Cosmological Investigations On Large And Small Scales

Abstract

The standard model of cosmology, founded on Einstein’s theory of gravity and on the Cosmological Principle (CP), has been understood as being successful in providing explanations for many observations so far. However, it relies on some yet unknown components; dark matter and dark energy. Apart from these two problems, there are some other interesting challenges for this model on both large and small scales. On large scales, some deviations from isotropy in the Cosmic Microwave Background (CMB) radiation have stimulated studies on testing isotropy using various extragalactic data. On small scales there are long-lasting issues with the predictions of structure formation in the standard model and the observed properties of the Local Group (LG) of Galaxies. The contribution of this thesis is testing the assumption of isotropy of the CP on large scales and launching a survey of dwarf galaxies for addressing the small scale problems. <br /> On large scales, first we probed the isotropy of cosmic acceleration using the magnitude-redshift relation of high redshift Type Ia Supernovae (SNe Ia). We found that although the deviation from isotropy is small, it is significantly aligned with the direction of the CMB dipole. This can be either due to uncleaned systematics, or some yet unknown phenomena. However, because of the non-uniformity of the sky distribution of the current SNe Ia data sets, it is hard to draw a firm conclusion. We then probed the isotropy of the distribution of the galaxy morphological types for the first time. We used the de Vaucouleurs morphological types of more than 60,000 galaxies from the HyperLeda database within a distance of about 200 Mpc. Surprisingly, we found a significant hemispherical asymmetry aligned with the celestial equator. Regardless of the significance of the difference in the distribution, the observed alignment would be a major challenge for the CP if the anisotropy is real. However since the asymmetry has a north-south alignment, it is most likely due to a systematic bias in the classifications of the morphological types or in the homogenization procedure of the catalog. Further studies are essential to reveal the exact source of this anisotropy. Future all-sky surveys can provide a large number of SNe Ia, and can deliver uniform measurements of galaxy morphological types for further testing of the assumption of isotropy.<br /> On small scales, we started the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) in which we use long exposure wide field images obtained by, as the name suggests, small telescopes to search for dwarf satellite galaxies around nearby Milky-Way-type galaxies. Increasing the number of known dwarf satellite galaxies outside the LG is necessary for testing the predictions of the standard model of cosmology in other galaxy groups. In the first application of the survey we managed to discover eleven low surface brightness (LSB) galaxies around six nearby Milky-Way-type galaxies. These LSB galaxies have similar properties to the satellite galaxies in the LG. The DGSAT will continue its operation and we are very optimistic that we can find more LSB galaxies outside the LG. <br /> The forthcoming huge amount of data in the next two decades from various large and small surveys will provide valuable information for studying the assumptions and predictions of the current standard model of cosmology and can enhance our knowledge about the Universe.