results show that this model has very interesting features allowing various scenarios of Universe evolution and in the same time perfectly fits contemporary observational data. It can describe exponentially expanding or finite expanding Universe and moreover, a Universe with phase transition of first kind. The phase transition occurs to a new, emerging at some time ground state with lower energy density, which affects significantly the Universe evolution.

gr-qc↗

arXiv2016

The Heun functions and their applications in astrophysics

The Heun functions are often called the hypergemeotry successors of the 21st century, because of the wide number of their applications. In this proceeding we discuss their application to the problem of perturbations of rotating and non-rotating black holes and highlight some recent results on their late-time ring-down obtained using those functions.

math-ph↗

arXiv2015

Numerical stability of the electromagnetic quasinormal and quasibound modes of Kerr black holes

The proper understanding of the electromagnetic counterpart of gravity-waves emitters is of serious interest to the multimessenger astronomy. In this article, we study the numerical stability of the quasinormal modes (QNM) and quasibound modes (QBM) obtained as solutions of the Teukolsky Angular Equation and the Teukolsky Radial Equation with appropriate boundary conditions. We use the epsilon-method for the system featuring the confluent Heun functions to study the stability of the spectra with respect to changes in the radial variable. We find that the QNM and QBM are stable in certain regions of the complex plane, just as expected, while the third "spurious" spectrum was found to be numerically unstable and thus unphysical. This analysis shows the importance of understanding the numerical results in the framework of the theory of the confluent Heun functions, in order to be able to distinguish the physical spectra from the numerical artifacts.

astro-ph.HE↗

arXiv2015

New results for electromagnetic quasinormal and quasibound modes of Kerr black holes

The perturbations of the Kerr metric and the miracle of their exact solutions play a critical role in the comparison of predictions of general relativity with astrophysical observations of compact massive objects. The differential equations governing the late-time ring-down of the perturbations of the Kerr metric, the Teukolsky Angular Equation and the Teukolsky Radial Equation, can be solved analytically in terms of confluent Heun functions. In this article, we solve numerically the spectral system formed by those exact solutions and we obtain the electromagnetic (EM) spectra of the Kerr black hole. Because of the novel direct way of imposing the boundary conditions, one is able to discern three different types of spectra: the well-known quasinormal modes (QNM), the symmetric with respect to the real axis quasibound modes (QBM) and a spurious spectrum who is radially unstable. This approach allows clearer justification of the term "spurious" spectrum, which may be important considering the recent interest in the spectra of the electromagnetic counterparts of events producing gravitational waves.

astro-ph.HE↗

arXiv2012

New results for electromagnetic quasinormal modes of black holes

The differential equations governing the late-time ring-down of the perturbations of the Kerr metric, the Teukolsky Angular Equation and the Teukolsky Radial Equation, can be solved analytically in terms of confluent Heun functions. In this article, for the first time, we use those exact solutions to obtain the electromagnetic (EM) quasinormal spectra of the Kerr black hole . This is done by imposing the appropriate boundary conditions on the solutions and solving numerically the so-obtained two-dimensional transcendental system. The EM QNM spectra are compared with already published results, evaluated through the continued fraction method. The comparison shows that the modes with lower $n$ coincide for both methods, while those with higher $n$ may demonstrate significant deviations. To study those deviations, we employ the $ε$-method which introduces small variations in the argument of the complex radial variable. Using the $ε$-method one can move in the complex $r-$plane the branch cuts of the solutions of the radial equation and examine the dependence of the spectrum on their position. For different values of $ε$, one can obtain both the frequencies evaluated through the well-established continued fraction method or somewhat different spectra calculated here for the first time. This result raises the question what spectrum should be compared with the observational data and why. This choice should come from better understanding of the physics of the problem and it may become particularly important considering the recent interest in the spectra of the electromagnetic counterparts of events producing gravitational waves.

astro-ph.HE↗

arXiv2011

Application of the confluent Heun functions for finding the quasinormal modes of nonrotating black holes

Although finding numerically the quasinormal modes of a nonrotating black hole is a well-studied question, the physics of the problem is often hidden behind complicated numerical procedures aimed at avoiding the direct solution of the spectral system in this case. In this article, we use the exact analytical solutions of the Regge-Wheeler equation and the Teukolsky radial equation, written in terms of confluent Heun functions. In both cases, we obtain the quasinormal modes numerically from spectral condition written in terms of the Heun functions. The frequencies are compared with ones already published by Andersson and other authors. A new method of studying the branch cuts in the solutions is presented -- the epsilon-method. In particular, we prove that the mode $n=8$ is not algebraically special and find its value with more than 6 firm figures of precision for the first time. The stability of that mode is explored using the $ε$ method, and the results show that this new method provides a natural way of studying the behavior of the modes around the branch cut points.

gr-qc↗