Testing large scale cosmology with MeerKAT

Abstract

The cosmic radio dipole is an anisotropy in the number counts of radio sources, analogous to the dipole seen in the cosmic microwave background (CMB). Measurements of the radio dipole with large radio surveys have shown that though the radio dipole is aligned in direction with the CMB dipole, the amplitudes are in tension. These observations present an intriguing puzzle as to the cause of this discrepancy, with a true anisotropy having large repercussions for cosmology as a whole. Measurements of the cosmic radio dipole with large radio surveys have often suffered from systematics in the data, hampering sensitivity and reliability of these measurements. In this thesis, I aim to measure the cosmic radio dipole with the MeerKAT Absorption Line Survey (MALS). Though sky coverage of MALS is low, with 391 pointings observed in total, the sensitivity and field of view of MeerKAT yields thousands of sources observed in each pointing. We perform a deep analysis of the complete set of processing steps, from observations to cataloguing, of ten MALS pointings, to characterise and quantify potential systematic effects which could hamper a dipole measurement. Using the noise characteristics of these pointings, we find that we can homogenise the catalogues to a deep enough level for a dipole measurement. We furthermore define Bayesian estimators that are able to perform a dipole measurement with the sparse sky coverage of MALS. Testing these estimators out on other radio surveys, we perform the most significant measurement of the cosmic radio dipole yet. Finally, we perform a measurement of the dipole using all MALS pointings, but find an effect in the data which causes a systematic variation in source density with declination, hampering a dipole measurement. Though we can account for the effect by extending our estimators, it comes at the cost of further uncertainties, which can only be remedied by reprocessing the data. However, combined with other measurements performed in this thesis and in the literature, there is a little doubt anymore as to the legitimacy of the radio dipole measurement, and we may look forward to further measurements which aim to uncover the cause of the dipole tension.