Dissertation Title: STRONG LENSING AS A METHOD IN THE STUDY OF THE EVOLUTION OF THE UNIVERSE

Author: Esmeralda GULIQANI 

• Institution: University of Tirana, Faculty of Natural Sciences, Department: Physics

• Field of study: NATURAL

• Publication date: 03/06/2026

• The dissertation is published in Albanian.

© Copyright: Esmeralda GULIQANI 

Published by the University of Tirana. Based on legal acts, regulations and policies of the UT.

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Briefing

The aim of this paper is to study the phenomenon of gravitational refraction and its role in understanding the evolution of the Universe, a topic that has gained particular importance due to the continuous divergence between the values ​​of the Hubble constant determined from measurements of the early and late Universe. In this context, strong lensing of quasars appears as a complementary and indispensable technique for measuring cosmological parameters. Although hundreds of lensed quasars have been identified, only a small fraction provide qualitative data for detailed analysis, so new space and ground-based telescopes have planned dedicated observations. This study evaluates the potential of the Roman telescope for detecting quasars lensed by galaxies along the line of sight. Using the Monte Carlo method, 200,000 events were generated and it was found that about one quasar in 150 is expected to be lensed, while about 85% of the cases are lensed by a single galaxy. For these events, the image positions, magnification ratios and time delays have been numerically calculated, considering three models for the lensed galaxy: SIS, NIS and SIE. The SIS model produces two images collinear with the galactic center. The NIS model gives three collinear images, where the middle image is not detectable. The SIE model can generate configurations with two, three or four images, where double quasars constitute the majority of cases. A linear dependence between the ratio of image positions and the magnification ratio has been observed for the SIS and NIS models. The influence of perturbations (flares) on the light curves of the lensed images has also been examined, which can produce variations that allow the measurement of multiple time delays. The study also includes an analysis of the effect of micro-refraction on the broad emission lines CIV, CIII] and MgII, which provide valuable information on the internal structure of quasars. These findings are important in the optimal planning of future observations with the Roman telescope, aiming to provide quality data for determining cosmological parameters and advancing knowledge on the evolution of the Universe.

Keywords: strong gravitational refraction, quasar, galaxy, observations, cosmology.

 

Abstract

The aim of this work is to study the phenomenon of gravitational lensing and its role in understanding the evolution of the Universe, a topic that has gained particular importance due to the persistent divergence between the values ​​of the Hubble constant determined from measurements of the early Universe and those of the late Universe. In this context, strong lensing of quasars presents itself as a complementary and essential technique for measuring cosmological parameters. Although hundreds of lensed quasars have been identified, only a small fraction provides high-quality data for detailed analyses, hence new space- and ground-based telescopes have planned dedicated observations. This study assesses the potential of the Roman telescope for detecting quasars lensed by galaxies along the line of sight. Using the Monte Carlo method, 200,000 events were generated, and it was found that approximately one quasar in 150 is expected to be lensed, while about 85% of the cases are lensed by a single galaxy. For these events, the image positions, magnification ratios, and time delays were numerically calculated, considering three models for the lensing galaxy: SIS, NIS, and SIE. The SIS model produces two collinear images with respect to the galaxy center. The NIS model yields three collinear images, with the central image not being detectable. The SIE model can generate configurations with two, three, or four images, where double quasars constitute the majority of cases. A linear dependence between the image position ratio and the magnification ratio was observed for the SIS and NIS models. Furthermore, the influence of perturbations (flares) on the light curves of the lensed images was examined, which can produce variations that allow the measurement of multiple time delays. The study also includes an analysis of the microlensing effect on the broad emission lines CIV, CIII], and MgII, which provide valuable information about the internal structure of quasars. These findings are important for the optimal planning of future observations with the Roman Telescope, aiming to ensure high-quality data for the determination of cosmological parameters and the advancement of our understanding of the evolution of the Universe.

Keywords: strong lensing, quasars, galaxy, observations, cosmology.