Probing Galaxy Clusters in Non-standard Cosmologies with Gravitational Lensing

Abstract

What is the nature of the dark sector in the Universe? This is one of the most fundamental questions in modern cosmology. The so-called standard model of cosmology (ΛCDM) fails to reproduce some observational data and thus exploring potential alternative cosmologies is of great importance for cosmology. In this research I probe two potential alternative cosmologies, ΛWDM and dynamical dark energy models, using both strong and weak gravitational lensing. Gravitational lensing is a powerful probe of the distribution of matter including dark matter. The mass of dark matter particles in the ΛWDM cosmologies is on the order of ∼ keV and the density fluctuations are suppressed on small scales. The evolving dark energy cosmologies assume that the role of dark energy is played by a dynamical scalar field rather than a cosmological constant with a constant energy density. The structures in non-standard cosmologies form and evolve differently from those in the ΛCDM model, and thus the internal properties and abundance of (sub)structures are not the same. Consequently, the strong and weak gravitational lensing which probe the matter distribution will differ. I find that both strong and weak lensing can probe for the presence of Warm Dark Matter. WDM clusters have slightly larger Einstein radii and lensing cross-sections than their CDM counterparts. The higher lensing efficiency in WDM cosmologies is a result of the physical size of dark matter haloes and their internal properties. WDM substructures are more extended than CDM ones, and hence they significantly alter the size of critical lines and caustics. The WDM substructures can also boost the cross-section for multiple images. In addition, they have a significant impact on the magnification distribution. Although the slope of the magnification distribution is the same for both WDM and CDM cosmologies, there is an excess in the magnification distribution of WDM model relative to that of the CDM one. In addition, the area of high magnification in the weak lensing regions of WDM models is larger than that of CDM ones. The matter in WDM cosmologies is more homogeneously distributed than their CDM counterparts, and hence there would be more matter in the beam. I also find that the impact of WDM on the weak lensing signature can be explored by means of the observed number density of galaxies as well as the reconstruction of shear and convergence. However, I find that the latter method is more reliable. Despite the difficulties with the method used to reconstruct the convergence, the spatial distributions of the reconstructed convergence in the WDM and CDM models show that the matter in the WDM cosmology is more homogeneously distributed than that in the CDM one. Strong lensing analyses in dynamical dark energy cosmologies showed that the Einstein radius and cross-section of φCDM clusters do not differ significantly from those of the ΛCDM counterparts. The φCDM clusters cannot reproduce the observations of 12 MACS clusters in the literature and thus the φCDM model cannot resolve the arc statistics problem. In addition, the strong lensing efficiency of φ(β = 0.050)CDM clusters is higher than that of their ΛCDM counterparts and the observed MACS sample. However, one cannot argue that φ(β = 0.050)CDM clusters can resolve the arc statistics problem, at least from these suites of simulations, as their masses are significantly higher than those of the observed MACS clusters.