Topics in cosmology

The universe is expanding and the contents in it determine the rate of the expansion. The Hubble constant H0, which is a measure of the current expansion rate, and the average mass density parameter of non-relativistic matter content Om have been measured by many different observers. We use median statistics to study these O m measurements and find 0.2 ≲ Om ≲ 0.35 at two standard deviations. This result was confirmed by the WMAP cosmic microwave background anisotropy measurements. Our analysis of the H0 measurements shows that the error distribution is non-Gaussian, so one should be cautious when interpreting errors on the values of H0.; The major contribution to the energy budget of the universe is the mysterious dark energy, introduced to explain the accelerating expansion. A popular dark energy candidate is a scalar field ( f ) with potential energy density V ( f ) ∝ f -alpha, alpha > 0, at low redshift z. We use two independent measurements to constrain the value of alpha. One is the compact radio source angular size versus redshift data, and the other is the statistics of strong gravitational lensing from the Cosmic Lens All-Sky Survey data. Small alpha values are favored in both cases.; More details about the history of the universe can be learned from the cosmic microwave background (CMB). For example, a primordial cosmological magnetic field induces Alfven waves, which in turn generate CMB anisotropies. A homogeneous primordial magnetic field with fixed direction induces correlations between the al-1, m and al+1, m multipole coefficients of the CMB temperature anisotropy field. Calculating these correlations for the first year WMAP data and comparing with simulations, we place 3 sigma upper limits on the strength of the magnetic field of B < 15 nG for spectral index n = -5 and B < 1.7 nG for n = -7.