Abstract
The existence of dark matter is a well established fact both due to galaxy rotation curves and also due to the CMB temperature anisotropies and the matter power spectra. According to the ΛCDM model, the so called standard model of cosmology, 22% of the energy content of the Universe is in the form of dark matter. There are several dark matter candidates. In this thesis we will study one of them; the axion. The axion is well established from particle physics, introduced as the solution of the strong CP problem of Quantum Chromodynamics. The aim of this work is to use astrophysics and cosmology to probe the properties of the axion. If axions are indeed present in stars, they will interact with the ordinary matter and thereby constitute a source of anomalous energy-loss. By studying the stellar evolution processes affected by this energy-loss, we can derive astrophysical bounds on the axion mass and coupling constant. Axions present in the cosmic plasma of the early universe would freeze-out through the standard WIMP scenario. We compute the present axionic density parameter from this process and compare it with the density parameter for CDM obtained from the 7-year WMAP data, which results in a cosmological bound on the axion mass and decay constant.