This doctoral thesis in physics at Luleå University of Technology is devoted to the phenomenology of compact stars, and theoretical models of their interior. Particle physics has provided fundamental concepts and details needed to develop a description of matter and the evolution of the universe. It is, however, difficult to obtain information about the properties of matter at low temperature and high density from such experiments. In this context, astrophysical observations constitute an important source of information, thanks to the high resolution of present and near-future terrestrial and space-based observatories. The density of matter in neutron stars exceeds that in atomic nuclei, and little is known about the nature of their interior. It is clear that the interaction between the smallest observed building blocks of atomic nuclei, the quarks, becomes weaker with increasing density. Matter should therefore dissolve into a state of nearly free quarks at high densities, and models of classical superconductivity advocate that this state is a superconductor. The argument for this is simple: a low-temperature Fermi system with a weak attractive interaction is unstable with respect to formation of Cooper pairs. It is not known if this state of matter exists in neutron stars, but models suggest that it is possible. The major part of the work summarised in this thesis is the development of a model of superconducting quark matter, and its consequences for the phenomenology of neutron stars and their formation in the collapse of massive stars. It is a Nambu - Jona-Lasinio model with self-consistently calculated quark masses and pairing gaps, which properly accounts for the beta-equilibrium and/or charge neutrality constraints in compact stars and heavy-ion collisions. Phase diagrams and equations of state of superconducting quark matter are presented, and the influence of different assumptions about the effective quark interaction is investigated. The effect of neutrino untrapping in hypothetical quark cores of newborn neutron stars is investigated, and phase diagrams for quark matter with trapped neutrinos are presented. While no evidence for the presence of quark matter in neutron stars exists, it is explicitly shown that observations do not contradict this possibility. On the contrary, the presence of a quark matter core in neutron stars can overcome problems with hadronic equations of state. In contrast to the expectation from more simple model calculations, the results presented here suggest that strange quarks do not play a significant role in the physics of compact stars. While there is some room for bare strange stars, such models suffer from low maximum masses, and the presence of a hadronic shell tends to render stars with strange matter cores unstable. Regardless of the nature of matter in neutron stars, general relativity and the standard model of particle physics limit their density to < 10^16 g/cm^3. In the light of established theory, any object with a density exceeding this limit should be a black hole. It is suggested in this thesis that if quarks and leptons are composite objects, as suggested, e.g., by the three particle generations in the standard model, a yet unobserved class of compact objects with extremely high densities could exist. As the hypothetical pre-quark particles are called preons, these objects are named 'preon stars'. The properties of preon stars are estimated, and it is shown that their maximum mass depends on the quark compositeness energy scale. In general, the mass of these objects should not exceed that of the Earth, and their maximum size is of the order of metres. Several methods to observationally detect preon stars are discussed, notably, by gravitational lensing of gamma-ray bursts and by measuring high-frequency gravitational waves from binary systems. To have a realistic detection rate, i.e., to be observable, they must constitute a significant fraction of cold dark matter. This condition could be met if they formed in a primordial phase transition, at a temperature of the order of the quark compositeness energy scale. Some unexplained features observed in spectra of gamma-ray bursts are discussed, as they are similar to the signature expected from a preon-star gravitational lensing event. An observation of objects with these characteristics would be a direct vindication of physics beyond the standard model.
Luleå: Luleå tekniska universitet, 2007. , 95 p.