No abstract

We discuss axion dark matter detection via two mechanisms: spontaneous decays and resonant conversion in neutron star magnetospheres. For decays, we show that the brightness temperature signal, rather than flux, is a less ambiguous measure for selecting candidate objects. This is owing principally to the finite beam width of telescopes which prevents one from being sensitive to the total flux from the object. With this in mind, we argue that the large surface-mass-density of the galactic center or the Virgo cluster center offer the best chance of improving current constraints on the axion-photon coupling via spontaneous decays. For the neutron star case, we first carry out a detailed study of mixing in magnetized plasmas. We derive transport equations for the axion-photon system via a controlled gradient expansion, allowing us to address inhomogeneous mass-shell constraints for arbitrary momenta. We then derive a nonperturbative Landau-Zener formula for the conversion probability valid across the range of relativistic and nonrelativistic axions and show that the standard perturbative resonant conversion amplitude is a truncation of this result in the nonadiabatic limit. Our treatment reveals that infalling dark matter axions typically convert nonadiabatically in magnetospheres. We describe the limitations of one-dimensional mixing equations and explain how three-dimensional effects activate new photon polarizations, including longitudinal modes and illustrate these arguments with numerical simulations in higher dimensions. We find that the bandwidth of the radio signal from neutron stars could be dominated by Doppler broadening from the oblique rotation of the neutron star if the axion is nonrelativistic in the conversion region. Therefore, we conclude that the radio signal from the resonant decay is weaker than previously thought, which means one relies on local density peaks to probe weaker axion-photon couplings.

We calculate predictions from a wide class of ''active'' models of cosmic structure formation which allows us to scan the space of possible defect models. We calculate the linear cold dark matter power spectrum and cosmic microwave background anisotropies over all observable scales using a full linear Einstein-Boltzmann code. The calculations are performed entirely in an Einstein-de Sitter cosmology. Our main result, which has already been reported, points to a serious problem reconciling the observed amplitude of the large-scale galaxy distribution with the cosmic background explorer normalization. Here, we describe our methods and results in detail. The problem is present for a wide range of defect parameters, which can be used to represent potential differences among defect models, as well as possible systematic numerical errors. We explicitly examine the impact of varying the defect model parameters and we show how the results substantiate these conclusions. The standard scaling defect models are in serious conflict with the current data, and we show how attempts to resolve the problem by considering non-scaling defects or modified stress-energy components would require radical departures from what has become the standard picture. ͓S0556-2821͑98͒01122-9͔ PACS number͑s͒: 98.80.Cq, 95.35ϩd

We investigate the dynamics of Q-balls in one, two and three space dimensions, using numerical simulations of the full nonlinear equations of motion. We find that the dynamics of Q-balls is extremely complex, involving processes such as charge transfer and Q-ball fission. We present results of simulations which illustrate the salient features of 2-Q-ball interactions and give qualitative arguments to explain them in terms of the evolution of the time-dependent phases.