Dr. Lavrentios Kazantzidis

Experience

Teaching Assistance

Winter Semesters 2017 – 2022     Classical Electrodynamics I (5th Semester Core Course – 52)
                                                              Department of Physics, University of Ioannina, Greece
Spring Semesters 2020 – 2022     Cosmology (Advanced Undergraduate Elective Course – 105)
                                                              Department of Physics, University of Ioannina, Greece
Spring Semesters 2018 – 2019     Gravity and Cosmology (Advanced Graduate Elective Course – M122) 
                                                              Department of Physics, University of Ioannina, Greece

M.Sc. Diploma Thesis

Title: Dark Energy from Generalized Gravitational Theories with a Time-dependent Newton’s Constant (scalar tensor theories).
Abstract: This work is focused on alternative forms of Dark Energy. The challenges that the ΛCDM model faces led to a variety of alternative models, such as extra dimensions, quintessence models, $f(R)$ extended gravity theories, scalar tensor quintessence models etc. The main goal of my master thesis is to investigate the cosmological dynamics for general scalar tensor quintessence field models. Firstly, we introduce the reader to the mathematical formalism of  Standard Cosmology and then, we study alternative theories that can play the role of Dark Energy such as quintessence models based on linear-negative potentials of the form $V (\phi) = −s \, \phi$. In the second part of this work we review the scalar tensor quintessence field models and their theoretical background. We investigate the equation of state parameter $w(z)$ for these particular models and we use the Union2.1 dataset of 580 SnIa as a validity test and to explore the observational consistency of the theoretical model and its predictions for the fate of our Universe. Finally we study qualitatively different potentials of the form $V = s \, |\phi|^{n}$ and use, once again the Union2.1 dataset as a consistency test of our model.  

You can see my full M.Sc. thesis here.

Ph.D. Thesis

Title: Cosmological Implications of Scalar Tensor Theories
Abstract: In this PhD dissertation we study the cosmological consequences of modified theories of gravity. Motivated by the theoretical and observational challenges of the concordance model ΛCDM, we use up to date cosmological data from both geometric and dynamical probes to constrain modified gravity theories and extract the relevant best fit parameters. In particular, we first introduce the reader to the mathematical formalism of standard cosmology and then focusing on one of the two major tensions that ΛCDM faces ($\sigma_8$ tension), we introduce a purely phenomenological parametrization for the evolving Newton’s constant $G_{\rm eff}$ and constructing an up to date compilation of growth data we extract its best fit parameters. Then, we study viable modified theories of gravities (such as f(R) and scalar tensor theories) in order to see if they have the potential to support the observed behavior the evolving Newton’s constant. Moreover, we examine other cosmological data (such as the low l cosmic microwave background data as well as the Pantheon compilation, i.e. the latest publicly available Type Ia supenovae compilation that is publicly available) to impose strong constraints on the phenomenological parametrization for $G_{\rm eff}$. Next, we study the constraining power (sensitivity) of a wide range of cosmological observables on cosmological parameters, showing that the sensitivity is actually a rapidly varying function of the redshift where the observable is measured and not a monotonically increasing function. In addition, we consider two late time gravitational transition dark energy models that have the ability to simultaneously tackle both the $H_0$ and growth problems and using the full cosmic microwave background data as well as some other up to date cosmological data, we obtain their quality of fit and compare it with the quality of fit provided by other well studied dark energy models that have been proposed as possible solutions in the literature as well as with the concordance model of standard cosmology. Finally, we study the impact of various modified gravity models in the sub-mm scales using the data of the Washington experiment.

You can see my full Ph.D. thesis here.